Hydroponics 101: How it ALL comes down to vapour pressure deficiency (VPD)

Water molecules at the surface of water in a container can have enough kinetic energy to escape the liquid and form vapour in the air above the water, creating a pressure within the container – the vapour pressure. As these molecules collide with the surface of the water, they return to the liquid phase by the process of condensation. When the number of molecules vaporizing and condensing per unit of time is equal, the pressure becomes constant and has a value that is characteristic of a liquid at a given temperature.

Developing and maintaining indoor growing and hydroponic systems all comes down the vapour pressures within a grow system and their direct impacts on plant growth and health. Like everything else, however, understanding these impacts starts with the basics.


1.1 A few botany basics

1.1.1 Plant anatomy

Describing the vegetative growth process of plants starts with a look at their basic vegetative structure (a branch of botany – the scientific study of plants), given in Figure 1.

Oregon State University

Water and nutrients travel from the growth medium to the roots (or sprouting seed) and up the stem through a vascular system that propagates water, minerals, and sugars to their intended destinations. Nodes are one of those destinations, where small clusters with high cellular activity and growth form buds that eventually turn into leaves, stems, or flowers. The entire process orchestrates the flow of water throughout the plant from the roots to the leaves, where it can eventually assist in photosynthesis or exit the plant via transpiration.

1.1.2 Leaf structure and function

Consequently, the leaf has a crucial role in these processes that allow a plant to breathe and grow. Within its deep green pigment lie several layers designed to carry out these specific functions, outlined in Figure 2.

leaf structure
Oregon State University

The external layers of the leaf make up the upper and lower epidermis. Sunlight hits the upper epidermis, which is covered with a protective, waxy cuticle, while the lower epidermis contains stomata – tiny pores on the under-side of the leaf that are controlled by guard cells (for aquatic plant leaves, the stomata are found on the upper epidermis). Guard cells open and close the stomata in response to environmental cues, controlling the movement of water, oxygen, and carbon dioxide into and out of the leaf as a result. Between the upper and lower epidermis is the mesophyll, which contains the chloroplasts where photosynthesis takes place. At the root of it all enter the xylem and pholem, which are those key components of the vascular system transporting water and nutrients throughout the plant. Together, each of these constituents of the leaf drives plant growth via three major processes: photosynthesis, respiration, and transpiration.

1.1.3 Photosynthesis, respiration, and transpiration

Photosynthesis allows plants to create food from sunlight, carbon dioxide (CO2), and water (H2O). Carbon dioxide enters the leaf through the stomata, while water enters either from the stomata or, in higher quantities from the roots and vascular system. Sunlight then provides energy for the chemical reaction of photosynthesis (shown in Figure 3) to occur in the chloroplasts.


The products of photosynthesis are oxygen (O2), which is released through the stomata, and glucose (C6H12O6), which is either stored for later use (in low light conditions, for example), or sent where it is needed (such as to developing fruit).

Oxygen and glucose are necessary for cellular respiration, which breaks down glucose in a “reverse-photosynthesis” reaction to convert its chemical energy into a form the plant can use to grow and reproduce. Unlike photosynthesis, respiration can occur in the absence of sunlight, at any time of day.

At the same time, water moves into and out of the leaf through the stomata along with the carbon dioxide and oxygen for photosynthesis and cellular respiration. Transpiration occurs when stomata open to release water, thereby pulling more water and essential minerals up through the plant from the roots.

Oregon State University

Optimal growth requires a delicate balance among each of these three major processes (outlined in Figure 4 ) to efficiently respond to environmental cues. In a particularly hot and dry environment, for example, stomata close to avoid losing excess water. Stomata also tend to close at night, when the absence of light means there is insufficient energy to power photosynthesis. In each case, plants retain water and, without access to carbon dioxide, must rely on glucose stores for energy via respiration to continue to develop and grow. Before too much water accumulates or glucose stores are used up, the stomata open up again. Similarly, when photosynthesis occurs at a significantly faster rate than respiration, the accumulation of its products will signal stomata to close and photosynthesis to slow or stop. When respiration occurs faster than photosynthesis can generate glucose, the accumulation of carbon dioxide and water from respiration will trigger the process to slow or stop photosynthesis. It’s all about maintaining a consistent environmental balance to control these responses.

Knowing how photosynthesis, respiration, and transpiration respond to each other makes it possible to efficiently control the environmental cues that trigger them to collaborate seamlessly on plant growth and development. In fact, this is one of the main advantages of indoor growing and hydroponics.

1.2 Environmental control factors for growing

1.2.1 Light, air, and temperature

We’ve seen the importance of sunlight as a source of energy for photosynthesis. While a seed can germinate in the absence of light, for a seedling to continue to grow, light is essential. Plants in direct sunlight tend to grow to be more compact, while those in shady environments grow to be more elongated. The amount, intensity, and duration, of light all affect the quality plant growth.

Besides energy from light, photosynthesis relies on carbon dioxide. As a result, air – and what it’s made up of – is a key environmental factor to consider for plant growth. Our atmosphere is constantly fed carbon dioxide from plant and animal respiration, decaying organic matter, combustion fuels, and volcanic activity.

Wind is an important part of how plants are exposed to air. While it can assist in processes like transpiration by accelerating the transfer of heat from leaf surfaces, too much wind can lead to excessive transpiration from evaporation and potential structural damage.

Light, air, and wind can also all have an effect on temperature, which directly affects plant growth and development. The temperature of our atmosphere depends on the transfer of heat from the Earth’s surface to the air. As a result, temperature is naturally always changing. It also directly influences climate, which determines what types of plants can grow in a specific location. Plants that grow in colder climate have what is referred to as cold hardiness, while those that grow in warmer climates are known as tender.

Together, these factors influence that intricate balance of photosynthesis, respiration, and transpiration required for plant growth. Other important environmental influences, however, can make or break healthy and productive plant development.

1.2.2 Nutrients

Plant growth and development depend on 17 essential nutrients, or elements, which are divided into three categories. The first is made up of those macronutrients obtained from air and water: hydrogen (H), oxygen (O), and carbon (C). The other 14 elements, however, must come from the growth medium (soil, for example). These are split into the remaining two categories, termed soil-derived macronutrients and micronutrients. The divide simply refers to the amount of each element a plant requires; macronutrients – nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), and magnesium (Mg) – are used in amounts above 0.1% of a plant’s total dry weight, while micronutrients are required in low concentrations, usually just a few parts per million (ppm) of a plant’s dry weight. These amounts are outlined in Table 1.

University of Idaho

The 14 soil-derived nutrients each play an essential role in the different plant processes and functions, as outlined in Table 2.

University of Idaho

Nutrient solutions are often used to enrich plant growth medium, thereby optimizing nutrient concentration availability and uptake by roots. As a result – like many plant processes- making these nutrients available really comes down to one vital resource: water.

1.2.3 Moisture and the hydrologic cycle

Water plays a vital role in plant growth and development through photosynthesis, nutrient transport and availability, and structure maintenance – keeping plants turgid. It also lowers leaf temperature through transpiration, drawing water from the roots to the top of plants and increasing nutrient absorption in the process. As a result, a large part optimizing a grow system involves optimizing water levels in the air, in the growth medium, and on the plant itself. In other words, while nutrients are essential to plant growth and development, how plants use these nutrients is just as important.

The moisture of a plant and its surrounding environment is central to its growth. In our environment, the constant movement of water between the oceans, land surfaces, and the atmosphere is called the hydrologic cycle, illustrated in Figure 5.

hydrologic cycle
Government of Canada

Water that leaves the atmosphere and falls down to Earth as precipitation joins surface water or makes its way to groundwater. Through evaporation and transpiration, that water then returns to the atmosphere for a full cycle.

Plants play a vital role in the hydrologic cycle, transpiring 5 to 10 times as much water as they can hold at once every day they grow. Consequently, the hydrologic cycle plays a vital role in plant growth ad development. Maintaining optimal levels of moisture and water vapour within a grow system encourages nutrient uptake and transport through transpiration for a stronger, healthier crop. So, how do we measure that water vapor among the various components of a grow system? That’s where VPD comes in.

1.3 VPD : A formula for success

1.3.1 What is VPD?

The Vapour pressure deficit (VPD) is the difference between the water vapour pressure at saturation (SVP) and the actual water vapour pressure at any given time and temperature. Air that is saturated has the lowest water vapour pressure, leading to the condensation of all that water on surfaces in the form of dew, for example. However, as temperature and humidity change, that vapour pressure increases with the evaporation of water from surfaces into the air. As a result, the difference between the two creates a deficit: VPD.

The VPD within an environment tells us how a plant will transpire. As we’ve seen, plants release water from their leaves via transpiration and absorb it from their roots so that the rate and control of transpiration directly affect water-nutrient delivery and growth. The whole process is dependent on temperature and humidity, which greatly affect the flow of water (think, hydrologic cycle) throughout an environment. An increase in temperature, for example, causes the evaporation of water from surfaces, which increases water vapour pressure in the air (or, the level of moisture in the air), to a certain extent, thereby increasing the VPD. The same is true for higher levels of humidity.

While the humidity of a plant’s environment does directly affect transpiration, it’s dependency on air temperature, leaf temperature, stage of growth, and the time of day makes it a difficult variable to set specific targets for at any given time. VPD, however, combines each of these factors into one reading, allowing growers to maintain ideal temperature and humidity levels for optimal transpiration and growth.

1.3.2 VPD targets for growth stages

When plants transpire too quickly, they can lose too much water and either be deprived of nutrients, or absorb a surplus of water in response, eventually leading to toxicity from an increased concentration of nutrients in solution; plants that transpire too slowly may not have access to sufficient nutrient levels. The balance of temperature and humidity required to achieve optimal transpiration and VPD is shown in green in Figure 6

Argus Ltd (2009)

Environments with high humidity and low temperature, or with low humidity and high temperature are in the red zone; they lead to transpiration that is too slow, or too fast, respectively. These red and green regions (yellow for warning zones) show the boundaries to healthy plant growth and development. The highlighted sections then change slightly with the different growth stages and associated needs.

Generally, younger plants and seedlings are still fragile and require low-strain environments to reserve energy for the development of strong roots. As a result, plants in this growth stage thrive in conditions of low transpiration and VPD from about 0.4 to 0.7 kPa. With more roots and some leaves, plants are then ready for a higher VPD to help boost nutrient uptake and delivery for growth. The ideal VPD for this group ranges from 0.7 to 1.1 kPa. For plants already flowered, a slightly higher temperature with lower humidity  is best with VPD values from 1.1 to 1.4 kPa. This allows them to take up more water while still being dry enough to avoid any rotting buds or fruit. 

Every stage of growth brings unique requirements. By adjusting temperature and humidity to meet these requirements and reach optimal VPD values, growers ensure the most efficient and productive environmental conditions for their crop – a major advantage of indoor growing and hydroponics. Achieving these target VPD values begins with understanding how they are calculated.

1.3.3 The VPD formula

The VPD in a growth environment can be calculated in 3 steps from the temperature  (To in degrees celcius) and % relative humidity (%RH) within a system. Because VPD depends on the water vapour pressure in the air at saturation (vpsat), determining that value at any given temperature is the first step to VPD calculation, and can be achieved using in equation 1.

water vapour pressure

Expressed in kPa, vpsat represents environmental conditions of full moisture (100% RH), where transpiration is eventually slowed or stopped with lack of evaporation. As the %RH decreases and varies, the water vapour pressure within a grow system (vpair, in kPa) can then be determined using equation 2.

water vapour pressure

The vapour pressure difference between these two environments gives us the final VPD value (kPa), as in equation 3.

vapour pressure deficiency

Together with the VPD values from Figure 6, these equations give system growers a chance to create and modify a prime environment depending on the various factors that affect plant growth and development. But to keep it simple, a calculator never hurts.

1.3.4 DiscountHydro VPD  calculator

Our quick and simple VPD calculator helps growers establish the optimal VPD target for a system given its various temperature and humidity conditions. Try it today!

Enter your system’s air temperature and %RH to obtain a VPD target value as well as your system’s associated vpair and vpsat values.

For a VPD value more representative of what your plants are experiencing, the canopy temperature (that of the vegetative covering they form) can be entered in place of the air temperature.  It’s all about establishing and hitting that VPD target.


Welcome to DiscountHydro: All the hydroponic equipment you need, online, at a discount. It’s that simple.

Check out our environmental controls today to optimize your VPD and get the best out of your crop.

Happy shopping and hey, happy growing.


This Post Has 5 Comments

    1. Chloe Dupuis

      Thank you! Stay tuned for more.

  1. Josh

    Thank you!! This is a great guide.

    1. Chloe Dupuis

      Glad you liked it! Stay tuned for more, and happy growing 🙂

  2. Ben J.

    Very informative.

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Hydroponics 101


Whether you are already familiar with it, or a complete beginner – you will gain something valuable from this post, as we are going to take a deep dive into:

  • What is hydroponics?
  • What are the benefits of hydroponics?
  • The best hydro systems of 2022
  • How to grow hydroponics plants
  • And much, much more!

Read our latest Blog

We've got your questions answered in our FAQ

What are 1020 trays ?

A standard size seed tray (also called a “1020” tray) is slightly larger than 10 inches by 20 inches (25 centimeters by 51 centimeters), with a depth of 2 to 2.5 inches (5.1 to 6.4 centimeters).

Made for propagating: 1020 Trays

1020 Trays are the perfect solution for propagating your clones and seedlings with rock wool, seedling mix or starter plugs. The 1020 trays are available without holes to catch excess water and help to keep starts or plants moist and also with holes, to drain off excess water from the base of the cutting or seed. Raised channels are incorporated in the base of the trays to help air circulate under your pot or insert tray.

Will hydrogen peroxide kill root rot?

Hydrogen peroxide on root rot

Hydrogen peroxide will oxidize all bad bacteria that causes root rot. However, it will also kill and inhibit good bacteria which is a bad thing in the case of root rot. This can lead to a problematic bacteria obstacle for the plant to overcome. A Peroxide treatment can be a good first measure to take when dealing with root rot.

root rot

What temperature should water be for hydroponic growing?

Water temperatures recorded highest brix (sugar to alcohol ratio found in the conducting networks within the plant) levels when Hydroponic reservoirs are kept from 17 to 21°C. When cloning plants, the temperature is something to keep in mind as well. This temperature window will facilitate healthy rooting too.

Is hydroponics better than soil?

Is Hydroponics better? If the gardener is consistent, with the right knowledge, hydroponics will ALWAYS yield a better plant. Hydroponics ensures optimal brix levels (sugar to alcohol ratio found in the conducting networks within the plant) and will allow the genome to express its full potential

What is the disadvantage of hydroponics?

The number one disadvantage of Hydroponics is mechanical failure. Pumps, timers and ballasts etc. other than that… NONE!

Why are my hydroponic plants leaves turning yellow?

Are your plants leaves turning yellow?

Always look to see if it is lower branches or upper branches that have the yellow leaves on them. Normally we will always see yellow first on the bottom leaves. And this will be a classic nitrogen deficiency. An expert gardener will know to look at the roots first and the fertilizer second.

How long can hydroponic plants go without water?

Depending on size of plant and atmosphere it lives in plants can normally only resist up to an hour without water in most hydroponic applications. However, soilless container gardening gives you much more room for error as you water it as you would a house plant…

How do you fix root rot in hydroponics?

Easiest way to fix root rot is to trim the damaged roots and get rid of all the slimy blackened roots you can. Work the root system through a solution of peroxide and then re-place/transplant into a system that has beneficial bacteria included to help the root system.

How does Hydroponics not drown Terrestrial plants?

Terrestrial plants that are have their roots submerged full time require roughly 1/3 of the root system to be exposed a terrestrial plant has no issue staying under the water with its roots at this point. If the water is stagnant any root growth going into it will die off as needed.

Do hydroponic nutrients go bad?

Yes. Hydroponic nutrients can go bad (out of balance) specially organic ones. Here we will talk about the synthetic ones. Always be sure that the liquid Is clear and see-through. Fertilizer should not be murky whatsoever. As for the Organics, just use your nose! If it turns your stomach, it’s bad!

Is an air pump necessary for hydroponics?

An air pump is always recommended however if you were using a organic additive this can sometimes cause problems leading to algae blooms pH problems ECTDS problems. They make sure D.O(dissolved oxygen) levels are at there best. Circulating with a pump can be a better solution to low oxygen reservoirs however they can also lead to heating the solution and create false readings do to magnetism.

Is rainwater good for hydroponics?

Rainwater can be good for Hydroponics if you find your well water is significantly high in mineral deposits. Key notes to look at for when collecting rainwater are the type of roof (metal, tar, etc.) and if there is a lot of wildlife that flies by in the area, as their feces can be problematic to the solution.

What are the 6 types of hydroponics?

Different types of hydroponics are:


(hooked up to a fish tank),


(supplied fertilizer and h2o is misted on roots with no aggregate)

Ebb & Flow

(plants are suspended above a channel that fills and drains with fertilizer solution several times daily)

Container Gardening

(generally using peat or coco in a pot whilst watering with fertilized solution as needed)

Deepwater culture

(plant sits in a cell above a reservoir whilst fertilized solution is aerated below with roots extending into lower cell)

Drip irrigation

(plant sits in a cell above a reservoir as fertilized solution is dripped across root zone).

Can you over water in hydroponics?

Overwatering and Hydroponics can lead to disaster. If your water cycles are not correct and the plant has not rooted into its aggregate properly it is a sure recipe for root rot. A cycle timer is your best friend!

How often should you change the water in hydroponic systems?

Hydroponic Systems

The frequency with which you change your hydroponic system water is important. We recommend changing it every 4 to 7 days; it is necessary to change the water of a hydroponic system frequently. Doing so will ensure a proper Harvest as well as water health (PH, Fertilizer balance and proper bacteria present.

Hydroponic Systems

Are electronic ballasts more energy efficient?

Electronic ballasts are more efficient at converting electricity into usable light. Since your power bill is based on kilowatt-hours and not efficiency, a 1000 watt electronic ballast will cost you about the same as a 1000 watt HID ballast to operate.

What are the major ballast differences between MH, HPS and electronic ballasts?

Ballast differences come down to frequency output to the lamp and energy conversion from electricity to usable light are the biggest differences between HID (MH & HPS) ballasts and electronic ballasts. HID ballasts produce a frequency of 60 Hz. Electronic ballasts vary from manufacturer to manufacturer, but the frequency produced can be 400x that of an HID ballast. HID ballasts produce more heat than electronic ballasts, thus making electronic ballasts more energy efficient. You will not, however, save money on your electric bill by using electronic ballasts. HID lighting has been available for 60+ years, while electronic ballast (especially 400 watt and higher) is a relatively new technology.

What is a Switchable Ballast?

Switchable light ballast grow lights allow using either a Metal Halide or a HPS lamp. To change from one type of lamp to the other, insert the appropriate lamp and set the switch. This enables growers to use the proper spectrums for vegetable growth (MH) and flowering (HPS) with a simple to use switch. The switchable ballast grow light kits that we carry include everything you needs.

What is an enclosed Ballast Grow Light Kits?

An enclosed Ballast light kit is a light system where the reflector and ballast are integrated in the same metal housing of these grow lights. A complete enclosed ballast grow light system would consist of a ballast/reflector reflector with and integrated socket & cord and light bulb.

What is a Remote Ballast Grow Light Kit?

A Remote ballast kit is a system where the Ballast or transformer is a separate unit from the reflector. These ballasts are extremely heavy and some models are made to be wall mounted while others can simply be placed on the floor. Most of the items we sell are lighting systems (kits) for growing plants, herbs, seeds, and for greenhouses applications. Many of the items we offer are sold as Kits with many different options and components. A complete remote ballast grow light system would consist of a ballast, reflector, socket & cord and light bulb. The ballast (transformer) is needed to ignite the bulb. The reflector or hood is what is used to direct the light and usually has a reflective insert to provide greater light intensity. Many of the reflectors now come with the socket & cord pre-wired into the hood.

What is the difference between a Remote Ballast and Enclosed Ballast Light System?

A Remote ballast kit is a system where the Ballast or transformer is a separate unit from the reflector. An enclosed Ballast light kit is a light system where the reflector and ballast are integrated in the same metal housing of these grow lights.

How do I know my MH or HPS bulbs are functioning normally?

It may take MH or HPS bulbs 10-15 minutes to come to full brightness.
During the first few hours of use, the light from the lamp might oscillate.
The light will decrease in intensity during the life of the lamp.
During the first hours, intensity of the light may fluctuate somewhat, which is normal. After 100 hours of “burn in” time, the bulb will continue to burn evenly for the remainder of it’s life (with normal aging reduction).

How often do I need to change my light bulb?

Change my Light Bulbs

HID (MH/HPS) bulbs should be replaced after 9 to 18 months of use. Although MH and HPS lamps will continue to light beyond 18 months of use, they will have lost up to 30 percent or more of their lumen output while consuming the same amount of electricity. The average life of a MH lamp is 12,000 hours for a 1000 watts lamp and 20,000 hours for a 400 watt lamp. The rated life hours for most HPS light bulbs is 24,000 hours. Most manufacturers rate their lamps by “Average Life Hours” and usually claim 10,000 to 24,000 hours. These ratings are based on when the lamp/bulb will completely fail to come on. They do not factor in loss of intensity or loss of color. MH and HPS light bulbs lose intensity and color through normal use. This is OK if you are lighting a warehouse, but when it comes to plant growth, these losses can mean wasted electricity and poor plant performance. Serious horticulturists recommend that replacing HPS or MH Light Bulbs after 6000 hours of use. This equates to using your light 16 hours a day for one year.

How long should I run my lights?

This depends on the type of plants and whether you have natural sunlight available to your garden. As a general rule, when you are in a vegetative stage of plant growth and you have no natural sunlight, run your lights 14-18 hours a day. If you have natural sunlight, it will vary because the sunlight may or may not be direct. It will take a little experimenting to find the best length of time to run your lights. If you are actively fruiting and flowering, the rule is to run your lights 12 hours a day if you have no natural light

Do I need to wear gloves when handling a (HID) Metal Halide or HPS Light Bulb?

Manufacturers do not indicate that gloves are required when handling MH or HPS Bulbs. However, it is recommended that your hands be thoroughly washed prior to handling these bulbs. However you should handle bulbs very carefully and should wear gloves.

Are HPS/MH grow lights safe to run in my home?

Metal Halide and HPS grow light systems are perfectly safe to run in your home. All of our grow light systems are manufactured with the highest quality and are CSA listed. The CSA listing indicates the grow light has been properly tested for safety.

What is the Lumen Measurement?

Lumen is a measurement of light output. It refers to the amount of light emitted by one candle that falls on one square foot of surface located at a distance of one foot from the candle. Traditionally, lumens have been the benchmark of a lamps ability to grow plants; meaning the brighter the lamp the better the plant. However, studies have shown that a broader color spectrum lamp will perform much better than a lamp with high lumen output, especially when it comes to plant growth.

What is Spectral Energy Distribution & PAR Watts?

The total visible spectrum is perceived by us humans as white light, but the “white light” is actually separated into a spectrum of colors from violet to blue, to green, yellow, orange and red made up of different wavelengths. Plants use the blue to red part of the spectrum as their energy source for photosynthesis. The different combinations and the relative intensity of various wavelengths of light determines the CRI of a light source.

Only part of solar radiation is used by plants for photosynthesis. This active radiation Photo synthetically Active Radiation (PAR) contains the wavelengths between 400 and 700 nano-meters and falls just within the visible spectrum (380 – 770nm). The light in this region is called PAR watts when measuring the total amount of energy emitted per second. PAR watts directly indicates how much light energy is available for plants to use in photosynthesis.

What is the Color Temperature or “K” – Kelvin Rating?

The K rating is a generalized form of addressing the color output of a Light Bulb. Color Temperature is not how hot the lamp is. Color temperature is the relative whiteness of a piece of tungsten steel heated to that temperature in degrees Kelvin. HPS has a warm (red) color temperature of around 2700K as compared to MH at 4200K, which has a cool (blue) color temperature. The higher the kelvin temperature gets, the bluer. 10k lamps seem to be a nice crisp white, while higher kelvin can go from a blue/white to very blue and lower kelvin seem more like that of sunlight (6500k). Metal Halide bulbs go up to 20,000K (commonly used in aquariums) providing the bluest light.

Color Rendering Index – CRI

CRI is a numeric indication of a lamp’s ability to render individual colors accurately. The CRI value comes from a comparison of the lamp’s spectral distribution to the standard (e.g. a black body or the daytime sky) at the same color temperature. The higher the CRI the more natural and vibrant the colors will look. A bulb with a CRI of 85 or higher is excellent being that the sun has a CRI of 100. Emperor lamps make 90-92 CRI bulbs that are used in aquarium, horticulture and other applications. Standard Metal Halide bulbs have a CRI of about 70, so only 70% of colors will be rendered correctly. HPS bulbs have a CRI of 22.

How is Light Measured?

The “color” of light sources comes from a complicated relationship derived from a number of different measurements, including correlated color temperature (CCT) or Kelvin temperature (K), color rendering index (CRI), and spectral distribution (PAR Watts). However, color is most accurately described by a combination of Kelvin temperature and CRI.

What is a conversion bulb?

A Light Bulb or Lamp that operates on the opposite ballast it was originally designed for. For example, a 940 watt conversion lamp is an HPS lamp that runs on a 1000 watt Metal Halide Ballast. There are also MH lamps that are designed to operate on HPS ballasts. These bulbs allow the grower to purchase the ballast of their choice and offer the flexibility of growing a variety of plant types by simply changing the lamp they need.

Can I run a 430 Watt HPS bulb in a 400 Watt HPS lighting system?


A 430 Watt HPS light bulb will run on a standard 400W HPS ballast (Ansi S51). However, you will only receive 400 watts of light output

What is the difference between MH and HPS Bulbs with regards to plant growth?

MH lamps provide more of the blue/green spectrum, which is ideal for leafy crops, and/or plants that are in a vegetative (actively growing) stage. MH lamps provide a more natural appearance in color and are typically the choice for plants that have little to no natural light available. HPS lamps provide more yellow/orange/red spectrum, which is ideal for most plants that are actively fruiting and flowering. In addition, HPS lighting is the choice for growers looking to supplement natural sunlight. Ideally, the horticultural will use MH to grow their plants and HPS to fruit and flower their plants. HPS Grow Lights are available in 250, 400, 600 and 1000 Watts. Metal Halide grow lights are available in 250, 400 or 1000 Watts.

Emits a Red/Orange Color Spectrum
Promotes fruiting, flowering & budding
ncreases plant growth during fruiting & flowering stages
Use as supplemental lighting (with natural sunlight) or as secondary lighting

Emits a White/Blue Spectrum
Promotes plant growth
Use for leafy vegetables such as lettuce or herbs
Excellent for seedlings
Use especially if no natural light is available
BT37 Shaped Metal Halide Pictured

What is HID Lighting: Metal Halide & HPS?

H.I.D. lighting stands for High Intensity Discharge, which is a special type of lighting that is much more intense than most other light sources available. HID lighting includes both High Pressure Sodium (HPS) and Metal Halide (MH) lighting. MH and HPS grow lights produce stronger, healthier seed starts, faster maturing plants, higher yields and increased flowering. HPS and Metal Halide lighting not only supplements sunlight but can replace it during the winter months. The light spectrum range produced by HPS or Metal Halide light bulbs enhances the natural light derived from the sun. In addition HPS and Metal Halide lighting is energy efficient and only requires about the same amount of energy as a standard kitchen appliance. The life of MH and HPS bulbs ranges from 6,000 to 20,000 hours depending on the wattage and bulb type.

What do the 3 numbers stand for in a fertilizer mix for example 20-20-20

These numbers represent the percentage of:
 Nitrogen (N)
 Phosphorus (P)
 Potassium (K)
 These are the 3 main elements in plant fertilizer.

Can I reuse my soiless mix after harvesting a crop?

You can but the possibilities of fungal attack, soil born disease, pythium, and nutrient build up are too high to risk. The soiless medium is so cheap and easy to replace so I definitely recommend not reusing the soil less mix. Spend the 25 $ and buy a new bale. Your plants will love you for it

What is the difference between a supplement and a nutrient?

Nutrients are stand-alone, in other words plants can be grown successfully with nutrients with out the need for Supplements. Supplements are not stand alone. Supplements are used in addition to nutrients and are designed for a specific task, e.g. bloom stimulation, root development, flavor, etc.

Root Stimulators – B’cuzz root, Root 66, Green Fuse Root Rootopia
Growth Stimulators – B’cuzz Grow, Green Fuse Grow
Flower and Fruit stimulators – Sugar Daddy, B’cuzz Bloom, Jurassic Bloom, B’cuzz PK 13/14, Green Fuse Bloom.

What are the different methods of controlling odor?

MountainAir Carbon Filters are the most effective choice for any grower who wishes to eliminate organic odors. The secret is in the carbon, which comes from a Pre Cambrian source making it the oldest and most effective carbon available. There are several brands of carbon filters on the market today, but almost all of them use coco coir carbon which doesnÕt filter as much of the odor and only lasts 8-12 months, and none can compare with the quality of the MountainAir carbon. Carbon filters can be used to clean outgoing air that is being vented out of a grow room or they can be used as a scrubber cleaning the air within the room. The size of your Carbon Filter is determined by the Maximum Watts of Light that you are running in your room

ONA Spray, Liquid, Gel, Mist and blocks are the simplest method of controlling odor. With the gel you simply remove the cap and leave it sitting in the room. In the case of the spray, simply spray into the air as necessary. After some time, the ONA Gel will begin to evaporate. When the container is half full simply add some of the liquid refill to reactivate the gel. Although quite effective in small areas, ONA products are usually not recommended for large areas, but because of the versatility of ONA product line they have implemented different model fan that can help disperse the scents throughout larger areas.

What does an air stone do?

An air stone helps to provide oxygenate the nutrient solution. This oxygen is extremely beneficial to the root zone and helps to promote fast, healthy growth as well as prevent disease. This is one of the main reasons that plants growing in a hydroponic system grow so much faster than plants in soil. If you are growing in soil you can still reap some of the rewards of oxygen by simply oxygenating your water before applying it to the soil.

Is a Reverse Osmosis system necessary?

RO water, or reverse osmosis water, is tap water that undergone a process to strip all the impurities out of the water. Most city drinking water contains high sodium (Na), boron (B) and fluoride (F) levels. These impurities, at high enough concentrations, can be extremely toxic to plants. These elements may also reduce the uptake of other nutrients and thus cause slight deficiencies that may not be visible. RO machines also remove important plant nutrients from the tap water, such as calcium and magnesium.

This is why Discount Hydro recommends using Magical when using RO water. The process of removing these impurities allows the grower to start with almost pure water (the RO process removes over 98% of the total dissolved solids). The grower may now add exactly what the plants require without pushing the ppmÕs too high. Plants grown with RO water, especially in a hydroponic system, will grow faster, yield more and exhibit fewer nutrient disorders then plants grown with tap water.

When and how often do I need to use CO2?

C02 should only be used when your lights are on, as plants only use CO2 during photosynthesis. C02 is most effective during the flowering stage, but BGH recommends using CO2 throughout the life of your plants for maximum results.

What are the benefits of adding CO2 to my grow room?

Many growers overlook the huge importance of CO2 to fast growing plants. CO2, along with light, are the two most important sources of food for plants. Plants take light and CO2, and through a process called photosynthesis, produce food for themselves. The nutrients that growers feed their plants are kind of like the salt and pepper, whereas the light and CO2 are like the meat and potatoes. The nutrients are necessary for photosynthesis to occur, but they are mainly a catalyst to allow the reactions to take place. In fact, if you were to analyze any plant, you would find that it consists of over 90 percent water, a few percent nutrients, and the rest is carbon.

Normal CO2 levels are between 300 to 500ppm (parts per million), depending on whether you live in an urban or rural area (we have almost 600ppm of CO2 here in Montreal!). Increasing these levels to 1500ppm can often have dramatic effects on your plants, including faster growth rates and increased yields. This is why it is so important to always have fresh air circulating into your grow room, or better yet, add supplemental CO2. We recommend you use quality north american made products such as Plug ‘N Grow.

What is the difference between MH and HPS?

MH lamps provide more of the blue/green spectrum, which is ideal for leafy crops, and/or plants that are in a vegetative (actively growing) stage. MH lamps provide a more natural appearance in color and are typically the choice for plants that have little to no natural light available. HPS lamps provide more yellow/orange/red spectrum, which is ideal for most plants that are actively fruiting and flowering. In addition, HPS lighting is the choice for growers looking to supplement natural sunlight. Ideally, the horticulturalist will use MH to grow their plants and HPS to fruit and flower their plants.

What size Fan do I need to ventilate my room?

Ventilate my room properly.

Fans are rated in cubic feet per minute or CFM, to figure out how many CFM’s you need; start by determining the size of your room ({L}ength times {W}idth times {H}eight equals total volume {cubic feet}. Once you know the volume of your room you will need to get a fan that will vent the total volume in four minutes or less. A room with dimensions of 14′ x 14′ x 8′ has a grand total of 1568 cubic feet. To find out how many CFM’s it takes to do the room in four minutes divide by 4. 1568 divided by 4 equals 392 CFM. For this we would recommend a Green Gold Typhoon inline fan 6” (420 CFM). You ultimately want to get your exhaust fans to a point where they only have to come on for a few minutes each hour to maintain a steady temperature and humidity, while still keeping the air fresh. MegaWatt offers a heatstat, coolstat, and a climate controller for use with these fans to help maintain environmental conditions.

How big of an area will my light cover?

The size of the garden area will determine the wattage you need. If we assume that the plants will get no sunlight, a 1000 watt light will cover about 7 x 7 feet of growing area. A 600 watt will cover 6 x 6 feet, a 400 watt will cover 4 x 4 feet, and a 250 watt will cover 3 x 3 feet. These sized areas would be considered the “Primary Growing” areas. These lights will light-up larger areas, but plants placed outside of the Primary Growing area, will stretch and bend toward the light; a phenomenon called phototropism. Keep these areas of coverage in mind when using multiple fixtures. The best results occur when the areas of coverage overlap.

Important Hydroponic Factors

The most important factor in hydroponics however, is the nutrient solution that must be mixed with water. Standard fertilizers are inadequate, because they lack some of the elements necessary that the plants would otherwise derive from the soil. Specially-formulated hydroponic fertilizer mixtures are required. These are widely available, but should be tested after dilution to ensure a pH of between 5 and 6. The nutrient solution should be changed every two weeks. In between changes, make sure that the volume is kept level by adding more water only, and not additional fertilizer formula. If water evaporates and the water level gets too low, the nutrient solution will become too rich and could actually burn roots.

Electronic vs. Magnetic ballasts

Electronic high-frequency ballasts increase lamp-ballast efficacy, leading to increased energy efficiency and lower operating costs. Electronic ballasts operate lamps using electronic switching power supply circuits. Electronic ballasts take incoming 60 Hz power (120 or 277 volts) and convert it to high-frequency AC (usually 20 to 40 kHz). Electronic ballasts are more efficient than magnetic ballasts in converting input power to the proper lamp power, and their operating of fluorescent lamps at higher frequencies reduces end losses, resulting in an overall lamp-ballast system efficacy increase of 15% to 20%

Are electronic ballasts more energy efficient?

Electronic ballasts are more efficient at converting electricity into usable light. Since your power bill is based on kilowatt-hours and not efficiency, a 1000 watt electronic ballast will cost you about the same as a 1000 watt HID ballast to operate.

Is the socket assembly sold separate for the Green Gold ballasts?

No, we have incorporated the socket assembly into all of our ballast to facilitate things. Please take into consideration that every mounted ballast comes with 15′ lamp cord and 8′ power cord. The lamp cord is 120V plug ready for the USA and may be used for either 120v or 240v in Canada.