The Best Distance to Have Lighting from Plants in a Hydroponic System

Growers often ask what is the best distance to have lighting from plants in a hydroponic system. The answer is whatever set-up emulates the best levels of sunlight that we can supply using artificial light or grow lights.

Various types of grow lights exist in the world of hydroponics, which are all explained in our Beginner’s Guide to Hydroponics.

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Briefly, the types of grow light available are fluorescent, High-Intensity Discharge. The most popular are metal halide, high-pressure sodium, and LED.

It is possible to bring too much light to bear on your plants, so hydroponic growers have always wondered what is the best or safest distance to place the grow lights from the plants. Too much intense light will hinder plant growth.

How do we know if a plant can be better with one distance than another? Growers can experiment to a certain extent, but the question of light distance will extend over the entire growth cycle; growers don’t have the time to try different lengths over several growth cycles to find out which is the best.

Following is a summary of today’s best practices for the most popular types of light we use in hydroponics.

What is The Best Distance to Have Lighting from Plants in a Hydroponic System?

It isn’t hard to put your grow lights too close to your plants, especially in a tight, enclosed environment such as a grow room or tent. Here are some possible tell-tale symptoms that indicate that your lights are too close to your plants.

The Grow Lights burn plants.

To risk stating the obvious, this occurs when plants are placed directly against the grow light or very near. The light’s heat dehydrates the plant tissue, which could be dangerous to the nearby plants.

The Grow Lights bleach plants.

This effect happens when the light intensity evaporates chlorophyll from the plants and usually occurs near the tops of plants.

Plants Seem to Hide from the Heat

The plants seem to fold over and bend so that they appear to shade themselves from the light source. This action happens with young plants under intense light; the curled-up appearance is similar to when the plant doesn’t get enough water.

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Plants Appear to be Stunted

This effect appears on the upper parts of plants nearest the light source; the plant’s growth would seem to be affected – or the plants would appear to stop growing altogether.

How to Adjust Your Grow Lights To Achieve The Best Distance to Have Lighting from Plants in a Hydroponic System

It would help to consider the light according to its distance from the canopy (the top of the plants’ height) downwards. The lighting intensity will be at its most extreme at the top if this is where you place the lights. The further down, the less light there is and the less effective the top-mounted grow lights will be.

So the trick is to position your lights, so they offer more or less equal intensity throughout and at every level of the plant. So the grow lights should be wherever needed; they could be positioned to the sides of the plant or even be mounted to throw light upwards from below.

Providing Optimum Light Using Grow Lights and Reflectors

The best light for a given space will provide sufficient light while keeping a safe distance from the plants. High-powered grow lights, around 1kW, are designed to provide light for extensive areas and work best where there is sufficient space to place them.

If areas are not adequately covered, consider using reflectors before buying more lights. You’ll want the overall effect to reduce the intensity of a single beam to several light areas. Think from the plants’ perspective.

You can also use several lower-powered grow lights spread out over the growing volume. The more lower-powered lamps you have, the fewer reflectors you’ll need (and the reverse is also true).

Experiment by setting up your lights and seeing which areas need more light, then adding to those by using more grow lights or more reflectors. Experience will be your best teacher.

Different shaped reflectors will result in differing lighting pattern results. The more you have at their optimum positioning will increase the efficiency of your hydroponic system because you’ll need fewer lights. Therefore you’ll need less in the way of electrical power usage, which equates to greater efficiency.

Consider the effect of the various light-absorbing materials and existing reflective materials in your grow room or tent. Grow tents come with interiors designed to reflect light, throw the light around more, and help increase efficiency by needing fewer lights (and preventing any buildup of hot spots).

According to the type of lights used, let’s look at what is considered best practice in working out the distance of grow lights from plants.

The figures below are general guidelines, and other factors (notably the number and type of reflectors used) will also affect this. In all cases, you should start at the furthest distance and work inwards as long as you see you’re doing no damage.

Always move your lights and reflectors around until you see what appears to be the best, even spread of good light throughout. It’s always impossible to give exact distances, so prepare to try things out in your growing area.

Best Distances for LED Grow Lights

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There is plenty of variance in LED grow light outputs, so LED lights are particularly difficult to pin down exactly how far or how close to place them.

LED grow lights have been in rapid development in recent years, and their appearance does not tell the whole story of how strong they can be; the red and blue lights (not the all-white variety) can be amazingly powerful. These lights don’t look as if they can give off much light, or heat for that matter, but they do and can burn plants very readily.

It would be best to start by placing the light array at a distance the manufacturer recommends. If you think any damage is being done to the plants, increase this distance. If not, edge the grow lights a bit closer to the plant. You should know if the plant is in danger by the feel of things and the appearance of the plant.

Note that distances will vary depending on whether this is for vegging or flowering plants. LED grow lights may be almost twice as close to flowering plants.

You also need to be aware that LED lights vary significantly in quality and that the highest quality LED lights are the most powerful. In all cases, you should follow the manufacturers’ guidelines. Generic charts show distance compared to how many watts output. Still, starting with manufacturer recommendations is safer and working from there with so many variables to consider.

Best Distances for HID (Metal Halide and HPS) Grow Lights

For metal halide grow lights, try using the back of your hand as a rough guide; hold your hand up next to the plant, palm inward so that the back of your hand faces the light. If this is too hot after 30 seconds, there’s a fair chance this will be too hot for your plants, so increase the distance. If there’s no discomfort at all, then you can go closer. You’ll want the back of your hand to feel the heat, but not painfully so.

The optimum distance for 150-watt metal halide lights would be between 10 and 25cm. For 400-watt metal halide lights, it would be about 18 to 45cm, and for 1000-watt metal halide lights, it would be about 30 to 75cm. But this is only a rough guide, and you should again seek manufacturers’ guidance for the specific light you use.

The distance for HPS (high-pressure sodium) grow lights will probably be the same minimum distance but slightly more maximum length than for metal halide lights.

Best Distances for T5 Grow Lights

Fluorescent lights are not as hot as other types of grow lights by their very nature. So these can be very close to the plants without causing any harm. But even here, it’s best to stick with the box’s recommendations and test this yourself. In practice, you’ll probably find that you can move very close to the plant (be careful with young plants). By all means, use the back of the hand test as above.

You should start with the fluorescent lights further away for seedlings and see how things go.

In conclusion, you should follow the manufacturer’s guidance and common sense with testing about 50/50. Eventually, you’ll want to end up with an evenly distributed light to all parts of the plant and work towards the most efficient system of its type.

Bear in mind that other things will be happening as well. The ventilation system will play its part, the lights may come with a built-in cooling system, and the oscillating fans you’ll have installed will provide better transpiration from the plant leaves.

Reducing the number of lights and increasing the number of reflectors or reflective surfaces will make for a more efficient system and lower costs.

Get all of this right, and you’ll have a controlled and enclosed system in complete equilibrium that you can be proud of while ensuring a recurring great crop.

Source: https://gardencourte.com
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