Light Distance from Canopy: Why the Maker's Manual Sets It

What distance from canopy means

Distance from canopy is the gap between the light's emitting surface and the top of the leaf layer. In a grow tent or on a shelf, the canopy is the upper layer of leaves that receives the light first. When a maker's manual mentions a distance, it is telling you where to hang the light above that leaf layer.

A grow light sends a fixed output from the fixture. As that light travels away from it, the same output spreads over a larger area, so the amount that reaches a single leaf changes. That is why the distance is part of the shopping question: a light hung high delivers less intensity to the canopy than the same light hung low. The exact numbers are on the maker's page for the model.

Why seedlings, vegetative plants and flowering plants need different distances

Plants at different stages are not equally tolerant of strong light. A seedling has a small leaf area and is easily stressed by intense light, while a flowering plant with a full canopy is built to take more. The grower's job is to give each stage the intensity it can use: gentler for young plants, stronger as the plant fills out.

The unit that describes light arriving at the canopy is PPFD, photons per second over a square meter. PPFD targets are the intensity ranges a stage is meant to receive. The targets are printed on the maker's page and in the maker's manual for the model, never in this site's catalogue. PPFD targets for seedlings, vegetative and flowering explains the term without numbers.

One way to read a listing safely is to remember that the distance is not a fixed number for all lights. Two lights with the same wattage can have different recommended distances because their optics and layout differ. The maker's manual for your model is the only source for the height.

What the listing states and what it leaves out

A grow light listing on this site states the light type, the wattage of the light, the coverage area, the chip maker, dimming, a timer and the brand. It does not state a hanging height. A missing height is normal: the seller is not the maker, and the mounting instructions live with the maker.

When a title or bullet list names a coverage area such as a 2x4 or 4x4 tent, that is the area the maker designed the light for, not the distance above the plants. Coverage and distance are separate questions. How to calculate coverage area explains the difference.

Reading the listing for the light type helps you know what to expect from the maker's instructions. A quantum board, a bar light and a panel light each have a different shape and a different way of spreading light. Quantum board vs light bars compares the types as listing terms.

Reading the maker's manual for the height

The manual is the only place the maker states the height. A manual is supplied with the light and is also posted on the maker's page. If the package has no manual, the maker's page is the next place to look. The steps below apply to any hanging light, whether it is a quantum board, a bar, a panel or a strip.

• Open the maker's page for the model and look for 'installation', 'mounting' or 'hanging height'.
• Read the distance values for each stage the maker names: seedling, vegetative and flowering.
• Check whether the maker gives the distance from the fixture to the canopy or from the floor to the fixture.
• Follow the maker's sequence: set the height first, then turn the light on.
• Keep the manual with the light. The distance can change when you move the light to another tent or change a mounting bracket.

When the distance cannot change: using a dimmer

Some setups have a fixed mount, such as a clip-on light on a shelf or a bulb in a ceiling socket. When the fixture cannot move, the intensity at the canopy is controlled by the dimmer, not by the height. Dimmable LED grow lights let you reduce the light at seedling stage and raise it as the plant grows.

The maker's manual states the dimming range and how the control works, whether a knob, a remote or an app. Dimming range is a separate listing term and is read from the listing. A light without a dimmer can still be moved, but the height range is set by the mount.

Adjusting the distance as plants grow

As a plant grows, the canopy rises. The grower's job is to keep the distance near the maker's figure, which means raising the fixture as the plant gains height. On a shelf or rack, that can be done by moving the light to a higher rung or by adjusting a stand. Grow light options for 2x2 and 3x3 tents covers lights that hang from a tent top.

Plants give visible signs when the distance is wrong. When a plant reaches for the light, the stems grow long and thin between leaves. When a plant is too close to a strong light, leaves can bleach or scorch. These signs are described in general terms by the University of Minnesota Extension page on lighting for indoor plants, which is listed in the sources of this article. The maker's manual is the first check before assuming the distance is the cause.

A light with a dimmer makes the adjustment easier: instead of moving the fixture a small distance, you lower the intensity a step and keep the fixture where it is. Height and dimming do the same job, and the manual for the model tells you which the maker intends.

Reading a PAR map from the maker

The maker's page for a model often includes a PAR map: a diagram that shows the light intensity across the coverage area at a certain distance. It is the maker's measured data, not a claim this site repeats. How to read a PAR map explains the diagram term by term.

When you compare two lights, compare their PAR maps at the same distance if the maker publishes both. PPFD vs PAR vs lux covers the difference between the units. The DesignLights Consortium page on horticultural lighting explains how makers qualify their performance claims.

What to pick for your job

Questions

What is distance from canopy?

It is the gap between the light and the top of the plant leaf layer. It controls how intense the light is at the canopy.

Why do seedlings need a different distance than flowering plants?

Seedlings take less light intensity than flowering plants. The maker's manual for the model gives the distance for each stage; this site does not state those distances.

Does the listing state a hanging height?

No. This site's catalogue states light type, wattage, coverage area, chip maker, dimming, timer and brand, not a hanging height.

Where do I read the recommended height for my light?

In the maker's manual and on the maker's page for the model.

Can I use a dimmer instead of moving the light?

Yes, when the fixture cannot move, the dimmer adjusts intensity. The maker's manual states the dimming range.

What if a plant looks like it is reaching for the light?

Long, thin stems between leaves are a plant's sign of too little light. Check the maker's distance figure, adjust the fixture, and watch the plant. The University of Minnesota Extension page describes the sign in general terms.