Under-canopy light: why lighting from below rescues the lower buds

Erweitertes Grow-Wissen 4 min read Updated Leave a comment
Reihen von Jungpflanzen in einer Gärtnerei
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Top buds fat, lower buds stuck at popcorn? That is rarely genetics and almost always light. Below the canopy only a fraction of what shines on top ever arrives. An additional light from below or from the middle of the plant changes exactly that. In our own production it works really well, and this post explains what happens, how to set it up and where the limits are.

Who it is for: Under-canopy light pays off when your plants grow taller than 60 to 80 cm and you already run a decent main lamp. With small plants or a sea of green it adds little. The basics on light and phases are in the beginner's guide.

Why lower buds stay small

Light does not fade gradually through the canopy, it drops steeply. Whatever the top layer of leaves does not absorb, the second layer catches, and 40 cm down often less than 10 % of the light is left. The lower buds are not lazy, they are starving. They only get sugar on loan from the upper leaves and stay loose, pale and low in resin.

Two ways out: either you remove what will never amount to anything down there (lollipopping), or you give those buds light of their own. Best is both.

What happens when you light from below

This is our observation over several runs, and it matches what research has measured on cannabis and on greenhouse crops:

  • The lower buds become firm and colourful. Instead of loose popcorn you get compact, fully ripened buds with visibly more resin.
  • The stems turn red and firmer at the bottom. A sign that the plant is processing light there and building supporting tissue.
  • The leaves claw downwards. It looks wrong at first, but it is the plant's intention: the fan leaves and the sugar leaves on the bud turn towards the light from below to catch it.
  • The harvest evens out. Less sorting between A and B grade, because the lower tiers deliver finished product too.

How to set it up

1

Clear the bottom

Without lollipopping the light from below achieves nothing. Remove the lowest shoots and leaves so that a free zone forms under the canopy in which the light can spread. A certain distance between lamp and plant has to be guaranteed. Otherwise leaves rest on the lamp, burn and block the light.

2

Choose the lamp

LED bars or strips with a full spectrum, ideally with some red for flowering, are the tool of choice. They stay cool enough for proximity to the plant and can be placed horizontally at pot height or vertically between the plants. As a rule of thumb: a quarter to a third of your main lamp's power. You do not need more, the lower leaves are used to low light.

3

Position and distance

The lamp belongs in the cleared zone, facing up or sideways into the plant. 20 to 30 cm to the nearest leaves is a good start. Watch the leaf tips during the first days: if they curl upwards or bleach, it is too close or too much.

4

Think about heat

Every extra lamp is also a heater, and this one sits right inside the tent. In summer consider whether you really want to bring that extra heat in or whether your exhaust can still cope. In winter it is almost a bonus: the light warms the root zone and the lower area where it is otherwise coolest.

5

Adjust feeding

More active buds means more demand. Stick to the feeding schedule, but keep an eye on EC and water uptake: plants with under-canopy light often drink noticeably more. If the lower leaves turn yellow despite the light, it is usually nitrogen deficiency in the last third of flowering, not the light. The deficiency guide helps to tell them apart.

The numbers behind it

  • <10 %
    light 40 cm down without extra lamp
  • ¼ to ⅓
    of the main lamp is enough below
  • 20 to 30 cm
    distance to the nearest leaf
  • +2 to 4 °C
    heat to plan for in the tent

What not to expect

  • No doubling of the harvest. The gain comes almost entirely from the lower tiers. Studies on cannabis with supplemental light below the canopy found more yield and higher cannabinoid content in the lower buds, the upper ones stayed as they were.
  • No substitute for a weak main lamp. If too little arrives on top, more light below does not help. Size the main lamp properly first.
  • Not a no-brainer in the heat. Above 28 °C in the canopy the extra light costs you more quality than it brings. Then rather off, or only as a supplement during the first weeks of flowering.

Setup check

Not sure whether under-canopy is worth it for your tent?

Send us tent size, main lamp and plant height in the chat, we will tell you honestly whether it adds up.

What comes next

Matching products

Sources

  • Hawley, D., Graham, T., Stasiak, M., Dixon, M. (2018): Improving Cannabis Bud Quality and Yield with Subcanopy Lighting. HortScience 53(11), 1593 to 1599. doi.org/10.21273/HORTSCI13173-18
  • Gómez, C., Mitchell, C. A. (2016): Physiological and Productivity Responses of High-Wire Tomato as Affected by Supplemental Light Source and Distribution within the Canopy. Journal of the American Society for Horticultural Science 141(2), 196 to 208. doi.org/10.21273/JASHS.141.2.196
  • Our own observations from BUDXXL production, several runs in 2025 and 2026.

Legal note: Since the Consumer Cannabis Act (CanG, 2024), private cultivation of cannabis in Germany is permitted for adults within the legally regulated framework (personal use, limited number of plants, no access for minors). This post is for information only and does not replace legal advice.

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