Under-lighting for cannabis – how additional lighting below and within the plant can change the grow

In classic indoor growing, the majority of light comes from above. This is logical, as sunlight in nature also hits plants primarily from above. However, in dense indoor crops, this creates a well-known problem: the upper canopy intercepts most of the light, while middle and lower flowers receive significantly fewer photons. This is precisely where under-lighting comes in. Technically, it is more precise to distinguish between sub-canopy lighting (light from below or from the bottom area of the plant) and inter-canopy lighting (light within the canopy or projected sideways into it). The goal is not to replace the main light, but to improve light distribution within the plant.

What under-lighting actually means for cannabis growing

In everyday growing, terms like bottom lighting, under-canopy lighting, side-lighting, and inter-canopy lighting are often used interchangeably. This distinction is useful:

Sub-canopy lighting illuminates the lower and inner parts of the plant from below or slightly from the sides at the base.
Inter-canopy lighting uses lights within or between rows of plants so that light reaches deeper into the canopy.

Both strategies follow the same basic concept: less shade, more usable light in the lower zones, and more uniform flower development across the entire plant. In a recent cannabis study, both sub-canopy and inter-canopy lighting led to a more even distribution of light within the crop.

Why under-lighting is interesting at all

The central problem with pure top-lighting is vertical light inequality. At the top, the plant is often oversupplied, while at the bottom, it lives in the shade by comparison. Exactly this inequality is described repeatedly in cannabis and greenhouse studies. A recent paper in *Scientific Reports* explicitly points out that cannabinoid and terpene content in flowers often decreases from the top to the bottom of the plant. This makes it understandable why additional lighting for the lower zones is even being discussed.

The biggest potential benefits of under-lighting

More even light distribution

The most well-documented benefit is the more homogeneous illumination of the plant. In the tomato sector, a *Frontiers* paper showed that a combination of top-light and inter-canopy light produced the most uniform light distribution within the crop. This same logic is relevant for cannabis: when lower and middle zones receive more light, they develop more uniformly.

Higher flower yield

For cannabis, there is now reliable data: in a 2025 study on medical cannabis, both sub-canopy and inter-canopy lighting increased the yield of dry flowers as well as the yield of cannabinoids and terpenes. The inter-canopy setup achieved the highest increase in dry inflorescence yield, while sub-canopy light was particularly efficient in terms of the ratio of increased yield to energy input.

Better lower bud quality

An important point is not just "more mass," but less quality drop-off in the lower area. An earlier *HortScience* study on cannabis already showed that sub-canopy lighting could locally increase THC and individual terpenes in the lower canopy area. At the same time, the overall pooled profile did not always change significantly. This is precisely why the most accurate statement is: under-lighting can improve lower flowers, but it is not an automatic guarantee for higher cannabinoid levels everywhere.

More standardization

Homogeneity is especially important for medical cannabis. The 2025 cannabis study showed that additional lighting within the canopy not only increased yield but also reduced variability between plants or flower batches. For standardized production, this is a real advantage.

What under-lighting cannot automatically do

Under-lighting is often marketed as a simple "yield booster." Research is more cautious. In greenhouse crops like tomatoes and cucumbers, there are many positive results, but not every study automatically finds increased yield. A *Frontiers* paper on intra-canopy lighting in tomatoes summarizes exactly this: in several studies, yield increased; in others, the differences were small or non-existent. For cannabis, the data situation is now clearly more interesting, but even here, the following applies: genetics, plant density, main light, climate, and plant architecture also play a role.

Under-lighting and photosynthesis – why the idea makes biological sense

The basic idea is simple: if lower leaves and flowers receive more light, they can contribute more to the plant's overall performance. Tomato studies show that inter-lighting can improve photosynthesis, chlorophyll status, and yield. These results do not come directly from cannabis, but they are biologically relevant because the principle is identical: more usable light in previously weakly illuminated zones increases the chance that the plant actually converts more light into biomass and quality.

When under-lighting can be particularly useful

Under-lighting is particularly interesting in these setups:

• for dense, tall indoor crops
• for cultivars that create significant shade in the lower canopy
• for medical or quality-oriented productions where homogeneity is important
• for setups where the grower has already well-lit the upper canopy and now wants to optimize the lower zones

Particularly in heavily layered crops, the drop-off in light toward the bottom is a known problem. This is exactly where sub-canopy or inter-canopy lighting can be usefully applied.

Which technology is suitable for under-lighting

In practice, LED systems are used almost exclusively. There is a clear reason for this: LEDs are compact, can be better accommodated in the lower or inner area, and generate less radiant heat than classic HID systems. Both the current cannabis study and the tomato literature use LED-based auxiliary systems.

What to watch out for with under-lighting

It remains supplemental light, not a replacement

The most important light source remains the main lighting from above. Under-lighting complements this but does not replace it. Both cannabis and tomato research show this: the combination of top-light and additional internal or lower lighting is usually the most useful.

Too much local intensity can be problematic

In greenhouse models, it was shown that intra-canopy LEDs can produce very high local light peaks directly near the lamps. This can lead to uneven adaptation within individual plant zones. This is precisely why under-lighting works best as a well-distributed supplement, not as brutal point-blank irradiation of individual leaves or buds.

More light can also shift metabolites differently

An important scientific point: more light does not automatically increase every quality value linearly. In the 2025 *Scientific Reports* paper on lower cannabis flowers, terpene concentration decreased in certain constellations with increasing light totals, while other light distributions showed positive effects. Light control is therefore more complex than "more at the bottom = always better".

Historical and scientific background

The idea of not only lighting plants from above does not come from the cannabis scene. In greenhouse production of tomatoes, cucumbers, and bell peppers, inter-lighting has been researched for years because dense crops develop strong shade zones. A *Frontiers* paper on tomato production describes that intra-canopy lighting has been intensely studied in recent years because it reduces reflection losses, improves vertical light distribution, and can provide yield advantages in several crops. Cannabis is increasingly adopting this knowledge today and applying it to flower quality rather than fruit yield.

FAQ – Frequently asked questions about under-lighting for cannabis

Is under-lighting the same as side-lighting?

Not quite. Under-lighting is often used as an umbrella term. It is technically more accurate to distinguish between sub-canopy lighting from below and inter-canopy/side-lighting within or to the side of the crop. Both are intended to better illuminate the lower and middle canopy areas.

Does under-lighting really bring more yield?

There is now reliable data for this in medical cannabis. In a 2025 study, both sub-canopy and inter-canopy lighting increased flower yield as well as cannabinoid and terpene yield; inter-canopy light achieved the largest yield increase.

Does under-lighting also improve lower buds?

Yes, there is good evidence for this. A *HortScience* study showed that sub-canopy lighting could locally increase THC and individual terpenes in the lower canopy area.

Is under-lighting useful for small grow tents?

It can be useful if the tent is densely planted and lower buds would otherwise be heavily shaded. However, it is more of an optimization technique than an initial mandatory purchase. First, the main light must be correctly set. This classification is derived from the research on supplemental versus replacement light.

Is under-lighting always efficient?

Not automatically. In the cannabis study, sub-canopy light was indeed very efficient in terms of certain yield metrics, but inter-canopy light delivered the greatest absolute increase in yield. What is more efficient therefore depends on the goal: maximum yield or best yield-per-watt ratio.

Conclusion

Under-lighting is not mandatory, but it is one of the most exciting lighting strategies in modern indoor growing. When used correctly, it can improve light distribution in the plant, upgrade lower buds, increase yield and homogeneity, and thus make a real difference, especially in quality-oriented or dense setups. At the same time, the technology is no miracle cure: it works best as supplementary lighting to an already well-planned top-lighting system.