LED Grow Lights for Indoor Plants: Cannabis, Tomatoes, Microgreens, and other vegetables

We sometimes stumble upon questions online like “can LED lights grow plants?” and “what type of lamp can I use to grow plants indoors?”.

We’ve seen people ask about all types of lights, everything from decorative LED strips to computer screens. So what works and what doesn’t?

In short: all lamps that emit light can theoretically grow plants. However, all plants have different light quantity requirements. Some plants do well in a shady corner while others need intense sunlight for the better part of the day. Plants’ light requirements stems from the environment they grow in naturally. A fern plant that lives on the bottom of a forest floor and doesn’t receive much light in the wild will not need much light when it’s cultivated at home either.

On the other hand, an avocado tree that thrives in a hot climate with plenty of sunlight will need a similar environment with lots of light if grown indoors.

How do we know how much light plants need? And for how long?

The relation of light quantity/intensity and duration is calculated with the so called Daily Light Integral, DLI. Light intensity, measured in PPFD umol/m2/s, and photoperiod, duration of light exposure, gives us a DLI value.

Taking once again the fern plant as an example, as it has gotten used to limited light, it’s DLI requirements are low. Around 5 DLI.

Tomatoes and peppers have opposite light requirements. Fruit (and bud) bearing plants in general need more light than non-fruiting plants as light is essentially the “food” plants need for their energy intensive processes, like producing fruits, Tomatoes and peppers do well in 20-30 DLI.

Cannabis is also a light-loving plant and given the ideal combination of water, nutrients, and CO2, it can take upwards of 50 DLI. However, for most home grow operations, 30-40 DLI is usually recommended.

The industry standard recommendation for hobby/semi-pro weed growing operations is 12-18 hours of light (depending on plant stage) and about PPFD 600-800 umol/m2/s for mature flowering plants. 

Here’s a great article about DLI as well as a DLI chart and examples DLI needs for plants:

https://www.ledtonic.com/blogs/guides/dli-daily-light-integral-chart-understand-your-plants-ppfd-photoperiod-requirements

Here’s a DLI map of United States, month by month:

https://www.researchgate.net/profile/Leah_English/publication/287215008/figure/fig4/AS:308032761286663@1450452141435/Maps-of-monthly-outdoor-DLI-throughout-the-United-States.png

Source: https://www.researchgate.net/figure/Maps-of-monthly-outdoor-DLI-throughout-the-United-States_fig4_287215008

Looping back to LED lights for indoor growing. As we understand the concept of DLI and that plants need different intensity of light, we estimate ourselves what light sources work and what sources don’t work for our indoor grow.

Here’s a Youtube video that tests different LED bulbs:

https://www.youtube.com/watch?v=QTD1qr7-S-4

LED bulbs ranging from 2.2W to Ikea’s 10W grow bulb are tested as well as a 28W T5 fixture. 

The video shows, for instance, that a 2.2W LED bulb at 6” emits a PPFD of 216 umol/m2/s in the center spot but about 3” from the center, the PPFD value is down to 25 umol/m2/s.

In other words, this lamp does emit very intense light and it does not cover a particularly large area. It would possibly be enough for a small herb plant like basil or oregano, or a 2×2” tray of microgreens.

A 10W LED bulb with 30 degree lenses is also tested. The lenses focus the emitted light so the results without lenses would be very different, a lot lower, but the tests shows that this lamp’s emitted light intensity reaches a PPFD of 710 umol/m2/s in the center spot at 9” and about PPFD 300 umol/m2/s 3.5” from the center. This light intensity would be enough to grow tomatoes, peppers, and even cannabis, to some extent, assuming the plant fits in a 7×7” area. Alternatively, four bulbs with this output would cover 14×14” (1.3 sqft).

Conclusion:

Plants will grow no matter the light source as long as the light source emits enough light. A computer screen or a low-powered LED light strip do not put out enough light to stimulate most plants, especially not fruit-bearing plants or cannabis. LED bulbs that are much more efficient at converting electricity into light can be used as a light source for indoor plants depending on what plant is grown. Generally speaking, the more light = the bigger plant/bigger harvest. This is true to some extent. On the other hand, if a plant is not getting enough light, it will grow poorly and may not produce fruits at all.

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