CFL Lights in a Grow Box, and
Do Plants Create Humidity?

by Fred
(Exeter, N.H. U.S.A)

I am a first time grower and I have a few questions. My grow box is 2'x2'x3' I have a total of 8 26 watt CFL's, 2 65 watt CFL's , with a 14 watt LED grow panel. There is a 4" intake fan (55 cfm) and a 4" outtake fan (55 cfm), and 6 sides of mylar. My first question....is that enough light for 4 tomato plants (my lights are 6500k for veg and 2700k replacement for the flowering phase). Second question, should I use the LED for the vegetative and germination stage? Third question, without any plants in my grow box and all the lights on I have a box temp of 68, a room temp of 64, and humidity at 35% in both. Will the plants create that extra 15% that I need to gain? I understand perfect humidity would be 50%-55%, is this true? Thank you very much!

Answer: Fred- tomatoes and most other garden plants do very well under 40 watts of light per square foot. I use this as a general guideline in just about every garden I set up or give advice on. You mentioned the "footprint" of your grow box is 2' x 2', giving you a garden area of 4 square feet. If you placed all of your CFL's into this space (338 watts total), your box would be lit at 84.5 watts/sq.ft..

With fluorescent lights (including CFL's), the light doesn't penetrate down into the plants very far. So, on one hand, the extra wattage will help the light penetrate down into the plants further, allowing you to grow slightly taller plants and still have them be healthy at the bottom. Instead of having a 12 inch effective growing distance from the lights, you may end up with an 18 or 20 inch effective growing distance from the lights (especially with mylar on all 6 sides of the box).

On the other hand, you have enough light in your grow box to take one of those 26 watt CFL's out and use it to grow your seeds/clones or small plants in the vegetative stage. This is what I would recommend. The vegetative stage requires less light than the flowering stage, but 14 watts will not be enough. As a general rule, I try to stay at 20 watts/sq.ft. or more for the vegetative stage.

One CFL would work well to grow four small plants up to about 8 inches tall from seed or clone. The 14 watt LED can be used for supplemental lighting in the veg area if you like. As always, the plants will benefit most from the light if you surround them with reflective material.

Since your box is only 3 feet high, you will certainly want to start flowering your tomato plants while they are still short....they will continue to grow during flowering. An 8 inch tall plant may end it's life cycle being 16 inches tall! Also, you want to make sure there is enough air movement INSIDE the box....this helps carry away oxygen when the plants exhale and also helps bring fresh CO2 in contact with the leaves for the plants to breathe in.

The ideal humidity is 50%, plus or minus 10%. In general, humidity that is too high is much more of a problem than humidity that is too low.... especially if your humidity level is only a few percent from being in the ideal range. If I were you, I wouldn't worry at all about your humidity level....but to answer your question yes, plants create humidity by exhaling water vapor along with oxygen.

This process is called transpiration- as water vapor exits from the leaves, it creates a little vacuum in the plant tissue, allowing the roots to suck up water (and nutrients). The plants may ADD humidity to your grow box, but if you have your intake and exhaust fans on constantly to control the temperature it will not be enough to raise the humidity level inside the box very much (maybe one percent).

As I said before, don't worry about the humidity level. Much more important to your success will be proper lighting, proper feeding, proper temperature control, and providing your plants a proper dark period when it comes time to force flowering. I hope this helps....Happy Growing!