There are a few critical stages for tomato seedling growth and providing lighting but seed germination isn’t one of them. In other words, a tomato seed doesn’t require light to germinate. But, by the same token it doesn’t require absolute darkness either. Normal room lighting is fine (the critical thing is bottom heat as the previous lesson pointed out.)
Once The Plant Has Two Seedling Leaves
At this stage, it’s critical to get the right amount of light to the plant. And that means full, strong sunlight. Without that, the stem will elongate and you’ll have a poor seedling right from the beginning. This can be “fixed” so it’s not the worst thing to happen but it’s better if you don’t.
Rule of Thumb
Once the seedling has two seed leaves, ensure the grow lamps are within 2-4 inches of the top of the seedling.
What’s the Deal With Light?
Think of a pebble tossed into a pond. Light is like that. The further away from the people’s drop space, the wider the circle. Similarly, the further away from the light bulb, the wider the light circle.
Let us start with a light that is rated for 100 watts of energy. If the light bulb is 2-inches above the plant, it may be putting almost the entire 100 watts onto the two leaves.
But we double the distance to 4-inches and the light-circle gets wider. (As in the pebble into the pond.)
Doubling the distance cuts the amount of light onto the leaf by half. Remember we only have 100 watts so if we make it cover a wider area, (from two square inches to four square inches for example) the amount of light per square inch is going to be cut in half.
If the seedling only receives half the light it needs, it’s going to stretch out looking for more light. You get straggly seedlings.
There Are Two Ways To Do This Next Step
The easy way is to tell you that you require at least 2 bulbs producing 600 watts each running for approximately 14 hours a day to light your tomato seedlings. So figure 1200 watts of light – whether you need two or three bulbs to do this – that’s your number.
And you want to keep those just above the leaf canopy around four to six-inches off the plant for the entire time they’re growing.
I have mine set to turn on at 6am (roughly when the sun goes up) and off at 8-9pm (to give the 14-ish hours a day. It’s far better to give too much light than not enough.
More light is better but you’ll probably do fine with the 1200 watts and 14 hours. (I use timers)
The more difficult way is to give you these references and you can figure that out on your own. 🙂 Frankly, I use a lot of bulbs – maybe even more than necessary – to avoid dealing with the math of this kind of thing. But if you’re the tinkering sort, have fun with these references.
Note these first two articles are about supplemental light in greenhouses – and plan accordingly. The third is about growing in grow chambers.
Evaluating Supplemental Light for Your Greenhouse from Cornell University
Measuring Daily Light Integral in a Greenhouse from Purdue University
Lighting Radiation Conversion from a lighting company Environmental Growth Chambers
The Balancing Act: Part 1
The first aspect of the balancing act to produce nice short seedlings is that the light levels need to be close enough to the seedlings to give them enough light. But far enough away so you can get the watering can into the plant to keep it damp.
Balancing Act: Part 2
Light isn’t the only thing that influences seedling height. There are other things as well so even if you get the light right, you may find yourself messing up some of these other systems (feeding, temperature)
True leaves are those that resemble the mature leaf of the tomato plant. When these start emerging, you’re not growing a seedling, you’re growing a tomato plant. And the light levels above are critical. If you thought it was easy to stretch a seedling, you haven’t seen anything. A fast growing young plant will even more quickly stretch out.
Again, it’s a bit of a balancing act. Keep the lights within 2-4 inches.
Having said all that
It’s not the end of the world if your seedlings aren’t stocky and well-branched. We’ll sort this out at the first transplanting and again on planting into the garden. Been there, do that.
At this point, I trot out my oft-repeated saying that this is gardening – not rocket science. I refuse to be obsessive-compulsive about seedling height.
If you’re going for world-record harvests, or want to sell your plants, it’s important. If you’re like me and simply want a great harvest so you can enjoy your own food, don’t worry about a bit of stretching here and there.