Last year I had a bit of trouble with “leggy” seedlings, where the plants sprouts out quickly and develops a long “leg” from soil to the cotyledon. The cotyledon is the clover looking leat that every plant has that, from my observation, looks about the same. The second leaf is the considered the first true leaf.
Anyway, as you can see in the picture below, the middle tray has some Brussels sprouts and cabbage and they are all “leggy”. I could not remember what to do, so I did some research and re-discovered that it has everything to do with not enough light.
I found a great blog post from Margaret Roach where she interviewed Dr. Thomas Bjorkman from Cornell.
“The stretching that home gardeners experience is almost entirely from insufficient light. There are a bunch of other factors (as you mention) that play smaller roles, but most people starting seedlings can safely concentrate on making the light brighter.”
When I measure the light intensity in the units that the plant uses, under regular fluorescent lights it is typically 50 to 100 µmol/m2.s [that measure, micromoles per square meter per second, quantifies the number of photons used in photosynthesis that fall on a square meter every second]. With Super High Output fluorescent lights spaced 2 inches apart, we can get 400 at about 6 inches.
On a cloudy day outside in the spring 500 to 800 is common. On a clear day in late May (when a lot of seedlings come up) the light will be 1,500 to 2,000. So even though a T-5 fluorescent grow light looks bright to your eyes, it is practically dark compared to what the seedlings are used to.”
Wow. So, even though my grow lights look bright to my eyes, they really are dim compared to the light from the sun. I remember a gardener friend saying he tries to get his seeds out into his greenhouse as soon as possible, and remember wondering if that was a good idea. Why not just leave them under the grow lights? Perhaps just natural arrogance on my part, or just not understanding the “why” of it, I didn’t really listen. Or it might just be intellectual laziness, that, as a scientist, I find despicable about my character. Sometimes, though, I just want a quick answer and not think deeply about things. Blah.
As far as the greenhouse, it is still a bit too cold to put seedlings out there right now, I think, but I am not really sure. I may just move my onion and leek seeds out there once they have sprouted. (I will save that for another post, actually, as there seems to be a lot of info on it out there.)
I came across this video from Gary Pilarchik, whose videos I have seen and found very useful. He explains in the video that it is very important to keep the grow lights 1-2 inches from the top of the soil before/while the seed is sprouting. When the seed sprouts, the maximum amount of light will be available, so that the plant will grow leaves and not stretch for the light. I remember learning this last year, but again, I just wasn’t sure of the “why” or if it would work, so I did not listen.
Last weekend, I just started another tray of celery, broccoli, cauliflower and collard seeds (the third tray back in the picture above), and I can see the cotyledon’s just peeking out of the soil. Once I learned this info, I literally raced downstairs to put them under my second grow light, and placed the lights basically on top of the tray. My concern is the heat may be too much, and so I will need to monitor the tray in the morning and evening. I guess I just need to know if this is going to work, and if there are any draw backs.