I have a small heated greenhouse with five 4' grow lights. My problem: How do I keep seedlings, especially tomato seedlings, from becoming extremely l...
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Growing Seedlings #756141
Asked June 14, 2021, 2:22 PM EDT
I have a small heated greenhouse with five 4' grow lights. My problem: How do I keep seedlings, especially tomato seedlings, from becoming extremely leggy? I have been keeping lights on just above all my seedlings 16 hours per day, and temperature at 70°. This causes leggy seedlings with many plants, especially tomatoes, which grow quite fast as seedlings. I have not been able to find the answer however, found one YouTube video that advises to drop temperature in greenhouse (especially for tomato plants) to approximately 45° once seedlings have sprouted. Since I start many different flowers and plants at different times due to the germination period required for each variety of plant, this poses my problem since germination temperature for most plants is at or near 70°. I'm thinking I have 2 solution options. Replace my current heating unit, which heats the air in the greenhouse, with heating matts, or build unheated cold frames to move seedlings into that can protect small seedlings from outdoor temperatures January through May. Not sure which option would be best to accomplish lowering temperature to slow growth of these small plants since some small plants need warmth to develop while some others can use lower temps to slow their growth. Please advise even if you feel the need to provide more options. I have a small, home designed and built 4'X6' greenhouse that I started 354 plants in this season which included plants for friends and neighbors. I need help especially with tomato plants since in only 6 weeks get as tall as 18" with main stems the size of toothpicks. They transplant into the garden fine, however, look terrible to give to my friends and neighbors.
Any ideas? Thank you for any help you can provide.
Multnomah CountyOregon
Expert Response
The constant temperature may have something to do with their stem caliper and height. But perhaps more important are how high the lights are above the plants and how widely they are spaced as they get older. Growing mixed sets of plants complicates height management since they are growing tall at different rates. The ideal is to have uniform batches of plants under a given light set and keep raising the lights to stay about two inches above the plant canopy. Transplant several times as they grow to keep any "excess stem" in the soil media to root. Tomato transplants also need to be spaced further apart as they grow (often moved up to the next sized pot with some of the "excess stem" covered by the media) to avoid competing with each other for the available light. Finally, there is good evidence that exposure to moving air from a fan strengthens stems and slows, to some extent, stem elongation. All that said, if they transplant well (and, as noted, tomatoes will root from the stem above the initial root system if they are planted a little deeper) your friends should love them. Hope this helps. Feel free to contact me directly if you have more questions. Chip Bubl OSU Extension/Columbia County <personal data hidden> <personal data hidden> cell
I plant whole trays of the same kind of plant. They are uniform. I also keep my grow lights no higher than 2" above the plant tops. I plant tomatoes in tall 16 oz cups in the bottom 1/3 soil, then add soil as they grow. When I transplanted into the garden for the past 70 years, I only leave about the top 3" to 4" above the soil surface. I have actually shown my friends and neighbors for the last 50 years how to do this. Most fail due to this because they are reluctant to cut the leaves off. They don't get it even when I plant my garden right in front of them! They seem to want a more visible plant at transplanting time. So currently after only 1 month with all of our plants in the ground, most of their tomato plants are 16" to 18" tall while mine are now 3' tall and actually blooming and forming small tomatoes. Most important right now due to the way I plant them into my earth garden, the stems are now as large in diameter as my thumb.
The reason I asked about dropping the temperature for tomato seedlings is evident with my answer. I have been doing all the things you are recommending for many years and continue to get tall, leggy tomato seedlings. I don't mind them being tall myself because it gives me the opportunity to put more of the transplant under the soil to encourage massive root growth, which helps in absolutely every aspect of a tomato's seasonal growth and production. Concerning soil, If you fail to take care of the soil and organisms in it during winter months, then those organisms may not be there to take care of your garden during the summer months. Over fertilizing during the growing season doesn't make up for that.
Since you mention "constant temperature may have something to do with leggy plants" could you be insinuating maybe a lower temperature at night (when plants seem to reach for light) as a possible partial remedy? I have monitored growth patterns and found that most upward growth happens during the 8 hours when the lights are off. So maybe this could be a solution.
