I've noticed that a few varieties of tomato seedlings have purple stems. This is a concern of mine because to the best of my knowledge they are all supposed to have green stems, which they all started out with. The growing schedule on all my plant varieties (tomato plants included) seem to be on time and are presently showing the cotyledons. No true leaves. The seeds have all been sown in a commercially available soil-less seed starting mix in mid April and germinated very nicely. All seedlings have been exposed to the same lighting and water schedule (+ as needed) to keep them moist. The seedlings are ~1 to 1.5 inches tall (and hopefully growing). The stem thickness is presently similar to a "2 or 3 times [x] a pencil/pen point," so not thick at all, maybe even a bit stretched, even with rotation. If you are seeking additional information to assist me in the diagnosis, please let me know. I will do my best by posting some photos soon. Thanks.
This time of year with smaller seedling plants purpling stems or underside of leaves is a sign of poor posphorus uptake. IE cold plants. Warm them some, or let summer over take them. This too shall pass.
Had the same problem with mine until they were repotted. Now there is no problem. Until reading Tom's post, I just thought it was normal baby tomato seedling behavior.
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If you are willing to cheat a bit, a pinch of NPK fertilizer dissolved in water (so the water is just barely blue) fixes the problem. I do use chemical fertilizer on my indoor starts, but not on my soil.
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Posts: 272 | Location: Upstate NY Zone 5 border with 4 | Registered: March 25, 2006
What you are most likely seeing is a Phosphorus deficiency most likely caused by a too cool soil temperature. There is no need, there never is a need, to use any synthetic fertilizer to solve this or any other plant problem. Warm the soil, or move the plants to a warmer location. If that does not solve the problem then you may need to look into other solutions.
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Posts: 2116 | Location: Central Michigan along the Lakeshore | Registered: August 28, 2004
Here's an update with photos to provide you with a better physical reference. The paper clip depicted in one of the photos measures 1.65 inches long. The photos have not intentionally had any color alterations.
Earlier this week, (May 12+) some of the tomato seedlings have begun to show their first true set of leaves. However the stems continue to be various shades of purple, ranging in color from variety to variety. None of the stems are thick, nor are they tall. They are all watered to keep the soil moist (+ as needed). The seedlings are brought outside in temperatures 65 degrees and higher and brought inside during the overnight. They have yet to be transplanted to larger containers. When should that occur?
Seedling varieties include, but are not all depicted in the photos are: Mortgage Lifter, Beefsteak, Sweet 100, generic larger red cherry etc.
Update: Throughout the week I transplanted the seedlings from the smaller germination trays into larger containers making sure not to disturb the root systems that were establishing. I also added additional soil-less mix so they will have room to spread and develop their roots. I made sure to create a soil line up to about the first set of true leaves (burying the stem pretty deeply) and placed them outside in full sun. I have yet to see any type of physical changes, but will keep you updated. I gather, they may need a few days to acclimate to their new surroundings.
