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I started some Brussels sprouts from seed, and on several of them I notice that the stem is very thin at the soil line. These ones are tending to lie instead of standing erect.

Is this something to be concerned about? How can it be prevented, and if possible can it be fixed?

(I have also noticed that the same thing happened to my melons, so it is likely not something specific to Brussels sprouts.)

UPDATE

I did as @benn suggested and buried them deeper. This turned out to be an informative experiment because less then 24 hours later several of the seedlings were exhibiting the same thin stem behavior at the new soil line! I wish I had marked which seedlings had this issue before I buried them deeper so I don't know if it is the same set of seedlings or not. But I do note that it is slightly less than half (there are 14).

Some additional information that may help with diagnosis.

  • U.S. Pacific northwest, Zone 8B
  • Seedlings were started inside and kept by a window on a heating mat in a roughly 6 in. diameter and 3 in. deep plastic "pot" (old takeout bowl)
  • Seedlings were sprouted on a seed starting heating mat and kept there till they were about 1 in. tall.
  • The soil is 1/3 compost, 1/3 coconut coir, and 1/3 vermiculite.
  • They have not received fertilizer.
  • They were moved into our greenhouse 1 month ago. The green house can get fairly warm during the day (80-90 degrees F) but cool at night.
  • They were watered by keeping the "pot" in a tray and watering the tray, allowing capillary action to pull water into the soil. They were rarely (but sometimes) top-watered.
  • The seedlings were moved to individual pots about 1.5 weeks ago.
    • A few days after this is when they started to show the thinning of the stems
  • About 5 days ago I started to keep them outside exclusively, but in a location where I could put them in the shade during the times when the sun was hottest.
  • When burying them deeper, I also added some more soil. I was out of the compost/coir/vermiculite mix so I used some standard potting soil.

I don't really believe they were getting a lot of sun initially, but certainly they had been for several days prior to burying them deeper and seeing the thinning happening again.

Image before update

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Images after update.

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  • Try to put them deeper. – benn Jun 3 at 7:58
  • @benn How can I be sure that if I do that it won’t just happen again at the new soil line? – SethMMorton Jun 3 at 14:48
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    Once they get their real leaves (so after these first cotyledon leaves), the stem will grow further from there. If you'll put the part under these cotyledons in the ground, the roots will also form from there. – benn Jun 3 at 16:18
  • @benn I did your suggestion - I hope it works well. I would still be interested in knowing the root cause of this and (hopefully) how to prevent it. – SethMMorton Jun 4 at 3:07
  • I suspect not enough light, but you have provided not enough info for a clear answer from anyone. – benn Jun 4 at 6:14

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