Take one out see how it responds being outside with a little air movement.

Jaden said:

Hey everyone I'm starting up a grow. My seedlings look a little too stretchy. But I can't take them out of their propagation dome because we are having a surprise cold snap. I have a 15 watt heating mat under them set to 23.5 celcius, and when the lights on the air temp is about 22 or 23. I had wanted to move them out of the dome and put a little fan on em to strengthen them up. How can I help strengthen the stems under the dome?
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You can just wiggle the cups every once in a while. Weaker stems may flop over and that's okay, it strengthens them.

Just fill out the cups with soil and get a fan in there asap. Even having the dome open a little with a fan beside it can provide some airflow. And if it's possible put the lights closer.

As tannedViking said, fill the cup to the top with more of what you started the plant in. Increase the light a little. When you transplant, plant deeper. Make some sort of seedling stand thats easily removable and plant deeper when you transplant.

Remember when u transplant those ladies, u can put most of that stem in ground they'll make roots.

WinJr63 said:

Remember when u transplant those ladies, u can put most of that stem in ground they'll make roots.

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Ladies? Half of them (give or take) are males. These seeds come from natural reproduction. I was hardening them and then came a surprise cold snap for a few days. If I take the dome off too much heat escapes and drops too low for my comfort. Maybe the seedlings comfort too. I didn't want to risk stalling or uptake problems... in the meantime they just stretched with no wind to strengthen them up. Sucks. But just a temporary problem.

Nice seedling stand! Good suggestions from everyone. I did work the seedlings back and forth lightly with my fingers to encourage them to strengthen up. One threatened to fall over completely not wanting to spring back upright without help. Temps are still rather low, but I will open some vents and let them harden off for a day or so, plus mound up soil tomorrow and see if thats enough to hold them. I had the lamp closer, (110 watt Starlight florescent ) but the leaves started going pale so I raised the S hooks up 2 chain links. About 3 inches or so. Once I get them more stable with soil/stands I can get a gentle breeze on them and this temporary problem can give way to new problems.

Its looking better. Gentle breeze from the fan just gently moving the tops. Blowing downward past the lamp to force any heat down where the seedlings are. Added a little soil, which works out well at this point. Planted the sprouts in light mix, which is almost a blank substrate only enough nutrients for a couple weeks. So when I added soil today it was a mix of 75% all-mix and 25% active living soil with biochar which is a small top dressing. This will start up a little organic activity slowly. I can add a little more active soil, and all mix as they get a little bigger. Those that get up-potted will get the full all-mix with 10-15% active terra preta style soil mixed into that. I'll probably pick up a bag of worm castings to mix in sometime in the near future to get the full living soil going. Put the light up a little higher to match their growth over the next week or so. Heating mat removed. Air temp is about 21, a little bit too cool but I'm not breaking out the heater. They will just have to grow a little slow until the ambient temperature gets better.

Clones in the background.