Answer: You describe a very common problem with seedstarting indoors. Seedlings will grow spindly if they don't receive enough light, and unless you have a greenhouse or grow lights that you can place directly above the seedlings, this stretching can occur. Heat can also cause weak stems because it increases the growth rate, and this is especially a problem if the light is not sufficient. So, providing more light and cooler temperatures would help. Also, make sure your seedlings have good air circulation around them because this helps the stems toughen up. If your seedlings have stretched too much already, the best solution is to reseed them. I've had to do this more than once, and I've always been happier with the results.
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