Answer: The height will vary depending on the type of plant. Some plants may have a mature height of only a few inches, others may have only one set of true leaves (eg squash) at that height. It will also vary on the amount of light they have been receiving. Plants in need of more light will reach or stretch and elongate beyond the normal height. Thinning is one way to increase the light they receive. When you thin, sometimes it is easier to trim off excess plants at the soil level using small scissors than it is to pull them out because pulling can disrupt the soil around the plants you wish to keep, especially if they are very crowded.
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