As soon as the first set of true leaves appear (leaves that look like the plant's normal leaves), it's time to give the seedlings room to breathe and grow. You only want one plant growing in each small cell or pot. When there's more than one, the roots will intertwine and they will compete for water and nutrients. Some gardeners carefully separate the seedlings and replant extras in other pots. But you may wind up with many more seedlings than you really need.
Peg Graft from Springfield, Mo., has found that thinning early with small scissors is the best way to thin seedlings without disturbing the roots of the survivors.
"I decide in advance exactly how many plants of each variety I want, and I plant two or three seeds in exactly that many cells. When it's time to thin, I snip off the extra seedlings with scissors at the soil line. That way I don't risk damaging the roots and I get exactly as many plants as I want."