Tomatoes started from seed generally take two to three weeks to form their first set of true leaves, usually when the seedling reaches 1 1/2 inches in height. True leaves differ from seed leaves, called cotyledons, in that seed leaves are long and narrow, while true leaves appear to be tiny versions of the mature plant's leaves. Cotyledons drop from the plant after the true leaves form.
About this Author
Michelle Z. Donahue lives in Washington, D.C., and has worked there as a journalist since 2001, when she graduated from Vanderbilt University with a B.A. in English. She first covered politics as a reporter for the weekly Fairfax Times newspaper, then for the daily newswire Canadian Economic Press, where she reported from the U.S. Treasury. Donahue is currently a freelance writer.