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 the Author
Michelle Z. Donahue has worked as a journalist in the Washington, D.C., region since 2001. After several years as a government and economic reporter, she now specializes in gardening and science topics. Donahue holds a bachelor's degree in English from Vanderbilt University.