Answer: Dahlias do not tolerate frost and need warm soil to grow well so the rule of thumb is to wait about one or two weeks after your average last frost date -- to some extent this will depend on the weather in any given year.
When you're ready to plant, choose a site that gets sunshine all day, and dig a hole about one-foot deep for each tuber, spaced about three feet apart. Mix 1/4 cup of complete fertilizer into the bottom of the hole, add about 4 inches of soil, and drive a 4-5 foot stake into the hole. Then plant each tuber, with the eye pointing toward the stake, and cover it with 3 inches of soil. Water well, and when the tuber sprouts, gradually fill the hole in with soil. Don't completely cover the emerging stem and leaves, but keep building up the level of soil. This method will give your mature dahlia lots of underground support. As the stem grows above soil level, tie it loosely to the stake. Dahlias growing in enriched soil won't need additional fertilizer throughout the season, but should be watered every week during the growing and blooming season. Plan to supply one-inch of water per week to each of your dahlia plants.
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