Answer: Dahlias are native to Mexico and Guatemala and should feel quite at home in your Arizona garden. Dahlias need a resting period and are usually dug and stored after the tops turn yellow at the end of the season. Although they begin from a single tuber, by the end of the summer the roots will have developed multiple tubers which should be divided and replanted the following spring.
There are hundreds of hybrids available, ranging in size from 6" to over 6' in height. All have the same growing requirements: Provide full sunshine, with some filtered afternoon shade in your hot summer climate, deep, rich, well-draining soil, and regular watering and feeding.
Dinner Plate dahlias are some of the largest plants and produce the largest flowers, so they'll need some extra care during planting. Dig a hole 12" deep and work peatmoss, composted sawdust, or other small textured organic matter into the soil. Then mix 1/4 cup complete fertilizer into the soil at the bottom of the hole. Add about 4" of amended soil, then place the tuber longways on top of the soil. Drive a stake into the hole for future support of the plant and refill the hole with the amended soil. As the plant grows, loosely tie the stem to the stake. Start watering regularly after the shoots are above ground, then side-dress with a high phosphorus fertilizer when flower buds appear. (Don't use a high nitrogen fertilizer or you'll have more foliage than blooms!) Mulch to keep down weeds and help the soil hold moisture.
Enjoy your dahlias!
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