NOTE: Written last fall and only just posted.
I have failed with dahlias so far because I have not paid good attention to their cultivation. Next year will be different! Here is advice from Anne Raver on what to do to have gorgeous flowers:
For Your Own Garden Palette
DAHLIAS like warm weather, so plant the tubers in late spring, just about the time you put in your tomatoes and cucumbers. (The ground should be about 60 degrees.) They need full sun, about eight hours a day, to grow sturdy and tall with many blooms.
The soil should be well drained, with plenty of compost or aged manure. Taylor Branch digs his holes about a foot deep. Some gardeners add a handful of bonemeal, mixed with a shovelful of compost at the bottom of the hole, before setting in the tubers and covering them with soil. Do not water; wait until sprouts appear weeks later.
The plants like to spread their branches, so dig holes 18 to 24 inches apart.
Christy Macy advises sinking an eight-foot stake a few inches from the tuber at planting time, because the tall plants tend to fall over. It also serves as a marker until those first sprouts appear.
“It’s such an exciting moment, when the dahlias break through,” Ms. Macy said. “Just an inch, but very substantive, not wispy, almost like a mini-tree.”
As the plants grow, tie the stems loosely to the stake, with jute or soft twine.
Dahlias prefer deep garden soil, but they will grow in large containers, with at least 12 by 12 inches of space per tuber, but choose dwarf or low-growing varieties rather than the giants. Keep the soil damp, since containers tend to dry out, and fertilize with a diluted fish or seaweed solution every few weeks.
Dahlias bloom in many shapes, from spiky to rolled petals, and from doubles the size of dinner plates to single-petaled one-inch flowers with button centers. The colors are magnificent, and many long-stemmed varieties make long-lasting cut flowers.
Sources include Swan Island Dahlias (800-410-6540 or dahlias.com), Ferncliff Gardens (604-826-2447 or ferncliffgardens.com) and Corralitos Gardens (831-722-9952 or cgdahlias.com). Order tubers by late summer or fall, before the best varieties are sold out; they will be shipped in the spring.