Dahlia

A dahlia is a commitment. The flowers are extraordinary — ten-inch blooms that hold in a vase for a week, colors from near-black to apricot to fuchsia — but the plant demands consistent attention from the day you set the tuber in the ground until the day you dig it up in fall. In most climates, that means treating it as an annual with extra steps: dig, dry, store, replant. In zones 8 and warmer, the tubers can stay in the ground year-round, which removes the hardest part of the job.

Plant the tubers about one week after your last frost, when the soil has warmed to at least fifty-five degrees. Lay them horizontal in a hole four to six inches deep, eye side up — the eye is the small bump where new growth emerges. Do not water immediately; a tuber in wet soil before it sprouts is prone to rot. Wait until you see green shoots, then begin watering regularly. Once the plant is growing, consistent moisture becomes critical. A dahlia that dries out in July will drop buds and produce smaller blooms.

The single most useful thing you can do is pinch the main stem when the plant reaches about twelve inches. Cut it back to just above the third or fourth set of leaves. The plant will branch, and each branch will produce its own flower stem. An unpinched dahlia grows tall and spindly with a single dominant bloom; a pinched dahlia becomes a productive bush that flowers for months. Tall varieties — anything over four feet — need staking. Drive the stake at planting time, before the roots spread, so you don't damage the tuber later.

Powdery mildew is the most common disease, showing up as white patches on the leaves in late summer when the nights turn cool and dewy. It is rarely fatal, but it looks bad and can weaken the plant. Spacing plants widely for airflow helps; so does watering at the base rather than overhead. Slugs tend to attack the young shoots in spring — check for them in the evening and remove them by hand, or set beer traps near emerging plants.

In colder climates, the tubers must be dug after the first frost blackens the foliage. Cut the stems back to a few inches, lift the clump carefully with a spading fork, and let it dry for a day or two in a shaded spot. Brush off loose soil — do not wash them — and store in a cool, dark place in barely-damp peat moss or sawdust. Check them once a month for rot or shriveling. A tuber that survived the winter is worth five new plants the following spring.