The first thing to remember when growing dahlias is that they can grow quite large. Parshall recommends planting dahlias near stakes in a six-inch planting hole that is deep enough to provide freeze protection underground. Plant the tuber with the eye--a little white bump--(figure G) facing up. Cover the tuber with a few inches of soil, put a little bone meal or blood meal over the top, and then add a small amount of time-release formula (figure H). As the plant grows, continue to fill in the hole to ground level, or even higher if additional support is necessary. To create big beautiful blooms, Parshall recommends disbudding or thinning the buds to concentrate the plant's growth into emerging buds below (figure I). Disbudding is really easy and should be done in the morning or late evening when there is a lot of moisture in the plant. Continue to disbud your plants throughout the growing season.
When the growing season is over, just a few weeks before the first frost, cut down the plants leaving about six inches of the plant above ground so that underground, the eyes of the plant will begin developing for the dividing process. Then two weeks later, dig up the clumps with a shovel or spade (rather than a garden fork). Dig a circle 10 to 12 inches away from the stem to gently unearth the tubers. Remove as much soil as possible without damaging the tubers. Carefully spray off the tubers with a hose.
If storage is a problem, and if the ground in your area doesn't freeze deeply, you can dig and divide in the spring. "The nice thing about dividing in the spring is that you can see the shoots coming out, sometimes one or two inches long, so there is no mistake you have a viable tuber," says Parshall. If you do dig and divide your dahlias during the spring, let the tubers sit for a few days before planting to let the cut surfaces heal.