DAHLIA CARE

Dahlias are only perennial in horticultural zones 9 and warmer. That means that here, in mid-Michigan, the tubers need to be replanted every spring and dug up in the fall for storage through winter. There are as many methods of storing tubers as there are gardeners growing them. Some outstanding information about tuber care can be found at the American Dahlia Society and at the Colorado Dahlia Society websites.

To show what is required to maintain the 3,000 plus dahlias that are planted at Dahlia Hill, we outline what we do during one year.

Tuber storage is a big issue with many growers. Ideally, dahlias need to be kept at 40 to 50 degrees with high humidity. Dahlia Hill has a workroom with a temperature and humidity controlled storage room where around 10,000 tubers are stored yearly. The tubers are checked each month to see if any rot or drying has occurred. Our storage season extends from late October to April.

In the last week of April the tubers are taken out of storage. Each tuber is checked and separated into bags of those with and without buds. Using a list of the available tubers, the Administrative Director and President design the layout of the Hill for the coming year. This is more complicated than it sounds. A balance has to be struck between planting enough of each variety and making sure the Hill will look its best.

After the plan is drawn up, the tubers are placed in plastic grocery bags marked with the row where they will be planted. Planting is undertaken only after the soil temperature reaches 50 degrees, usually the second or third week of May. First, 6" deep trenches are dug with a blade on a tiller. The rows are spaced five feet apart in the center of each terrace. Bonemeal and organic compost is mixed into the soil at the bottom of each trench. The tubers are planted according to the plan, one foot apart. One to two inches of dirt is gently raked over the tubers.

Through June and July, volunteers remove weeds and pull more dirt over the tubers as the shoots grow. Cutworms, leafhoppers, wireworms, japanese beetles and spider mites are the main pests dealt with during these months. Only organic controls and fertilizers have been used at Dahlia Hill since 2005. Quick-release water connections on each terrace enable us to control spider mites by blasting them with a high-powered spray of water using a watering wand attachment. In dry, hot weather this needs to be done twice a week or more. Insecticidal soap misted on the leaves weekly keeps the leafhoppers to a minimum. Japanese beetles and cutworms are hunted daily and dropped into soapy water. We are experimenting with several organic cutworm controls - fortunately, the plants usually recover from cutworm attack. A control for wireworms hasn't been determined, other than to destroy them when found. Encouraging healthy soil and thereby, healthy plants is our main strategy for controlling pests.

The dahlias start to bloom in mid-July. Taller plants are staked using green jute twine and five foot pieces of 1/2" rebar painted dark green. Deadheading is done weekly. Soil is packed around the base of the stems to help support the plants. The programmable drip irrigation system waters deeply twice a week during dry weather.

Hard frosts have occurred as early as September 25th and as late as October 22nd. After frost kills the foliage we wait one to two weeks, allowing the tubers to harden. All stakes and twine are removed, then a hedge trimmer, pruning loppers or pruning saw are used to cut the stalks to about 6" above the soil line. The stalks are placed between the rows and chopped coarsely with a sharp shovel blade.

Care has to be taken when digging the dahlia clumps. Some varieties have long, slender tubers that break easily under the weight of the soil. As much of the dirt as possible is shoveled off the clump first. Then two people, one on each side, use shovels or forks to dig up the clump. The digging tools have to be placed at least a foot away from the stalk to avoid cutting through the tubers. Clumps are never shaken or knocked on the ground as this can break the tubers. A hose is used to thoroughly wash off the clump, which is placed with others of the same variety into a plastic garbage bag. The bags are marked with the variety and put into the workroom.

Tubers are divided in the fall - storing them undivided would require too much room. Dividing all of our tubers can take three to four weeks. Each tuber variety is taken out of its bag and the clumps divided into separate tubers, each with some of the stem attached. This is where the "eyes", or future stems, will develop. The tubers are counted and marked with an indelible pen or pencil and soaked for a few minutes in a 1:50 solution of bleach and water. While still damp, the tubers are placed in grocery bags and completely covered with vermiculite. The full bags are put into cardboard boxes and put on shelves in the storage room.