Dahlia tubers like well drained, medium ph soil that has at least half a day of sun; full sun is best. Peat moss and organic matter with some bone meal are good soil additives. Work in well 2 weeks or more before planting, or apply in fall prior to sping planting. Try not to use too “hot” of organic matter. A commercial fertilizer with equal parts nitrogen, phosphorous and potassium, such as 16-16-16, works well at planting time, but work it well into the soil so as not to burn the tuber sprouts.
April and May are ideal times to plant new tubers, as long as the ground is not too wet or cold. Dahlias will rot if the ground is too wet or cold and we cannot replace them if the tubers have been subjected to this type of treatment. It is better to wait until the weather has warmed than to plant too early. The tubers should be planted on their side with sprouts pointing upward 3 inches deep and covered gradually as they sprout to give them a good strong stalk and to protect the tuber ball from freezing under the ground in winter. A stake can be put in at planting time for tall dahlias. Do not water tubers at planting time unless ground is very dry, they have moisture in them to get started. Use a good slug,insect/cutworm bait. Earwigs and sowbugs can eat the new sprouts, so be sure to inspect any damage carefully to be sure what is eating the plant if you have a problem.
Fertilize with a generic fertilizer when planting, like a 16-16-16. When plants are 12 to 18 inches high fertilize with a commercial or organic fertilizer that is low in nitrogen, but high in phosphorous and potassium to promote blooms, such as 5-10-10. Again, read label content or ask at your garden store. Fertilize again when the plants are starting to bloom well. It is preferred to not fertilize after the first of September or so as the roots could get soft.
If you pinch the center of the sprouts out when the plants are 6 to 8 inches high, this will allow more sprouts and gives a much bushier plant, and of course, more flowers. Water the plants thoroughly and deeply every week, more if temperatures are real hot and no rain has fallen.
Cut dahlia flowers in the cool of the day. Bring indoors and place stems in 2” of hot water, 160 F. Leave in the hot water until it cools or until the stems turn dark at the ends. Flowers can be re-cut and arranged as desired. The hot water gives them a longer life and can even liven up wilted blooms. A commercial additive and a couple drops of bleach added to the water of fresh flowers helps as well.
Insects can be a problem. 2 spotted spider mites attack the plant from the ground up and turn the leaves yellow and then brown as they suck the juice out of the plant. A commercial miticide or insecticidal soap can be used. Green spotted beetles (bean beetles, cucumber beetles or 7 spot) can be a pest to the blooms, use of a commercial insecticidal soap, in conjunction with an insecticide will help reduce these pests. Fungicides may be necessary if mildew is a problem, usually in the Fall, but also if you water heavily on the leaves. Please follow all chemical label directions carefully.
In Oregon and other states where the winters are often mild, it is not necessary to dig up the tubers every year. If they were planted in well drained soil 6 inches or so deep they should be fine. Cut the dead stalks back and add 3 to 4 inches of a mulch over the top of the clumps. Leaves, sawdust, straw or most any type of mulch can be used. You can also add a plastic cap or strip of plastic over the bed before applying the mulch for added protection from winter rains. In March as the weather warms take the mulch off and slug bait real well, then wait for the sprouts to appear. The tubers can be dug at this time and separated if desired.
If they need to be dug before winter, lift them carefully so as not to break their necks. Cut the stalk off close to the clump but don’t cut off the new eyes on the tubers. Wash all the dirt off and let them air dry for a couple of days. They can be stored in clumps or cut apart.
Storage of tubers can be a challenge. They must be stored at around 40 F. with some humidity. If they are too warm and dry, they will shrivel and won’t sprout well. If they are too warm and wet, they will rot. If they are too cold they will freeze and rot. A box lined with a breathable plastic liner layered with clean peat moss, fine sawdust (pine works well), vermiculite or clean potting soil, then covered to prevent evaporation works well.
The garage works well for a storage unit until freezing weather hits, then you must either heat the garage or move them to a nonfreezing spot temporarily. Talking with other gardeners offers any number of ways to store tubers, and they all may work. Find a way that suits you and stick with it.
When you divide clumps, save all tubers with eyes, even small tubers grow well. You can leave several tubers together also. Discard all tubers with broken necks. Locate the eyes on the stalk end and cut with a sharp knife. Not all tubers will have eyes. Eyes can be tiny bumps or can look like slightly indented craters. After cutting they can be dipped in a liquid fungicide or dusted with a bulb powder. Let the cut edges dry overnight before storing to prevent rot. Check stored tubers several times during the winter to be sure all is well. Discard any rotten ones.
Watch the video below to see cutting in action.