Many gardeners love dahlias for their wide range of colors and large, showy size. But the drawback to dahlias in northern climates is that they can’t tolerate winter’s cold, so the tubers must be dug out each fall and planted outdoors each spring when all danger of a frost has passed. This means that it can be well into summer before northern dahlia growers see blooms. Luckily, greenhouse owners can get flowers much earlier than this by starting their dahlias in the greenhouse in February or early March.
You’ll need to set up in your greenhouse one or more dahlia potting trays, about 6” deep. (I use wallpaper-paste trays with drainage holes cut in the bottom, but you can use regular 6” pots with one tuber per pot.) The soil should be slightly acidic, with a pH of about 6.5. Add a little peat moss if the soil is more alkaline. I also add a little compost and bone meal to the mix to help promote growth.
Some gardeners store their dahlia tubers for the winter in sawdust or sand, but I’ve found they do just as well simply placed in trays under a greenhouse bench. At planting time, to propagate your dahlias, carefully break up the clusters of tubers, making sure that each separate tuber has at least one eye to produce a new plant stalk. Then plant each tuber in a potting tray or pot to a depth of 4 to 5 inches and water it in. Use just enough water to moisten the soil, not soak it. Too much water can rot the tubers. Keep the trays at a temperature of about 60oF (18oC).
The trays will also require about eight hours of sunlight daily, which is impossible in winter, so you’ll need to provide additional lighting. This can be done with 100-watt-equivalent LED lights. You may think that light is irrelevant because the tubers are underground, but the light warms the soil and tricks the plants into growing.
When stalks begin to show, start fertilizing the plants. At this early stage, I prefer a fertilizer diluted to about a quarter of full strength. I fertilize once a month until all danger of frost is over and the dahlias are ready to be planted outdoors. I add a bit more bone meal to the garden soil when planting (about a teaspoon in each hole), and I stake the stalks to reduce wind damage. Then I simply wait for an early display of magnificent dahlia color.