Start waking up dahlia tubers about 4 to 6 weeks before your last expected frost date. This head start gives your tubers time to grow roots and green shoots indoors before warm weather arrives outside.
I begin the process each spring by laying my tubers on a wide flat tray filled with lightly damp potting mix. I set the tray in my sunniest spare room near a south-facing window and keep the room at 65°F to 70°F (18°C to 21°C). Within one to two weeks little pink and green nubs push up from the crown. That first sign of life after months of winter storage still gets me excited every year. The whole setup takes about ten minutes and costs nothing beyond a bag of potting mix.
Starting dahlia tubers indoors gives your plants a major jump on the growing season. While your tubers build roots and shoots in the warmth of your home, the soil outside is still too cold for planting. By the time you move them outdoors these pre-started plants have 4 to 6 weeks of growth behind them. You get blooms 3 to 4 weeks earlier than tubers planted straight into the garden. That extra month of flowers makes the small effort worth it every time.
UAF Extension says you can pot tubers indoors 7 to 8 weeks before your last frost date for the earliest possible start. Place each tuber in a pot with the eyes facing up. Cover with about 2 inches (5 centimeters) of potting mix. This longer indoor period works great for short-season climates where you need every extra week of bloom time you can get.
Watch for dahlia tuber sprouting by checking your trays every few days after you set them up. The first shoots look like small pink or green bumps on the soil surface. Do not water much at this stage. Too much water before roots form is the fastest way to rot a tuber indoors. Give the soil a light mist only when the top inch feels dry to your touch. I check mine every other day and mist about once a week during this early phase.
Once your sprouts reach 3 to 4 inches (7 to 10 centimeters) tall you can start thinking about the move outdoors. Wait until your soil passes 60°F (15.5°C) before transplanting. Harden the plants off for about a week first. Set the pots outside for a few hours each day and bring them back in at night. Add an extra hour of outdoor time each day. This slow shift prevents shock and keeps your plants growing strong.
The whole waking process takes just a small amount of effort for a big payoff in your garden. A tray, some potting mix, a warm room, and a little patience are all you need. Your pre-started dahlias will tower over the ones planted straight into the ground. You get weeks of extra flowers before the first frost shuts everything down in fall. I have never gone back to direct planting after seeing the difference that indoor starting makes.
Read the full article: Dahlia Tubers: A Complete Growing Guide
