Pre-sprouting: If you’ve overwintered your Dahlia tubers, I’ve found it’s best to confirm tuber viability by pre-sprouting before planting to assure your success for growing these beautiful flowers. About 4 weeks before you want to plant the tubers in pots or in the garden, remove the tubers from their cool winter storage and place snugly on a tray with an inch of peat, vermiculite, potting soil, or any combination. Cover the tubers lightly with the media you’ve chosen, mist with water to have the media slightly damp but not wet, and place in a warm and/or sunny area. Damp media keeps the tubers hydrated while they are sprouting but wet media can cause the tubers to rot. If heating mats and grow lights aren’t available, a warmer area in the house or by sunny windows should work too. I’ve found warming the tubers up is more important than light for pre-sprouting.
Once the eyes have sprouted, you’ve confirmed the tubers are healthy and ready to plant (see directions for planting below). I’ve planted tubers anytime between when the sprouts are just visible up to when the sprouts are several inches tall. All Dahlias do not sprout at the same rate and you may find some varieties need a couple more weeks than others so be patient!
Planting: Dahlias prefer at least 8 hours of direct sunlight a day; less sun produces taller plants but less blooms. In hot climates, dahlias like morning sun and afternoon shade. Though dahlias are only winter hardy in zones 8-11, gardeners in zones 2-7 can simply plant dahlia tubers in the spring. They will grow quickly and the plants will be blooming by mid to late summer.
Amend beds with 2-4 inches of high quality compost or well rotted manure, a light dusting of bone meal and a balanced organic fertilizer. Mix all ingredients into the soil to distribute evenly.
Dahlias like warm (60 degrees minimum), well-drained soil. Dig a hole about 4”-6” deep and lay the tuber horizontally with the sprout facing up, spacing the tubers about 18-24” apart and then cover with loose soil. It’s best to stake dahlias at the same time as planting so you don’t have to worry about damaging the roots. Staking your dahlias will keep your flowers clean and off of the ground and you will have more flowers. After the sprouts appear above the ground you can start watering. Don’t cover the tubers with mulch until after the soil has warmed up and don’t fertilize when planting.
Growing: Once your dahlias have started to grow leaves, you need to start watering regularly. Depending on where you live, how fast your soil drains, and the temperatures should all be factored into how much you water your dahlias. During the hot summer months you may water 2-3 times per week and 30-60 minutes per day.
Fertilize your dahlias with a low nitrogen fertilizer when the plants are around 6” tall, then again every month until frost. Do not overfeed your dahlias. High nitrogen fertilizers result in a lot of foliage and not a lot of blooms.
You can pinch or top your dahlias to have bushier plants with strong stems. To do that, cut off the center shoot just above the third set of leaves when the plants are 8”-12”.
The best time to cut flowers is in the morning when the buds aren’t too tight. Cut the flowers with clean, sharp pruners and place in water right away. The flowers will stay fresher if you change the water everyday. Don’t forget to remove the old blooms so the plant will produce more blooms until frost. The more you cut dahlia flowers, the more they will bloom.