Grow dahlias in full sun. Because they’re extremely cold sensitive, it is important to wait to plant them until the ground has warmed to 60 (15.5), normally about 2 week after your last spring frost. Lay tubers in planting holes horizontally, 4 to 6 inches (10 to 15 cm) deep, and do not water until they sprout up through the soil. I grow them in double rows in beds that are 3 feet (1m) wide, spacing plants 18 inches (46 cm) apart.
Once any foliage emerges from the ground, water deeply 2 or 3 times a week for at least 30 minutes. After plants reach 1 foot (30 cm) tall, give them a hard pinch by snipping out 3 to 4 (7.5 to 10 cm) inches of the growing centre to encourage low basal branching and increased stem count, and longer overall stem length.
Slugs and snails are dahlias number one enemy. Put down bait about 2 weeks after planting, or as soon as you see foliage emerging from the ground, and then periodically through the season. I use Sluggo, an organic option that’s safe for both children and pets.
But midsummer, plants will get tall and require stalking to keep them falling over. I recommend the corral method, which involves placing a metal T-post every 10 feet (3m) along the outside of the beds and then stringing a double layer of baling twine from post to post to corral the plants in. For home gardeners with just a few plants, place tall, sturdy posts next to tubers at planting time so you can tie stems to them as they grow.
In most areas, winters are too cold to leave tubers in the ground to perennials, so after blooming, you need to dig them up and store them. After a few frosts in the fall, lift the tubers clumps out of the ground with a pitchfork, being careful not to slice through the clumps as you go. Was the clumps to remove all of the excess soil. (I use a strong hose to do this.) Then dip the camps into a 5-percent bleach-water solution and lay them out to dry in a cool garage or basement for a day or two.
Dividing dahlias every year, because their tubers grow quickly, and when they get too large they can rot or become too heavy to lift and store. Once the clumps have dried after washing, split them in half with sharp pruners so you’re left with two smaller, more workable pieces. For a viable tuber, it is essential that the eye or eyes (swollen growing nodes) are connected to a complete, unbroken tuber. If you accidentally break one, just toss it.
In my early days I tried to save many of these, but they always ended up rotting; its better to be ruthless on the front end. With a little practice it gets pretty esy to spot eyes and separate tubers with accuracy and speed. After dividing, store the tubers in a medium such as slightly dampened peat moss or sawdust in a newspaper-lined box, or wrap them individually in pieces of plastic cling wrap. Keep them in a cool, dry area with a temperature of 40 to 50 F (4 to 10 c) such as a basement or garage for the winter. Check monthly throughout the winter, and toss any that show signs of rot.