If you don’t know how to grow dahlias, you’re missing out on a long-blooming flower that fits nicely into perennial borders and begins blooming in mid-summer.
They flower prolifically late summer and autumn when many other annuals and perennials have finished blooming.
Modern dahlias flowers range from dwarf to giant, and come in forms that resemble daisies, cactus, pompoms, water lilies, buttons or zinnias and more. There are thousands of named varieties, in a wide array of colors, except true blue and black.
These plants are also suberb cut flowers. In fact, the more you cut them, the more they bloom.
Buying dahlias: tubers or plants?
Dahlia tubers ready for planting
Along with lilies, begonias and gladiola, dahlias are classified as summer flowering bulbs, although they are actually tubers.
You can buy dahlia tubers online – the choicest cultivars are available by mail order.
Or you can get them as bedding plants pre-grown in nursery pots, as many garden centers carry summer bulbs grown in pots like annuals.
How to grow dahlias: planting tips
Dahlias are easy to grow, but cannot take frost or survive cold winters.
To pre-start dahlia tubers yourself, put them into pots with a growing mix indoors six weeks before planting in the garden, and you will get a jump on the season and earlier flowering.
Plant them into the garden only after you’re sure that frosty nights are over for the season. A good time is when you would normally plant tomatoes.
Sunlight: Dahlias need a sunny location and grow best with at least eight hours of direct sunlight, but in hot southern regions they prefer morning sun and afternoon shade.
Soil: The soil temperature should be around 60ºF (15ºC) at planting time. Dahlias prefer well drained soil, so if you have a heavier soil, add coarse sand, peat moss or bagged manure to improve soil texture for better drainage.
How to grow Dahlias – planting depth: Plant dahlia tubers horizontally about 6 inches (15 cm) deep and 18 to 24 inches (45-60 cm) apart.
If you are going to stake your plants (larger types benefit from staking), put stakes in at planting time to avoid damaging tubers. Tomato cages also work well, and should be put in when planting.
Water: Dahlia tubers don’t need to be watered after planting. Once the plants are growing for best blooming, water once or twice a week during hot, dry weather.
Fertilizer: Use a low-nitrogen fertilizer. Vegetable garden fertilizer is ideal. Look for a percentage of potassium and phosphorous such as a 5-10-10, 10-20-20, or 0-20-20. Apply fertilizer about four weeks after planting and repeat three to four weeks later. Don’t overfeed.
Pinching: To encourage the growth of bushier plants with better stems for cutting, pinch out the center shoot just above the third set of leaves.
How to grow dahlias: overwintering tubers
You can treat dahlias as annuals, but they can be dug up after the first frosts kill off the stems and leaves and kept through the winter for replanting the following year.
Store the tubers in a cool, frost-free place. This is a great way to keep your favorite cultivars going. Here’s more information on how to overwinter dahlias and other summer-flowering bulbs.