What Grows In Mary's Garden

A garden full of dahlia blooms.

If you want your neighbors to think you're an award-winning gardener, try planting dahlias in your garden this year. Just when everything else starts to show the strain of midsummer, dahlias begin a showstopping performance that lasts through fall. Dahlias need just a little bit of special care in some climates, but the rewards far outweigh the work.

About Dahlias

Buying: Dahlias are sold as tubers, but they may also be available planted in pots at garden centers in the spring. For the most interesting selection, visit your nearest dahlia society.
Cold hardiness: Dahlias are considered true perennials only in zones 6-7 and warmer. In cooler climates, they can be treated like annuals, or you can dig up and store the tubers over the winter. Cold hardiness for dahlias depends on the variety, so check the label when buying.
Size: Dahlias range from foot tall dwarf varieties to towering 8' giants.
Quality: Look for plump, firm tubers with no spoiled spots and at least one eye. Unlike potatoes, dahlia tubers have eyes only on the top, where the stem meets the root.

Variegated red and yellow dahlia bloom.

Dahlia Growing Conditions

Full sun: 6-8 hours a day is ideal, although if you live in a hot climate you may want to give them a little cooling afternoon shade.
Soil: Loose, fertile, well-draining, slightly acidic soil. Dahlias put out tiny feeder roots that require light soil that holds moisture without being soggy. Clay soil should be amended with compost, sand, and/or humus.
Water: Dahlias need regular water but will rot in heavy wet soil.
Fertilizer: Dahlias need lots of nutrients, especially when blooming. Rich soil and balanced organic fertilizer will help.
Cautions: Dahlias are very susceptible to rot and freeze, so keep this in mind when choosing your planting site.

Red red dahlia bloom.

How to Plant Dahlias

When to Plant: Plant dahlias in spring, after all danger of frost has passed. If the weather has been soggy, wait until the soil dries out a little.
Soil Preparation: Till the soil about a foot deep. Work in plenty of well-rotted compost, sand, or humus. Avoid manure or fresh compost – it's too high in nitrogen. If you're buying soil, make sure it hasn't been treated with weed preventers since dahlias are very susceptible to herbicides.
Planting: Plant dahlia tubers about 2' apart and 6" deep, with the eyes facing up.
Watering: Ideally, you shouldn't water new dahlias, but let the spring rains work their magic. If it's already hot and dry, you can give them a light watering.
Support: Taller varieties need stakes for support. Install a bamboo stake or tomato cage when planting, so that you don't cause damage later.
Identify: Tag plants with the name of the variety to prevent confusion if you dig them up to keep over the winter.
Mulch: Add a couple of inches of mulch to hold in moisture.

Pale yellow dahlia blooms.

Dahlia Growing Tips

Fertilizing: Feed dahlias about once a month with an organic fertilizer lower in nitrogen and higher in phosphorus and potassium, such as a 1-2-2 ratio. With enough compost, your dahlias will do fine without fertilizer.
Watering: Make sure your dahlias get an inch of water per week. Keep track of rainfall so that the water is consistent.
Deadheading: Clip off spent blossoms to encourage more blooming.
Disbudding: While optional, some gardeners remove many of the flower buds to focus the plant's energy on fewer, larger and showier blooms with long stems. The flower buds come in clusters of three. If desired, remove the two side buds and allow the middle one to grow, and keep pinching off any more side buds that form along the end of the stem.

Variegated pink and yellow dahlia bloom.

Pinching: For a more natural look, pinch back dahlia plants when small to encourage branching.
Dividing: Dahlias can be divided in fall or spring. Dig up the tubers and use a sharp knife to cut into sections, making sure each section has at least one eye. Allow tubers to dry overnight before replanting.
Overwintering Outdoors: If dahlias are hardy in your zone, simply cut them back after the first heavy frost and apply an extra layer of mulch to protect them from the cold. Remove the extra mulch in spring.
Overwintering Indoors: In colder climates, dig up and store the tubers in a basement or garage, then divide and replant them in the spring.
Weeding: Hand pull weeds around dahlias, or smother with mulch. Don't use any sprays.
Cut Flowers: Dahlias make stunning bouquets! Cut the blooms in early morning for best results.

Further Information

American Dahlia Society (dahlia.org)
How to Grow Dahlias (dahlias.com, PDF 151Kb)
Dahlia Do's and Don'ts for Beginners (scdahlias.org)