- Don't be in a hurry to plant; dahlias will struggle in cold soil. Ground temperature should reach 60°F. Wait until all danger of spring frost is past before planting. (We plant them a little after the tomato plants go in.)
- Select a planting site with full sun. Dahlias grow more blooms with 6 to 8 hours of direct sunlight. They love the morning sunlight best. Choose a location with a bit of protection from the wind.
- Dahlias thrive in rich, well-drained soil. PH level of your soil should be 6.5-7.0, slightly acidic.If you have a heavier soil, add in sand, peat moss or bagged steer manure to lighten and loosen the soil texture for better drainage.
- Bedding dahlias can be planted 9 to 12 inches apart. The smaller flowering types, which are usually about three feet tall, should be spaced two feet apart. The taller, larger-flowered dahlias should be spaced three feet apart.
- The planting hole should be slightly larger than the root ball of the plant and incorporate some compost or sphagnum peat moss into the soil. It also helps to mix a handful of bonemeal into the planting hole. Otherwise, do not fertilize at planting.
- Avoid dahlia tubers that appear wrinkled or rotten. A little bit of green growth is a good sign. Don't break or cut individual dahlia tubers as you would potatoes.
- Plant them whole, with the growing points, or “eyes,” facing up, about 6 to 8 inches deep. The crowns should be just above soil level.
- Tall, large-flowered cultivars will require support. Place stakes (five to six feet tall) around plants at planting time and tie stems to them as the plants grow.
- Large dahlias and those grown solely for cut flowers are best grown in a dedicated plot in rows on their own, free from competition from other plants. Dahlias of medium to low height mix well with other summer flowers. If you only have a vegetable garden, it's the perfect place to put a row of dahlias for cutting (and something to look at while you're weeding!).
- Dahlias start blooming about 8 weeks after planting, starting in mid-July.
- Some gardeners start tubers indoors a month ahead to get a jump on the season.
- Do not water the tubers right after planting; this encourages rot. Wait until the sprouts have appeared above the soil to water.
- Do not cover the dahlias with mulch or bark or sprouting is more challenging; apply slug and snail bait to avoid pests.
- There's no need to water the soil until the dahlia plants appear; in fact, overwatering can cause tubers to rot. After dahlias are established, provide a deep watering 2 to 3 times a week for at least 30 minutes with a sprinkler (and more in dry, hot climates).
- Dahlias benefit from a low-nitrogen liquid fertilizer (similar to what you would use for vegetables) such as a 5-10-10 or 10-20-20. Fertilize after sprouting and then every 3 to 4 weeks from mid-summer until early Autumn. Do NOT overfertilize, especially with nitrogen, or you risk small/no bloms, weak tubers, or rot.
- Like many large-flower hybrid plants, the big dahlias may need extra attention before or after rain, when open blooms tend to fill up with water or take a beating from the wind.
- Bedding dahlias need no staking or disbudding; simply pinch out the growing point to encourage bushiness, and deadhead as the flowers fade. Pinch the center shoot just above the third set of leaves.
- For the taller dahlias, insert stakes at planting time. Moderately pinch, disbranch, and disbud, and deadhead to produce a showy display for 3 months or more.
- Dahlia foliage blackens with the first frost.
- Dahlias are hearty to zone 8 and can be cut back and left in the ground to overwinter; cover with a deep, dry mulch. Elsewhere, the tuberous roots should be lifted and stored during the winter. (Some readers find, however, that dahlias will survive in zone 7 if the winter isn't too severe.)
- Slugs and snails: Bait 2 weeks after planting and continue to bait throughout the season.
- Mites: To avoid spider mites, spray beginning in late July and continue to spray through September. Speak to your garden center about recommended sprays for your area.
- Earwigs and Cucumber Beetle: They can eat the petals though they do not hurt the plant itself
- Powdery Mildew: This commonly shows up in the fall. You can preventatively spray before this issue arises from late July to August.
Taking Up the Tubers
In cold regions, if you wish to save your plants, you have to dig up the tubers in early fall and store them over the winter.
Dahlias may be hardy to USDA Zone 8. There they can be left in the ground to overwinter. In areas that get frost, including most parts of Zone 5, a killing frost—or a touch of frost—can help the bulb to shut down/go dormant.
- Foliage should be cut back to 2 to 4 inches above ground and lifting and separating should be completed.
- Gently shake the soil off the tubers.
- Cut rotten tubers off the clump and leave upside down to dry naturally.
- Pack in a loose, fluffy material (vermiculite, dry sand, Styrofoam peanuts).
- Store in a well-ventilated, frost-free place—40 to 45 degrees F is ideal, 35 to 50 degrees F is acceptable.
- Take out the tubers in the spring, separate them from the parent clump, and begin again.
- If this all seems like too much bother or you do not have the right storage place, skip digging and storing, and just start over by buying new tubers in the spring.