Dahlia Planting Guide
Everything You Want to Know About Dahlias!
Dahlia is a genus of tuberous plants that are members of the Asteraceae family; related species include the chrysanthemum, daisy, sunflower and zinnia. They grow from small tubers planted in the spring (they are like a potato). Picking a favorite dahlia is almost impossible. As well as coming in a rainbow of colors, dahlia flowers can range in size from petite 2-inch lollipop-style pompoms to giant 15-inch “dinner plate” blooms. Most varieties grow 4 to 5 feet tall.
Most growers plant dahlias from tubers (which is what is sold on this site) that are an exact genetic copy of the plant it came from. Dahlias grown from seed are genetically unique- every seed from a plant is different, even within the same seed pod. There are thousands of varieties that have been grown and cultivated for years. I have started working on my own dahlia breeding to create unique cultivars for future growing!
WHEN TO PLANT DAHLIAS
- 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.)
- Some gardeners start tubers indoors in containers a month ahead to get a jump on the season. Medium to dwarf-size dahlias will do well in containers.
- Order dahlia tubers in early spring. This gives gardeners in colder zones time to get them growing in a sunny window. Or, skip the potting and simply plant the tubers in the ground after the spring weather has settled and the soil has warmed.
CHOOSING AND PREPARING A PLANTING SITE
- 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. The pH level of your soil should be 6.5-7.0, slightly acidic.
- If you have a heavier (clay) soil, add in sand, peat moss, or aged manure to lighten and loosen the soil texture for better drainage.
- 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!).
HOW TO PLANT DAHLIAS
- Each tuber you receive will have at least one “eye” (bud) or a little bit of green growth. Don’t break or cut individual dahlia tubers as you would potatoes.
- Bedding dahlias can be planted 9 to 12 inches apart. The smaller flowering types, which are usually about 3 feet tall, should be spaced 2 feet apart. The taller, larger-flowered dahlias should be spaced 3 feet apart. If you plant dahlias about 1 foot apart, they make a nice flowering hedge and will support each other.
- The planting hole should be slightly larger than the tuber 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.
- Dig a hole that’s about 4-6 inches deep. Set the tubers into it, with the growing points, or “eyes,” facing up, and cover with 2 to 3 inches of soil (some say 1 inch is adequate). As the stem sprouts, fill in with soil until it is at ground level.
- STAKE YOUR TUBER AT THE TIME OF PLANTING! If you wait to stake your plant, you risk damaging the tubers once they are growing. It also allows you to remember where you have planted.
- 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.
- Dahlias start blooming about 8 weeks after planting, starting in mid-July.
- Do not water the tubers right after planting; this encourages rot. Wait until the sprouts have appeared above the soil to water.
- Do not bother mulching the plants. The mulch harbors slugs and dahlias like the sun on their roots.
- Add slug bait around the plant once growth appears. Slugs & snails love dahlias and will be happy to eat your new growth.
- 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).
- 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.
- 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 blooms, weak tubers, or rot.
Pinching, Disbudding, and Staking
- When plants are about 1 foot tall, pinch out 3-4 inches of the growing center branch to encourage bushier plants and to increase stem count and stem length.
- If you want to grow large flowers try disbudding —removing the 2 smaller buds next to the central one in the flower cluster. This allows the plant to put all of its energy into fewer but considerably larger flowers.
- 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.
- 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.
- Deer: Find a list of deer-resistant plants to grow around your dahlias.
- Powdery Mildew: This commonly shows up in the fall. You can preventatively spray before this issue arises from late July to August.
Dahlias are beautiful in a vase. Plus, the more you cut them the more they will bloom. To gather flowers for a bouquet, cut the stems in the morning before the heat of the day and put them into a bucket with cool water. Remove bottom leaves from the stems and place the dahlias in a vase. Put the vase in a cool spot and change the water and trim stems daily. The bouquet should last about a 5-7 days in a cool environment.