How to Grow Dahlias from Seed in 7 Easy Steps
Dahlias charm in summer with diverse flower forms in a spectrum of eye-catching colors. Starting the beloved bloomers from seed lets us grow them in numbers and creates an opportunity for surprise as bees craft new selections. Garden expert Katherine Rowe explores how to grow dahlias from seed for bright surprises in the garden and the vase.
Contents
Dahlias are a long-time favorite with exceptional bloom forms and a lasting season of color. They delight in the ornamental border, container, and fresh floral arrangement. They also draw pollinators and other beneficial insects with their large, showy flowers, especially those with open centers and easy access to nectar and pollen.
Dahlias create the mood by adding drama, elegance, and whimsy depending on the flower form. They range in size from tiny pompons to towering dinner plates. They perform well as annuals, growing quickly in a single season. Pair them with geranium, agastache, salvia, asters, and coneflower for an extended display from summer to fall.
Growing dahlias from seed lets us select unique varieties and sow them in large numbers economically. Seeding also gives the opportunity for a totally new plant, unique to your garden, as bees pollinate them on their garden travels. Because hybrids don’t grow true to type from seed, the offspring won’t necessarily reflect the parent plant.
You’ll come away with an exciting combination and surprise bloom. If it’s one you love, store tubers over the winter for the next growing season.
Growing Dahlias From Seed
To get a direct clone of your favorite dahlias, stick with tubers and cuttings. But for a novel, relatively easy, and economical way to grow a number of plants, seeds are a great option. You can order seeds from reputable retailers or collect your own at season’s end. When flowers fade, deadhead and let them dry. In late summer and early fall, let some dry on the stem for clipping after frost. Then, crack open the pods for seed collection and winter storage.
Start dahlia seeds indoors to promote development before transplanting after the final frost. A seed tray or suitable low, flat container with drainage holes makes for handy seeding. Disinfect previously used seed-starting containers with a bath in soapy water, followed by a dip in a bleach or alcohol solution. Sanitizing supplies helps ward off lingering fungal spores and pests.
For a bleach solution, use one part bleach to nine parts water. Give a rinse with clean water following the dip. To use rubbing alcohol, use a spray bottle or cloth with isopropyl alcohol (70%). There’s no need for rinsing or water dilution with this treatment.
Use a moistened seed-starting mix or regular potting mix to fill trays. Use one free of added fertilizers. Soil blocks are an innovative way to create self-contained “pots” of the soil medium to house developing seedlings. They make for easy transplanting from tray to pot for dahlias.
Plant Selection
The American Dahlia Society classifies dahlias into groups like decorative, ball, orchid, and cactus, all based on petals and shapes of intricate symmetry. Their nearly perfect petal arrangements create a beyond-imagination array of species and varieties. Over forty recognized species exist, each with architectural blooms for outstanding beauty.
Most dahlias are available as tubers (fleshy, starchy root structures with bud eyes ready to grow). Seeds will take longer to develop but form small tubers over the growing season for future collection. Experiment with different seeds from online or local retailers to find your favorites. Smaller, more compact varieties are quick to develop with versatile garden placement and no need for staking.
Dahlias are toxic to dogs and cats, so keep them out of range of curious pets.
Choose the Growing Location
Dahlias prefer climates with cool nighttime temperatures and warm days. On average, 55-60°F (13-16°C) night-time and 70-80°F (21-27°C) daytime temperature are ideal. When choosing your planting site, opt for one with rich, well-draining soils. Sun-loving dahlias thrive with at least six hours of sunlight daily. In hot climates or intense summer sun, offer afternoon shade protection. Dahlias tolerate partial shade, but too much leads to a lack of flowering and vigor.
Ideal soils are loose and crumbly, with good aeration and moisture retention. Amend the soil with compost, leaf mold, aged manure, or pine bark at planting. Mix these with ⅔ native soil. This is especially important in poor soils like sand or clay. Compost helps with aeration, moisture retention, drainage, and nutrition.
In addition to bedding features, dahlias make gorgeous container specimens. Use a high-quality potting mix for good drainage, aeration, and moisture retention. Ensure the pot has ample drainage holes and place it in a sunny spot protected from intense afternoon sun in hot climates.
Tall stems are susceptible to wind damage. Shelter them from windy zones when feasible. Staking helps stabilize tall bloomers.
Start Dahlia Seeds Indoors
Start dahlias from seed indoors four to eight weeks before spring’s anticipated final frost. For gardeners with a long growing season, direct sow them after frost passes for later blooms. Starting them indoors gives seedlings a headstart on development, with strong roots and leafy stems for faster growth and blooming after transplanting.
To start dahlia seeds indoors, fill a tray with potting mix. Scatter the seeds on top of the media and lightly cover with no more than 1/2 inch of soil. Moisten thoroughly after sowing and keep the media moist throughout the germination process (soil should not be wet or soggy).
Place trays in a warm, bright spot. Temperatures between 65-70°F (18-24°C), or a heat mat at that setting, promote sprouting. Dahlias tend to germinate well in slightly cooler conditions than some other summer bloomers. Expect to see sprouts in 7 to 10 days or within two weeks as they spring up intermittently.
Tending Seedlings
Once sprouting occurs, move the tray to a sunny spot in the same temperature range. Or, employ grow lights to indicate longer day-length periods for growth. Keep the developing roots evenly moist.
When seedlings reach a few inches tall, pot them up to larger cell packs or four-inch pots. Let them continue to develop for about four weeks before transplanting.
Hardening Off
Tender seedlings benefit from gradual acclimation from the protection of indoor growing situations to garden conditions. Before transplanting the young plants, place them in a protected outdoor area for a few days to a week. Slowly move them to conditions mirroring their new garden location. Bring them inside on chilly nights.
Protect them from strong winds, cold temperatures, and afternoon sun in the process. Hardening off sets plants up for success at transplanting.
Transplanting
Dahlia seedlings are frost-sensitive and tender against cold conditions. Wait to transplant them until a few weeks after the final frost, when temperatures are consistently warming. The American Dahlia Society recommends treating them as you would a tomato or other heat-loving selection.
Watch for slugs and snails who prefer tender new shoots. Hand-pick them early in the day or evening when they’re most active. If numbers increase, bait them with beer or soda traps by filling a shallow dish and placing it at soil level.
Irrigate and Fertilize
Dahlias need consistent moisture and regular water throughout the growing season, usually once or twice weekly. However, they won’t tolerate wet feet. Ensure good drainage and water deeply. Increase frequency during dry spells. Containers dry out more quickly than in-ground plantings, so check water needs regularly.
In the ground, dahlias benefit from surface watering like drip irrigation or soaker hoses. Avoiding overhead saturation helps prevent foliar diseases. Watering deeply rather than frequently staves off slugs and other pests.
Dahlias grow quickly and use a lot of energy to produce their outstanding blooms. Nutrient-rich soils ensure health and vigor all season long. To boost nutrition, use an all-purpose organic fertilizer shortly after planting and then monthly until mid-August, when growth slows. A 5-10-10 or 5-20-20 ratio helps foster blooming without sending too much nitrogen into leaf and stem production. A 1-1-1 or 5-5-5 are other balanced, low-grade options.
If you opt to fertilize, begin applications three weeks after planting the seedlings. A slow-release granular minimizes monthly feedings and lasts about two months. They disperse nutrients gradually as granules degrade. For slow-release, apply the few weeks after installation and again in mid-summer.
Staking, Pinching, and Deadheading
Many dahlias require staking for their tall stalks and heavy blooms that weigh down hollow stems. Install stakes at planting time or before plants are over one foot tall. Tie stems to the stakes with soft, natural twine and continue tying them off as they grow. Twine or netting between stakes around a group is useful if you’re growing a number of dahlias in the same area.
Dahlias benefit from light pruning and pinching as they grow to encourage side branching. Removing the growth tip from young plants promotes a bushier form with a leafier, less leggy habit. It also creates more stems for more buds and blooms. Pinch off the growing tip when plants reach 12 inches tall with three or four pairs of leaves.
Regular deadheading promotes flowering, with less energy going into seed production. Clip dahlias for cut flower arrangements anytime, enjoying the beautiful buds. Cut the blooms when they’re fully open for a lasting floral show. Leave some blooms on the stems in late summer to develop seeds for collecting.
In zones 7-10, dahlia tubers overwinter outside, though you may choose to dig tubers to store them inside rather than risk a cold winter. Dahlias from seed will have small clumps of tubers. Dig and store them together in a cool space for the next growing season.
