Learn About Dahlias

How to Sow

Dahlia may be grown from seed sown early indoors and transplanted outside after frost, or sown directly in the garden after frost, or grown from potted plants or tubers.

Sowing Seed Indoors:

  • Sow indoors 8 weeks before last frost using a seed starting kit.
  • Sow seeds ¼ inch deep in seed starting soil
  • Keep the soil moist at 70-75 degrees
  • Seedlings emerge in 14-20 days
  • As soon as seedlings emerge, provide plenty of light on a sunny windowsill or grow seedlings 3-4 inches beneath fluorescent plant lights turned on 16 hours per day, off for 8 hours at night. Raise the lights as the plants grow taller. Incandescent bulbs will not work for this process because they will get too hot. Most plants require a dark period to grow, do not leave lights on for 24 hours.
  • Thin to one seedling per cell when they have two sets of leaves.
  • Seedlings do not need much fertilizer, feed when they are 3-4 weeks old using a starter solution (half strength of a complete indoor houseplant food) according to manufacturer’s directions.
  • Transplant hardened-off seedlings to the garden after the frost.
  • Before planting in the garden, seedling plants need to be “hardened off”. Accustom young plants to outdoor conditions by moving them to a sheltered place outside for a week. Be sure to protect them from wind and hot sun at first. If frost threatens at night, cover or bring containers indoors, then take them out again in the morning. This hardening off process toughens the plant’s cell structure and reduces transplant shock and scalding.

Sowing Directly in the Garden:

  • Direct sow after danger of frost has passed.
  • Select a location in full sun with good rich moist organic soil. The more sun dahlias receive, the more flowers you will get. In hot climates, a bit of shade during the hottest part of the day will protect plants from heat, however.
  • Prepare the bed by turning the soil under to a depth of 8 inches. Level with a rake to remove clumps of grass and stones.
  • Most plants respond well to soils amended with organic matter. Compost is a wonderful form of organic matter with a good balance of nutrients and an ideal pH level, it can be added to your planting area at any time. If compost is not available, top dress the soil after planting with 1-2 inches of organic mulch, which will begin to breakdown into compost. After the growing season, a soil test will indicate what soil amendments are needed for the following season.
  • Sow seed ¼ inch deep.
  • Firm soil lightly, water and keep evenly moist.
  • Seedlings emerge in 14-20 days.
  • Thin plants to 18-30 inches apart, depending on the variety, when seedlings are 1 inch high.

Planting Dahlia Tubers

  • Plant tubers when you receive them in spring.
  • Select a location in full sun with good rich moist organic soil. The more sun dahlias receive, the more flowers you will get. In hot climates, a bit of shade during the hottest part of the day will protect plants from heat, however.
  • Prepare the bed by turning the soil under to a depth of 8 inches. Level with a rake to remove clumps of grass and stones.
  • Most plants respond well to soils amended with organic matter. Compost is a wonderful form of organic matter with a good balance of nutrients and an ideal pH level, it can be added to your planting area at any time. If compost is not available, top dress the soil after planting with 1-2 inches of organic mulch, which will begin to breakdown into compost. After the growing season, a soil test will indicate what soil amendments are needed for the following season.
  • Dig a hole six inches deep and wide enough to accommodate the tuber. Place the whole dahlia on its side (do not cut up dahlia tubers).
  • Cover with 2 inches of soil. As the plant grows fill in the hole until it is even with the rest of the garden.
  • For larger varieties inset the stake you will use to support them the same time you plant the tuber to avoid damaging the roots.

Planting Potted Plants:

  • Select a location in full sun with good rich moist organic soil. The more sun dahlias receive, the more flowers you will get. In hot climates, a bit of shade during the hottest part of the day will protect plants from heat, however.
  • Prepare the bed by turning the soil under to a depth of 8 inches. Level with a rake to remove clumps of grass and stones.
  • Most plants respond well to soils amended with organic matter. Compost is a wonderful form of organic matter with a good balance of nutrients and an ideal pH level, it can be added to your planting area at any time. If compost is not available, top dress the soil after planting with 1-2 inches of organic mulch, which will begin to breakdown into compost. After the growing season, a soil test will indicate what soil amendments are needed for the following season.
  • Dig a hole for each plant large enough to amply accommodate the root ball.
  • Set level with the surrounding soil. Fill with soil to the top of the root ball. Press soil down firmly with your hand leaving a slight depression around the plant to hold water.
  • Water thoroughly, so that a puddle forms in the saucer you have created. This settles the plants in, drives out air pockets and results in good root-to-soil contact.
  • Use the plant tag as a location marker.
  • For larger varieties inset the stake you will use to support them the same time you plant the tuber to avoid damaging the roots.

How to Grow

  • Keep weeds under control during the growing season. Weeds compete with plants for water, space and nutrients, so control them by either cultivating often or use a mulch to prevent their seeds from germinating. 
  • Mulches also help retain soil moisture and maintain even soil temperatures. For annuals an organic mulch of shredded leaves lends a natural look to the bed and will improve the soil as it breaks down in time. Always keep mulches off a plant’s stems to prevent possible rot.
  • Keep plants well-watered during the growing season, especially during dry spells. Plants need about 1 inch of rain per week during the growing season. Use a rain gauge to check to see if you need to add water. It's best to water with a drip or trickle system that delivers water at low pressure at the soil level. If you water with overhead sprinklers, water early in the day so the foliage has time to dry off before evening, to minimize disease problems. Keep the soil moist but not saturated.
  • Until plants become established, some protection from extreme winds and direct, hot sunlight may be necessary. Good air movement is also important.
  • After new growth appears, a light fertilizer may be applied. Keep granular fertilizers away from the plant crown and foliage to avoid burn injury. Use low rates of a slow release fertilizer such as Flower-tone, as higher rates may encourage root rots.
  • Deadhead regularly to keep plants blooming.
  • Monitor for pests and diseases. Check with your local Cooperative Extension Service for pest controls recommended for your area.
  • In the north just after frost kills the foliage, cut back the foliage, dig up the clumps, remove the soil and allow to dry for at least an hour. Do not wash roots. Store them over the winter in paper bags filled with slightly moistened vermiculite in a cool but frost free, well-ventilated location. Check throughout the winter that they do not dry out. For larger flowers do not divide root clumps.
  • When flowering ceases in Zone 8 and south, cut the plants to 6 inches and mulch them. Dig and divide clumps every two to three years to keep them vigorous.

Growing tips

  • Compact bedding dahlias are ideal for container gardens and for edging beds. Use the border types to add height and color to gardens in late summer to fall. Dahlias also make beautiful cut flowers.
  • Gardeners who wish for fewer but larger flowers should disbud their plants.

Common Pests and Problems

Common Disease Problems

Aster Yellows: Plants are stunted, develop witch's brooms (excessive growth), petals turn green and become deformed. This virus-like condition is spread by leafhoppers. Burpee Recommends: Remove infected plants and control leafhoppers. Remove weeds in the area which serve as alternate hosts to the disease.

Botrytis: This fungus causes a grey mold on flowers, leaves, stems and buds. It thrives in cool wet weather conditions. Burpee Recommends: Remove affected plant parts, avoid watering at night and getting water on the plant when watering. Make sure plants have good air circulation. Contact your Cooperative Extension Service for fungicide recommendations.

Damping Off: This is one of the most common problems when starting plants from seed. The seedling emerges and appears healthy; then it suddenly wilts and dies for no obvious reason. Damping off is caused by a fungus that is active when there is abundant moisture and soils and air temperatures are above 68 degrees F. Typically, this indicates that the soil is too wet or contains high amounts of nitrogen fertilizer. Burpee Recommends: Keep seedlings moist but do not overwater; avoid over-fertilizing your seedlings; thin out seedlings to avoid overcrowding; make sure the plants are getting good air circulation; if you plant in containers, thoroughly wash them in soapy water and rinse in a ten per cent bleach solution after use.

Powdery Mildew: This fungus disease occurs on the top of the leaves in humid weather conditions. The leaves appear to have a whitish or greyish surface and may curl. Burpee Recommends: Avoid powdery mildew by providing good air circulation for the plants by good spacing and pruning. Contact your Cooperative Extension Service for fungicide recommendations.

Root Knot Nematodes: Microscopic worm-like pests that cause swellings (galls) to form on roots. Plants may wilt or appear stunted. This is a serious problem in many Southern states. Burpee Recommends: Do not plant into infested soil. Try planting ‘Nema-Gone’ marigolds around your plants.

Common Pest and Cultural Problems

Aphids: Greenish, red, black or peach colored sucking insects can spread disease as they feed on the undersides of leaves. They leave a sticky residue on foliage that attracts ants. Burpee Recommends: Introduce or attract natural predators into your garden such as lady beetles and wasps who feed on aphids. You can also wash them off with a strong spray, or use an insecticidal soap.

Cyclamen Mite: These mites damage plants by sucking juice from stems and leaves. They multiply rapidity in hot, dry weather. They can only be seen using a magnifying glass. Plants will look distorted and stunted, and may not bloom. Flowers will be distorted, streaked and blotched. Leaves can become cupped, curled, dwarfed and thickened. Burpee Recommends: Discard plants that are severely infested. Avoid working with infested plants. Keep plants watered in dry weather. For heavy infestations consult your Cooperative Extension Service for insecticide recommendations.

Leafhoppers: Leafhoppers cause injury to leaves and stunt growth. They also spread disease. Burpee Recommends: Remove plant debris. Use insecticidal soaps. Consult your Cooperative Extension Service for other insecticide recommendations.

Snails and Slugs: These pests leave large holes in the foliage or eat leaves entirely. They leave a slime trail, feed at night and are mostly a problem in damp weather. Burpee Recommends: Hand pick, at night if possible. You can try attracting the slugs to traps either using cornmeal or beer. For a beer trap, dig a hole in the ground and place a large cup or bowl into the hole; use something that has steep sides so that the slugs can’t crawl back out when they’re finished. Fill the bowl about ¾ of the way full with beer, and let it sit overnight. In the morning, the bowl should be full of drowned slugs that can be dumped out for the birds to eat. For a cornmeal trap, put a tablespoon or two of cornmeal in a jar and put it on its side near the plants. Slugs are attracted to the scent but they cannot digest it and it will kill them. You can also try placing a barrier around your plants of diatomaceous earth or even coffee grounds. They cannot crawl over these.

Spider Mites: These tiny spider-like pests are about the size of a grain of pepper. They may be red, black, brown or yellow. They suck on the plant juices removing chlorophyll and injecting toxins which cause white dots on the foliage. There is often webbing visible on the plant. They cause the foliage to turn yellow and become dry and stippled. They multiply quickly and thrive in dry conditions. Burpee Recommends: Spider mites may be controlled with a forceful spray every other day. Try hot pepper wax or insecticidal soap. Check with your Cooperative Extension Service for miticide recommendations.

FAQ

Do I need to deadhead my plants? Yes it is important to keep dahlias deadheaded to ensure continuous blooming.

How do I get the biggest flowers on my dinner plate dahlias? To get the largest dinner plate dahlias remove side buds and allow only one but per stem to develop. Do not divide the tubers, the larger the tubers the bigger the flowers.

What pollinators do dahlias attract to the garden? Dahlias attract butterflies and bees as well as other beneficial insects

Can I start dahlia tubers inside? Yes, start dahlias about 6 weeks before last frost. Plant in pots large enough to accommodate the tuber, use a good quality potting soil. Plant two inches deep. Keep the soil barely moist but not wet.

What is the difference between seed dahlias and tuberous dahlias? Seed dahlias tend to be smaller single daisy types that are great for borders and beds. Tuberous dahlias tend to have larger flowers, and they usually don’t come true from seed. They are propagated by the tubers.

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