Lush, bright and beautiful, dahlias grow from tubers and bloom in the summer season, flowering until the frost. They originate in Mexico and South America and are hardy to USDA zone 8. In the north we grow them as annuals or dig them right after a frost.
Tubers come in all shapes and sizes, and all it takes is one "eye" on a tuber to grow. Tubers can be round like a potato, or long and skinny, or oval shaped, each variety can have differently shaped tubers. Often the dahlia tuber clump we are sending you will have more than one "eye" and will send out multiple growing shoots. Dahlias can be planted outside when the soil temperature is at least 60 degrees. Here in Connecticut, that is late May, about the same time tomatoes are planted outdoors. This is important, because planting in cold soil can cause your dahlia tubers to rot.
Starting Dahlias Indoors
Gardeners in colder areas can give dahlias a head start and get them to bloom sooner by planting the tubers in a pot indoors in a warm sunny window at 65-75F. Use a 5-6" pot for each dahlia clump. Fill your pot with a good quality, free-draining potting soil. We like to soak the tuber in water for 1 hour before planting to hydrate the root; this makes them activate and grow sooner. Plant your tuber on its side, with the stem end pointing upward, under 1-2 inches of barely moist soil. You will see last year's stem cut back, this is the top of your tuber. Don't worry, dahlias are very adaptable and laid on their side, under a little soil, they will find the right way to grow. Be careful about watering. If the soil is just a little bit moist, wait to water until it begins to grow. Once the shoots are growing, water as needed to keep them from wilting. Plant them out in the garden when the soil is warm and the dahlia is sturdy, or transplant to a larger patio pot, (5 gallon size) for outdoor growing. Be sure to harden off the plants before you transition them to outdoors, the leaves can burn if not accustomed to bright outdoor sun.
Soil and Sunlight
Dahlias thrive with 6-8 hours or more of sun in the North, and in the South, plant where you have afternoon shade. They are best in a sunny area with well drained, fertile soil that has been amended with compost. Slightly acidic soil is fine, pH of 6.5 to 7.0. Dahlias can share a sunny bed with herbaceous or itoh peonies and bring you summer color that lasts to the first frost.
Planting Directly Outdoors
When your soil is 60F, you can plant dahlias directly in the garden. The tubers will start to show small bumps or sprouts which may look like white or green buds erupting from the center stalk of the light brown tuber. Lay the tuber on its side, with the sprouts pointing upward. If planting directly in the warm garden bed, dig a hole about 6 inches deep in well worked soil and cover 4-6" of soil. It is recommended to put a sturdy stake in with the initial planting, so you won’t damage the root if you stake later in the season. If you use mulch, keep it away from the crown and stems. Dahlias are sensitive to fungus, so let them breathe. Space dahlias 18-24" apart. If planted too close there is a greater chance for disease issues.
Watering and Fertilizing
Planting in a hot dry climate is different from northern growing. Dahlias in hot climates will need water for the unsprouted tuber. Water lightly once or twice a week, you do not want to dry out the young tuber as it is getting started. Once growth is above ground, you will need to water to keep some moisture in the soil. Watering depends on the temperature and how fast the soil dries out, so you will have to pay attention.
Planted in the north, be spare on watering until the new growth appears above the ground. Excess water will decay the roots. Once spring temperatures warm up, the green growth will accelerate. If no rain is in sight, you will need to water June-September. It may not be every week, it all depends on the weather.
Once green growth appears, we have found dahlias love the Neptune's Harvest fish/seaweed fertilizer or North Country Organics Pro-Gro granular fertilizer every 4 weeks through the summer. We prefer organic products to feed the soil and the plants for everything we grow at Cricket Hill Garden.
Pinching, Pruning and Staking
The minimum you need to do is stake the main stems of your dahlias. Stems are brittle and will break in the wind and rain. Flowers are not as attractive left sprawling on the ground, and will decay faster. So for best results, get them staked at planting time. It is very hard to stake them when they get overgrown and tangled. A very large plant might need multiple stakes to tame and support the growth. Soft jute twine is fine to tie up the stems. Tie securely but loosely, so not to cut into the stem.
To get longer stems and a bushier plant, pinch out the center shoot when the dahlia has its third set of leaves. Pinching will create more stems and flowers for cutting and is recommended. We have grown dahlias with just a basic staking and have been thrilled with the flowers. If you want to maximize the flower size, prune out the side buds to make the main flower a true exhibition size. The large dinnerplate dahlias get to be 7-9 inches with no pinching, so that's already impressive! It's up to you to decide how fussy you wish to be in tending them. Remove spent flowers as they fade each week of summer and your plant will put more energy into making new buds. Deadhead dahlias and they will keep blooming to the frost. Here in Connecticut we have flowers until the October frost.
Cutting Dahlia Flowers
Choose flowers for cutting early in the day before it is hot. Check the back of the petals for fresh flowers, those that show wilting or dryness are too old for cutting and will not last in a vase. It's important to cut them when they are almost fully open, as the buds do not open much after cutting. Place them immediately in water and keep them cool and let them fully hydrate before arranging. Use a floral preservative to extend the life of the cut flowers.
Pests and Diseases
Watch out for aphids and spider mites in summer. These can be sprayed off with a steady stream of water or use insecticidal soap. We have not had much of a problem with this. Depending on your garden, slugs can be an issue, so watch for signs of damage. Powdery mildew can occur in a wet summer, showing as white powdery coating on the leaves. If addressed early you can arrest the growth of the mildew with a sulfur spray or try the Monterey Complete Disease Control.
When the Season is Over
The frost will blacken the foliage, but wait a few days to dig the dahlia tubers. If they have happily grown all season, you will triple the size of the tubers and can keep them for the next spring. Cut them back to 4-5 inches of stem, dig carefully, and gently shake off the soil. If needed, wash off excess soil. Allow to dry for a day or two before storing. We have had good results by storing in cardboard boxes with barely moist sawdust, sand or peat moss. Keep boxes cool and dark at 40-50 degrees all winter. Do not let them freeze. Check for dryness and shriveling every month in winter. If you see they are dry, water the packing material very lightly. This is the simplest method for storing, but if you have many tubers, you may want to further trim and store only selected tubers. See the website of the American Dahlia Society for more information.
Cricket HIll Garden www.crickethillgarden.com 860-283-1042