Horticulture and Crop Science
2021 Coffey Rd., Columbus, Ohio 43210-1086
Growing Dahlias
HYG-1245-92
Jack Kerrigan
Dahlias are considered one of the most spectacular garden flowers. They are also thought to require a great deal of care. Both of these statements are true to some extent. There is a great variety of form in dahlias, from the showy dinner-plate size to the bright, little single ones. Dahlias require some special care, especially lifting for winter storage. However, with minimum care, beautiful dahlia blooms will grace your garden from July until frost. Dahlias should be planted in a sunny location. The soil should be rich and well-drained with a pH of about 6.5. Wait until all danger of spring frost is past before planting.
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. Dig the planting hole slightly larger than the root ball of the plant and incorporate some compost or sphagnum peat moss into the soil. Plant dahlias so the crowns are 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.
Dahlias tend to have shallow roots so be careful when cultivating. About the end of June, mulch plants with well-aged compost, aged manure, peat, or straw. Water plants regularly if rain is insufficient but avoid wetting the foliage. Dahlias require a rich soil; a commercial fertilizer low in nitrogen, such as a 5-20-20, worked into the soil a month before planting is good preparation. As plants grow, a monthly feeding will encourage large blooms. If using a granular fertilizer, water it in thoroughly after application. Do not fertilize dahlias after mid-August.
When the plants reach a height of three to four inches, pinch the terminal bud just above the second pair of leaves. This will produce two main stems. When flower buds are visible, begin disbudding. This will increase the bloom size of the leader bud and improve the flower stem formation. Flower buds come in three's. The central leader bud will produce the largest bloom. There will be a smaller flower bud on each side of the leader bud. Remove these side buds by pinching or cutting to the base of the leaf axil. Further down the same stem, two more buds will be found in the leaf axils and these, too, should be removed.
Flowers should be cut when fully opened. Early morning cutting will provide the longest lasting cut flowers. As soon as you cut the bloom, plunge the stem into water. Major pests of dahlias include aphids and slugs early in the growing season, and mites in mid- to late-summer. Thrips, earwigs, caterpillars, and wasps may occasionally attack dahlias. Consult your county Extension agent for recommended controls. Leafspot and dahlia wilt also can be problematic. Should the leaves yellow in a random pattern, submit a sample for diagnosis. A plant with a viral infection should be removed from the garden immediately. Also, destroy plants with abnormal or deformed crowns.
Lift dahlias after the first frost or before the end of October. Before lifting tubers, remove all ties. Stakes should be cleaned, repaired, and stored. Attach name labels to the base of the main stem of named cultivars. Cut the foliage off so that there remains a three to four inch stem on the roots. Because tubers may have spread quite a distance, begin digging far enough from the plant so as not to damage them. A spading fork or spade works well for this task. For easier lifting make several cuts into the soil with the fork or spade around the clump and gently pry to raise the tubers intact.
Once the tubers are out of the soil, remove as much soil as possible without damaging the tubers. Turn the tubers upside down to drain the stem and allow the soil to dry. When dry, remove the remainder of the soil. Soil may also be washed from the tubers with water under pressure. Cut off any small roots. Remove and discard tubers that are damaged or diseased, and dust any cut surfaces with sulfur. Place the tubers in wooden flats, bushel baskets, or cardboard boxes that have a few inches of peat moss, sawdust, or vermiculite in the bottom. Then cover them with the same packing material, leaving the stems exposed. A little moisture may be added to prevent shriveling. Place the packed tubers in a dry, 40 F to 55 degrees F location.
Check tubers in a few weeks and again in January for signs of shriveling or fungal infection. Add a small amount of moisture if shriveling has occurred. Do not expect 100 percent survival.
Bring the tubers out of storage in March or April and locate eyes on each tuber. With a sharp knife, divide the tubers with a portion of crown attached, so that each piece has an eye. If eyes are not evident, place the tubers in moist leaf mold, peat, or soilless mix. In a week or two the eyes will appear. Pot the divisions in a sterilized, soilless mix or porous potting soil with the crown above the potting medium. Provide the potted divisions with maximum sunlight and a temperature of about 55 degrees F. Water when the potting mix dries to a depth of one inch. Good ventilation will help prevent disease.
Cuttings may be taken from new growth but do not cut below the first set of leaves. Cuttings should be three to four inches long.
Dahlias may also be propagated from seed. This method is used most often for bedding dahlias. Because of cross-pollination, they will not come true to cultivar. Harvest seeds in September and October. Cut the ripened flower heads open and lay them out to dry on shallow trays. When dry, the seeds will readily separate form the chaff. Store them in sealed containers. Seeds can be sown indoors in February or March; germination takes about one week.
Source: Ohio State University Extension
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