Mendocino Coast Botanical Gardens, 47 acres of botanical bliss fronting the Pacific Ocean

Growing and caring for Dahlias

Dahlias are easy flowers to grow and will produce, relative to size, more flowers than any other plant. Native to the sunny foothills of Mexico and Guatemala, dahlias require open, sunny conditions. It is important to understand dahlias grow from tubers and are not bulbs. Bulbs, such as lilies, daffodils, tulips, can be planted in wet, cold soils in the fall. Dahlias must be planted in warm, well drained soils at spring time, about the same time you would plant your vegetable garden.

An area that receives at least 8 hours of direct sunlight is best. Hot climates are an exception to this as dahlias should be planted where they will receive morning sunlight, but not hot afternoon sun. No matter what climate you have, do not plant in areas that are shaded all day. If you do, the plants will be tall and spindly, blooms weak headed, and blooming sparse.

Plant between mid-April and late June. Place the tubers horizontally, with the sprout (eye) pointing upwards, 6 inches deep, 36 inches apart, and 2 inches in front of a stake. Place fine soil around and under the eye of the tuber and fill in the hole with about three inches of fine soil. Encourage upward growth by adding soil as the sprout grows. Allow only one sprout to grow from each tuber. Many people have excellent success with tomato cages instead of wood stakes. Do not water the tubers after planting as spring soil has enough moisture to promote proper growth and spring rains will provide enough water. Do not use bark dust for mulch or in the soil. It does not allow the sun to warm the soil properly and tubers will not sprout as they should. Bark dust also increases soil acidity, which is harmful to the dahlias.

Well drained is important; clay and sandy loam amended with steer manure, leaf compost, and organic material is best suited for dahlias. Prepare the beds a few weeks before planting. Steer manure or a commercial 5-10-10 fertilizer and a quarter cup of bone meal should be applied to the bed before planting. If you are planting in deck containers or pots, use garden soil, or a mixture of 2 parts garden soil and 1 part potting soil. Dahlias planted in 100% potting soil will dry out too often causing poor bud formation.

Do not water until the sprouts begin to appear above the ground. This gives the tubers a chance to form roots which can absorb water. Watering too early will increase the chance of rotting tubers. After plants are established, a deep watering once or twice a week is necessary during warmer, dryer weather or in hotter climates. Proper watering will promote better blooming. Over-watering to keep the soil damp in pots may result in rotting tubers.

A light application of well-balanced commercial fertilizer every three weeks is recommended. High nitrogen fertilizers such as composts, fish fertilizers, or other high nitrogen fertilizers are to be avoided as they promote weak stems, small blooms, or no blooms, and tubers that rot or shrivel in winter storage. It is recommend to use a high percentage potassium and phosphorous fertilizer such as 5-10-10, 10-20-20, or 0-20-20. It should be applied within 30 days of planting and repeated about 6 weeks later. Potassium and phosphorous break down slowly and will become available to your plants during peak blooming.

Pests and Diseases
Snails and slugs eat the tender shoots as they try to break through the ground in spring. Bait may be needed to prevent these pests from eating up the tender new shoots. Cucumber beetles feed on the flower centers. They can be trapped using yellow sticky traps or simply tap the flower over a bucket of soapy water. Gophers may gnaw on the tubers.

Pruning and Maintenance
To encourage lateral growth and increase the number of flowers produced by the plant, pinch out the top of the plant when the third or fourth set of leaves have appeared. To elicit fewer but larger flowers, pinch off all but the largest bud in each cluster. This practice is called disbudding. Peak blooming of dahlias occurs from late August through September.

After the first frost, cut down the tops (leaving a few inches of stem) and dig carefully with a spade or fork so as not to break the necks of the tubers. Remove most of the soil and mark the clump with some form of identification for the next year. Turn clumps upside down so that any moisture in the hollow stem will drain out. You may divide tubers at this time; they are easier to cut and the eyes are visible before the tuber dries out. When dry, tubers should be scrubbed, treated with a fungicide, and covered in peat moss, dry sawdust, medium sand or vermiculite to prevent shriveling. Store them in a cool, dry area until next spring. If you have well-drained, sandy soil you may carefully cut back the tops to about 6 inches above the ground and then dig and divide in March.

In the spring, when the tubers sprout, divide them with a sharp knife, into single tubers with one strong eye or sprout on each tuber and plant as per foregoing instructions.

Weeds may be kept down by mulching with leaves, bark or grass clippings. To promote shorter, bushier plants with better stems for cutting, pinch or cut out the center shoot when the plant is about 20 inches tall.

For more information on dahlias, see the website for the American Dahlia Society at