This Month in the Garden: Plant Dahlias During Springtime for Late-Summer Blooms

This Month in the Garden Dahlia Planting & Care
It’s time for another “This Month in the Garden” and it’s Dahlia planting season! In colder climates such as North America, Dahlias are treated as tender perennials or annuals, which are mainly planted as tuberous roots during springtime (April or May). Dahlias add beautiful blooms to the garden from mid-summer through fall, all the way through the first frost, with a growing season of approximately 120 days. These beautiful perennials come in a multitude of colors and sizes from the giant 10-inch blooms of the “Dinnerplate” variety to the smaller 2-inch “Pompon” style, rounded blooms. On average, most varieties of Dahlia grow to a height between 3 and 5 feet tall and are winter hardy in USDA zones 8-11, where they can stay in the ground. Dahlia do tend to thrive best in cool, moist areas, such as the northeastern U.S., but have been known to be grown in warmer climates such as Florida and Texas when sheltered from the extreme heat.

VARIETIES OF DAHLIA: There are thousands of Dahlia varieties that exist today, all which were derived from the original 35 genus of Dahlia found in the highlands of Mexico and Central America. Today, there are approximately 42 species existing, 25 which came from the wild. Basic cultivars of Dahlia available include the larger “Decorative” Dahlia and “Dinnerplate” Dahlia, “Pompon”(rounded) Dahlia, Cactus and Semi-Cactus flowered dahlias (spiky), Waterlily dahlias, Peony-flowered dahlias and Daisy dahlias, with new color combinations introduced every year. Since there are more than 57,000 registered cultivars of Dahlia to choose from, I would recommend purchasing your selections through a reliable website or nursery that has photographs and descriptions of each plant. In the meantime, here is some important information on the planting and care of these gorgeous cultivars.
Cactus Dahlia
WHEN AND HOW TO PLANT DAHLIA: In cooler climates, plant Dahlia tuberous roots when ground temperatures reach 60 degrees Fahrenheit and all danger of frost is gone. As a rule of thumb, schedule planting around the same time, of just after, tomato seedlings are able to go outside. Choose an area of full sun (6-8 hours of direct sunlight daily) in a rich, but well-drained soil with a pH of 6.5-7.0 (slightly acidic). Add sand or peat moss to a heavier soil to allow for better drainage and air circulation. Be sure to shelter plants slightly by placing them in a location that has protection from the wind. Another tip is that Dahlia do well with morning sunlight.
Semi-Cactus Dahlia
PLANTING DEPTH: When planting Dahlia, place tuberous roots approximately 9 to 12 inches apart either in the ground or in a planter. Varieties growing more than three feet tall should be spaced two feet apart and larger, taller growing varieties should be placed three feet apart. Generally, the taller and larger the blooms, the more space they need to grow. There is a smaller Bedding variety which is ideal for planters. Inspect your Dahlia for any wrinkles or rotting of the tuberous roots and plant only those that look healthy and that may have signs of growth. Dig the hole wider and deeper than the root and place some compost and a little bone meal into the planting hole to ensure a good start. Position the root about six to eight inches deep with the “eyes” (growing surface) facing up. It is recommended not to mulch over your newly planted Dahlias or water too heavily until growth starts to sprout.

Waterlily Dahlia
GENERAL CARE: Once your Dahlias are established water two to three times a week for approximately 30-40 minutes at a time and adjust according to climate. Dahlias benefit from an application of a low-nitrogen liquid fertilizer such as a 5-10-10 or 10-20-20. Fertilize after sprouting and then every 3 to 4 weeks from mid-summer until early autumn, but do not over-fertilize. As Dahlias get taller, some of the larger varieties may require staking to keep blooms from falling over. Bedding Dahlias generally do not require any kind of staking if you deadhead any spent blooms and pinch back growth to keep them full.
Pompon Dahlia
PESTS & DISEASES: Dahlia are generally free of pests, but should be regularly monitored for any signs of distress. They can be a target of insects such as Japanese Beetle, slugs, snails, mites and aphids. Diseases include powdery mildew, verticillium wilt and Dahlia smut, which mostly result from over-watering of the foliage. Some growers recommend being proactive and tend to use a slug repellent at time of planting to prevent any issues from ground insects as the plants are trying to develop.
Decorative Dahlia
STORAGE OF DAHLIA: In warmer climates (zone 8 and above) Dahlias can be cut back and left in the ground to overwinter. If doing so, cover the location of the tuberous roots with a 2-3 inch layer of dry mulch for protection. Elsewhere, (zones 7 and under) the tuberous roots should be lifted and stored during the winter. Remove any remaining foliage before digging them up and carefully remove from the soil, not to break the root. Once dug, gently shake off any soil and pack the dry tuberous roots in a loose fluffy medium such as vermiculite. Store them over winter in a well-ventilated frost-free space, such as a garage, and start over in spring.

Ball Dahlia

Dahlia Classifications (Source: Nineteenth Supplement to the International Register of Dahlia Names 2007)

For more Information on Dahlia Varieties visit:

As Always…Happy Gardening and Happy New Year!

Author: [email protected] Guide to Northeastern Gardening, © Copyright 2018. All rights reserved