Summer Care for Dahlias

Credit: Arthur Chapman

How to care for big, bold bloom from your summer dahlias

There are few people, even non-gardeners, who aren’t familiar with the glorious flowers of dahlias. Easily the most popular summer-blooming flowers among commercial growers and gardeners alike, their beauty, vigour and easy-care nature make them a great value for the gardening dollar.

While I emphasize easy-care, there are definitely a few things about dahlias you should know. My first experience with them was many years ago in the small garden of my first home. Like so many new dahlia growers, my first mistake was not providing these plants with enough room to grow without smothering all other life forms in their general vicinity.

It’s important to know that the more than 2,000 varieties of dahlias available mature to a height of anywhere from 30 centimetres to three metres. And as a general rule most dahlia varieties will grow to a width of at least 50 per cent of their mature height. So, when selecting dahlias, be sure to check out the height and width to which they will grow so that you can ensure accurate placement in your flowerbeds. Tip: Keep each dahlia bulb in its original package until you’re ready to plant it. As tubers, nearly all dahlias look the same.

Dahlias are actually quite low-maintenance and easy to grow. With the exception of some of the smaller border varieties, they’re best grown in full sun directly out in your flowerbeds. Smaller varieties may be grown in containers.

Taller-growing dahlias look exceptional toward the back of a flowerbed or up against a south-facing fence or wall, where they add depth and colour. The flowers themselves tend to face the sun, so planting them in full sun, but in a northern or eastern area of your yard, will assure you the greatest visibility of blossoms from your house.

Get Growing
Although dahlia tubers are generally planted earlier in the season, there is still time in late May and early June to get them going.

When planting tubers, I normally like to dig the hole to at least twice the planting depth (approximately 45 centimetres) and fill the base of the planting hole with a combination of compost, mushroom manure and peat moss. The planting site itself should be well drained, but the planting medium must retain a certain amount of moisture to support the massive amount of foliage produced by most dahlia varieties. I also recommend mixing 500 grams of a low-nitrogen, slow-release granular fertilizer (such as 8-20-20) into two cubic feet of the planting medium – that’s about two cups of fertilizer mixed into a small wheelbarrow filled with soil.

No matter how you care for your dahlias throughout the growing season – good or bad – you’ll likely experience success. One procedure that I definitely recommend is the removal of a few of the young shoots shortly after they have emerged from the soil by pinching them out with thumb and forefinger. Limiting the quantity of shoots a dahlia tuber produces will maximize the quantity of blooms from each plant. Leaving only five to 10 emerging shoots will actually ensure larger blooms as well as a more plentiful bloom count by season’s end. This simple task can be extremely worthwhile and rewarding.

If your preference is to create a full and bushy plant, you can do so by pinching off just the tips of those remaining five to 10 emerging shoots after they’ve grown to a height of approximately 20 to 25 centimetres.

One other maintenance issue that is integral to growing dahlias is staking the plants. Gardeners sometimes use a tomato cage as a support system. This works well for medium-height varieties, but for taller-growing varieties you should combine the tomato cage with a number of 1.8-metre wooden or bamboo garden stakes to which you can affix the major dahlia stalks. For best results and the least potential for tuber damage, position your support structure of stakes and/or tomato cage immediately after planting each tuber.

Disbudding to Optimize Blooms
As the season progresses and your ultimate goal of large blossoms looks imminent, you may want to remove a few of those flower buds. Three flower buds will develop at each leaf axil; periodically pinching out one or two of the three buds will encourage the remaining flowers to be larger, with longer stems. This process is known as disbudding, and is often practiced by exhibition growers.

Dahlias, like all plants, can experience the occasional pest problem. Aphids favour dahlias, however, if an infestation is noticed early enough it can generally be controlled by washing away the insects using a steady stream of water from a garden hose. Weevils, too, sometimes leave telltale holes in dahlia leaves. Unless the problem is severe, I generally let them be.

Dahlias should be given a deep watering once a week – approximately half an hour of water application will do it if the plants are rooted in a good soil mix. They also benefit from a good feeding every two weeks using a water-soluble flowering-plant fertilizer mixed at the recommended strength.

As your dahlia specimens bloom, be sure to deadhead, as this will encourage additional blooming from the plant. As an added bonus, cutting blossoms for indoor enjoyment will further force bud set. So make a point of savouring a few of those large, luscious dahlia blooms indoors in a vase; they create a wonderful bouquet and are relatively long-lived as cut flowers. Enjoy, and remember: Protect the earth and compost year-round!