Dahlias are simply brilliant. Anyone who has grown dahlias will tell you that they start flowering mid summer and continue until the first frost. Varieties are available in all colors and shapes, far beyond what can be achieved in the rest of your garden. Dahlias are a horticultural import from Mexico that acclimatized to the British climate in the last couple of centuries. Our winters do marginally increase the amount of care dahlias require, and although that slightly discourages some gardeners, it is a small price to pay for their stunning appearance.
Before we go any further, we must set the record straight: dahlias do not grow from bulbs. While promoting dahlias, many people evoke the tropes of bulbs, and assuming dahlias grow from bulbs. Dahlias grow from tubers, which are rounded ponytails of multiple thin, brown, and scaly “carrots.” These tubers are the food storage structures for dahlias and provide all the nutrition dahlias need to grow for an entire season.
Purchasing Tubers
From late winter, tubers can be purchased. Popular varieties quickly sell out, so buying earlier is better. Shrivelled tubers sell out, so bought tubers should be firm. Tubers should be plump and should have eyes. These are found at the old stems which are the points from which new shoots will sprout. If you want, dahlias can be purchased from the winter for spring delivery at the Bloms Bulbs website. If you order from here, there’s a better variety of dahlias, rather than what is left at the local garden center.
Starting Them Off
In Britain, tubers are best started in pots. March and April are better months to do this, rather than planting directly in the cold ground. Late frosts will kill the young shoots, and while the tuber will survive, it sets the plant back for weeks. Plant the tuber in multipurpose compost with the old stem just showing. Water them sparingly until new shoots appear. Grow the tubers in a light and frost free area until the end of May.
Planting Out
Dahlias should be grown in areas with a lot of sun. They will sulk if there is insufficient sun. Plant dahlias in areas with as much sun as possible. Dahlias prefer soil that has good drainage. Heavy clay soil can be improved by adding a lot of compost or well rotted manure. Dahlias will use this later in the season, too.
Most regions have their last frost in late May or the first week of June. Dahlia tubers can be planted 15cm deep and spaced about 45cm apart, or more if they are larger varieties, as dahlias spread. Dahlia tubers also take up space, as they grow quickly, and fully grown dahlias can be quite large. It is better to plant stakes with the tubers rather than waiting, as stakes must be dangerously forced into the ground to be set, and stakes should be planted to support blooming dahlias. Supine dahlias are a sad sight.
The Rewarding Dahlia
Complete fertilizing to dahlias late in the season for rapid blooming extensions. Fertilizer should be distributed more generously as flowering approaches. Potash should be deep watered once or twice each week. A mulch of fresh compost helps keep moisture and suppress weeds.
Dahlia flowering can be stimulated by pruning blooms. Give wilted or damaged blooms a pinch to rupture the defense in the center of the wilted bloom. It is easier to distinguish wilted from unopened blooms by shape, conical for wilted and globular for unopened.
Slugs and earwigs
Snails and slugs love dahlia shoots and can strip a plant in a single night. Barriers, traps, and patrols work to protect sprouts during spring. Once dahlia shoots grow strong, slugs leave. Earwigs eat dahlia petals. A good, upturned flower pot, stuffed with straw, and standing on a can can helps trap earwigs along with daily check and clear. Infestations of virusing, spreading aphids, show signatures of streaks and distortion on a dahlia and are best to be dug and removed.
What to do in winter
In autumn, the ultimate question of dahlia care, is do you dig the tubers, or do you leave in the ground? The answer is: it depends. In gardens with milder climates, dahlia tubers can be left in under a dry mulch. Since the wet is what usually kills, and not the cold, it can be safe to leave dahlia tubers. However, in the colder areas, or heavy, in the ground soil, it is best to dig the tubers.
Wait for frost to blacken the foliage before lifting. Cut the stems to about 15 cm. Spray around the edges to avoid bruising the tubers as you lift the clump. Dry the clumps upside down for a week or two. Store frost free in barely damp compost for box and check monthly for rot. Divide the stored clumps in spring as the eyes begin to swell. Each piece requires a tuber and an eye to promote growth. A piece of old stem is also necessary for the division.
Selecting Cultivars
There’s so much to choose from when selection dahlia varieties that it can be a little chaotic. There are dahlia varieties that range in a height from 30 centimeters to 2 meters, in bloom size from a neat pompom to a dahlia that has blooms the size of dinner plates, as well as dahlia varieties that bloom in single flowers to those that are fully double and much more. Here are a few that are good to start with.
- An old favourite that bees love, Bishop of Llandaff, has a single red flower that blooms from dark bronze foliage
- Café au Lait’s blooms are large and soft in a creamy color that are highly requested for weddings
- Art Deco blooms are very happy in a large pot
- Round and rich dark purple, ‘Thomas Edison’ blooms glow in the fading light
Dahlias can, and are, used as cut flowers, and the more blooms that are cut, the more that will be produced. A row of dahlias can fill a home with cut flower blooms from July until the frost. Most people will tell you that 2 dahlia varieties will easily become 7, and that is how the hobby starts.