According to the late Christopher Lloyd, whose Great Dixter garden featured multiple dahlia cultivars, dahlias are best in in late summer, when they are valuable in: “stiffening a garden that might easily dissolve into an amorphous froth of Michaelemas daises, and when the mellow quality of autumn sunlight best suits their warm colours.”
This is undoubtedly true, but I think there is even more to dahlias than that. They are, to my mind, one of the best all round garden plants.
Having been a stalwart of many a 1950s and 1960s garden, they gradually fell out of favour. But now, like Eames chairs and G-plan sideboards, these retro garden stars are back.
Dahlias originate from the mountainous areas of Central America and Mexico. There are about 30 species of dahlias and thousands of different cultivars.
They are tuberous perennial plants, grown in gardens for their showy flowers which last from midsummer to late autumn.
Most Dahlia cultivars have mid to dark green leaves, although some modern varieties have dark red-black foliage.
Here I’ll give you the full year round rundown on how to grow dahlias, from planting in spring, through summer care and over-wintering.
Dahlias are borderline hardy. Flowers and foliage will be cut back by the first frosts. But the tuberous root system can survive winter temperatures down to around -5° C, if conditions are not too wet and a thick layer of mulch is applied.
Where winter temperatures are colder, or where otherwise desired, the tubers should be lifted once the first frosts have hit.
The tubers should be cleaned up and, in the first instance, stored upside down so that water can drain away from the hollow stems. This lessens the risk of rotting of the tubers during storage.
Once the tubers have dried out, it is worth dusting them with an anti-fungal powder. They should then be stored in a dry growing medium or dry sand, preferably in a slatted wooden box to allow air to circulate, in a dry and dark place where temperatures don’t dip below freezing.
Stored dahlia tubers can be replanted once the risk of frost has receded in spring.
Dahlias prefer fertile humus–rich soil with good drainage in full sun. They should be fed with a nitrogen rich fertiliser in early summer to promote bushy leaf growth.
Once the flower buds start to appear, in midsummer, they should be fed with a high potash/potassium fertiliser as this promotes flower growth.
Most cultivars, other than the bedding varieties, will require staking. It is advisable to get a sturdy frame in place early in the growing season and to tie in stems as the plants grow out.
One of the great attributes of dahlias is that they can flower for a long period between midsummer and the end of autumn. However, the key to extended flowering lies in deadheading. See this separate article on deadheading dahlias for some important deadheading tactics.
Pests and diseases
Dahlias can be troubled by aphids, red spider mites, caterpillars, earwigs and slugs.
They can also be prone to powdery mildew and mosaic virus. Certainly in my experience, powdery mildew is the biggest potential problem and not one I’ve found very easy to deal with.
Conventional wisdom says that it occurs in hot and dry conditions where plants are tightly packed together. Therefore a good watering regime, mulching and wider spacing may help.
There is a bit of work in growing dahlias, but to my mind it is well worth it. Dahlias are large enough to add presence to a border or to stand alone as a featured plant. The multitude of different varieties offers countless different flower shapes and colours.
Best of all, dahlias provide us with a focus for the whole growing season. First, a sense of anticipation, as they gradually grow and bush out into their allotted space. Then, the grateful appreciation of their floral display and, finally, the careful attention we give to deadheading and pruning, as we seek to eke out their beauty for as long as the seasons will allow.
For a great selection of dahlias, check out what is on offer at DirectGardening.com – you’ll find quality plants at great prices.
Brickell C (ed), 1998, the Royal Horticultural Society A-Z Encyclopedia of Garden Plants
Dorling Kindersley, London.
Buczacki, S., & Harris, K. M. (2005). Pests, Diseases & Disorders of Garden Plants (Collins Complete Photo Guides) Harper Collins UK.
Lloyd, C. (2001) The Well-tempered Garden Wiedenfeld and Nicholson, London