Dazzling Dahlias

Credit: International Bloembollen Centrum Hillegom, Holland

Without doubt, the colourful fireworks of a summer border are dahlias, some of which really look like giant starbursts.

The original 30 species of this genus are native to the mountains of Mexico through Colombia. They are named for Swedish botanist Andreas Dahl (1751 – 1789), who was a student of Linnaeus. They first arrived in Europe in 1789. The Aztec people consumed the fat, succulent tubers for their medicinal properties (in fact they are still eaten by the Tunebo nation of Colombia), and it was thought they might be a new and exciting food crop for Europe. Trials were carried out at the Jardin des Plantes in Paris, but the European palate found the tubers bitter and nondescript.

The Empress Josephine, however, recognized the beauty of the flowers and had dahlias widely cultivated in her world-famous gardens at Malmaison. Their popularity has spread throughout the world and now there is a staggering count of over 20,000 cultivars.

Some avid plantspeople still like to grow some of the wild species. One of these is Dahlia merckii, which grows to 2 m (6 ft.) and is topped by white to purple flowers. It makes a bold statement in the back or middle of a border. But be warned: a friend who grows it in her garden on Bowen Island finds that it freely seeds itself about and can be a bit invasive, particularly in the hot dry summers we have experienced lately.

Dahlias are great performers in the garden, but they do have some particular requirements. Lifting the tubers, staking the plants and giving them lots of space and sunshine are some of these, depending on the type.

Dahlias are perennial in their native habitat but tender in our B.C. climate, so they are treated as summer bedding plants: the tubers are lifted in fall, stored and replanted each spring.

Dahlia tubers resemble elongated potatoes, but they do not have dormant buds distributed all over them as a potato does. In a dahlia, the growth bud is right at the top, just next to where last year’s stem was. When the first frost of late fall has blackened all the top growth, cut it back to about 15 cm (6 in.). Carefully lift the tubers with a wide-tined fork. If the clumps are quite large, this is the time to divide them as the stems are softer and easier to cut while they are still green. Use a very sharp knife to carefully cut down through the stem, splitting it in half with a few tubers attached to each portion. Thoroughly wash the tubers with water and set them aside on a shelf with good air circulation for a day or two. Once they are dry, store in boxes of peat in a cool, dry, frost-free place for the winter, an ideal temperature being 4°C (40°F). Try to remember to attach labels to each set of tubers so you will be able to identify the colours next season.

When it comes to the division of dahlia tubers there seems to be quite a passionate debate as to when they should be separated. And like in so many aspects of gardening, there really are no rules. My choice is based on my horticultural training where we always split in the fall. However, if you prefer to wait until spring, that is okay, too.

You can also propagate dahlias from cuttings. When I was an apprentice gardener in the UK some 40-odd years ago, we grew a lot of dahlias. To help build up stock of the rarer cultivars, we’d take softwood cuttings of the new shoots when they were just over 20 cm (8 in.) in length. We made a clean cut just below the leaf joint and removed the basal pair of leaves, leaving a cutting of just under 15 cm (6 in.) long. We set them in a mixture of 50/50 peat and sand in a heated propagating frame where they quickly rooted. Cutting out the tops also encouraged the plants to send out many more side branches, resulting in fuller plants.

Because dahlias like heat to get them started, I plant the tubers in pots around the middle of April. Put the pots in a cool greenhouse if you have one, but a table near a window in a spare room of the house will work just fine. Getting a jump on the season will give you some nice plants to bed out in late May.

Choose a sunny open position for your dahlias (they are definitely not good for shade). They are very heavy feeders, so soil rich in organic matter is a must. At planting time, put a good shovelful of well-rotted manure or compost in the base of each planting hole and mix it in thoroughly.

Dahlias are vigorous growers, so in a mixed border, allow just under a metre (30 in.) in diameter so they can develop freely. There are, of course, dwarf cultivars, but the big, showy ones need lots of space. Many people grow dahlias for show and set aside a separate area for this purpose. Once they are established and are growing happily they will need to be supported by stakes and preferably mulched to help keep the soil moist.

My favourite dahlia is ‘Bishop of Llandaff’. I grow him in a pot on my deck but feel he would be much happier in a garden. It is a smaller-flowered, almost single dahlia producing rich, dark-red flowers with a scattering of yellow florets around the centre. What makes it even more dramatic is the dark, blackish-red foliage. In August and September it is very showy. While not always easy to find in garden centres, a close runner-up to ‘Bishop of Llandaff’ is another single red with dark foliage called ‘Rood’. It doesn’t, however, have the tiny yellow petals in the centre.

Next in line for me are the ball or pompon types, and one that looks good enough to eat is ‘Highflyer’. It starts out as a deep-pink bud and develops into a superb bloom with outer petals of the palest pink and a deeper pink centre. It would look absolutely stunning interplanted with the perennial baby’s breath (Gypsophila paniculata) – like a giant wedding bouquet!

‘Golden Scepter’ is another good ball type with tight, custard-yellow flowers. I find that the ball types make excellent cut flowers for a summer arrangement.

Cactus-flowered dahlias look very jolly in a mixed border, particularly towards the end of summer when all hues seem to happily blend together in the season’s crescendo. ‘Jeanne d’Arc’ is one of these with its apricot/pink spiky petals. There are two excellent white cactus types: ‘My Love’ is pure white and ‘Playa Blanca’ is ivory, making it a perfect companion for spiky perennials like the yellow Verbascum chaixii and the white form, V. chaixii ‘Album’.

If hot pink is your colour, look for ‘Fascination’, an almost-single prolific bloomer with contrasting dark foliage.

The decorative types have such big flowers they most definitely need to be grown in a large garden. A friend of mine has such a garden in Vancouver and has created island beds of dahlias that can be viewed from a terrace behind her house. While the garden gets plenty of sun, the backdrop is towering, dark-green native cedars. Believe me, those island beds filled with dahlias look like flamenco dancers in a green courtyard!

If you want to make a dramatic statement, two very showy decorative dahlias to seek out are ‘Ferncliff Inspiration’, with massive rose-pink blooms, and ‘Grand Prix’, with strong, bright-yellow petals whose tips look like they’ve been dipped in white paint.

David Tarrant of the UBC Botanical Garden is a well-known gardening expert, author, and host of Spring, currently on HGTV.

PHOTOS: Terry Guscott; John Glover