How to Grow Dahlias

By Steven Wedd

Where to Plant

* dahlias need more than half a day of full sun to grow strongly
* shelter from strong winds is desirable
* good drainage is essential as tubers rot in soggy soil – raise beds if this is a problem

Preparing the Soil

* dig the garden bed to the depth of a fork or spade
* place stakes 70 to 80cm apart – stakes need to be at least 150cm above the ground
* apply a shovelful of cow manure or compost at each stake
* a handful each of pelleted poultry manure and blood and bone, as well as some dolomite and sulphate of potash at each stake is beneficial
* apply the above 2 to 4 weeks before planting and dig in well

Planting

Dahlias will flower in 8 to 12 weeks from planting, and can be planted as soon as danger from frost is over. If planted in September you’ll have flowers before Christmas!
* plant tubers approx. 10cm deep with the eye or shoot close to the surface
* mulching with straw, sugar cane etc. will save weeding as well as conserve water

Growing

Dahlias are both big feeders and big drinkers so you’ll get the best flowers from fertilizing and watering them well.
* when plants have 4 sets of leaves the main growing shoot can be snapped off (best done in the early morning) – this is known as ‘stopping’ and creates strong side shoots
* if tubers send up a lot of shoots you can break off all but the strongest two which will make tying up much easier and give better quality flowers
* if larger blooms from ‘dinner plate’ varieties are desired then limit the number of side shoots to 4 – 6 by breaking some of them off
* watering deeply weekly is better than light watering daily – during really hot weather water more frequently if plants are wilting badly
* a handful of complete fertilizer applied to each plant at approx 30cm tall will help
* at this time start tying plants to the stake using a figure of eight – baling twine is good as is old stockings; avoid string as it can cut into the stem
* as buds are forming start foliar feeding with seaweed, fish emulsion and/or a soluble chemical fertilizer
*disbudding is the single most important tactic for getting large blooms on strong stems – when large enough to handle pinch out the side buds and shoots, leaving only the main centre bud
* to achieve even longer, stronger stems (for the vase or for showing) pinch out the side shoots from the next one or two leaf nodes below the flower – stems below this will then grow stronger

Pests and Diseases

Healthy dahlia bushes are very desirable to grasshoppers and the blooms can be susceptible to grubs. If a few blemishes and bite marks don’t bother you then you probably won’t have to worry. There is a range of specific insecticides available. ‘Wettable sulphur’ appears to control powdery mildew which may occur late in the season.

Maintenance

Dahlias will continue to flower for at least 3 months if you keep cutting them. This may either be picking for the vase or simply cutting off old flowers (deadheading). If you don’t do this the plant produces seeds and believes it’s purpose has been achieved, and so stops growing more flowers. It’s best if old flowers are removed at least twice a week.

Preparing for next season

By around late April/May your dahlias will stop flowering and start to look pretty sad. It’s important to let them die down completely at this time as they are storing food in the tubers ready for next season’s growth. If you’re happy for them to shoot when they are ready in Spring, simply cut them back and leave them in the ground. However, plants will be stronger if dug up and the clumps divided each year. Use a shovel (not a fork) and lift gradually from different points around the clump. Excess soil can be washed off gently. Store them in a cool shady place, preferably in wood shavings or potting mix so they don’t dry out.
When eyes or shoots are evident in Spring, use a knife or cutters to separate single or small clumps of tubers, being careful not to damage the ‘neck’ in any way. Part of the old stem is needed as this is where the 'eyes' are.

Dahlias will still grow and flower prolifically with a lot less care and effort than is suggested above, but for truly ‘wow factor’ results the more you put in, the more you’ll get out of it.