Dahlias are enjoying a come-back, which is why so many of us have been marvelling at their many different forms and colour this summer and autumn. But, like all good things, the dahlia flowering season must come to an end, and around the country we are bidding ‘au revoir’ to these bright florals. If you have grown dahlias in your garden, and want to delight in their blooms again, next season, what you do now is important. It also involves some decision-making. Use the advice below to help you help your dahlias.
Look the other way!
Dahlias look messy as their flowering and growing season slows or comes to an end. In colder regions, they look especially tatty once they are cut back by the first light frosts. However, it’s important you don’t snip away this above-ground material immediately, as it’s what feeds the tubers below, and gives the plant the strength to grow and bloom again, next season. If you have tied taller dahlias during their growing season, the ties will help hold the spent foliage and stems in place, which will go some way towards mitigating the ‘mess.’ In reality, though, your job is to look away until such time as you can cut back the remains of the plants.
Cutting back
If you live in a mild, frost-free region, your dahlias may never die back completely. They may even flower occasionally through winter if you don’t cut them back. But if you want to tidy them thoroughly, and ‘winter them down’, cut back their stems to around 5cm at the end of autumn, and mulch thickly around the base. If you live in a mountainous part of the country (think Central Otago or Central North Island), or in a region with very wet winters (or if your soil is poorly drained), cut back the dead stems to within 10 cm of the base of the plant. Then make your decision about whether or not to ‘lift’ the tubers (see below).
To lift or not to lift – that is the question!
Regardless of where you live in the country, it’s wise to lift (carefully dig up) your dahlias ever 2 years. This is because, left any longer, dahlia tubers quickly grow into heavy clumps. In order to keep blooming well, the tubers will eventually need to be divided (split into smaller sections), and the larger the clump they have grown into, the more difficult the splitting will be.
Now for the more difficult decisions: if you live in a mild, frost-free region, lift according to the age of your plants. If you live in a mountainous, heavy-frost but low-rainfall region, you may choose to mulch heavily over your dormant tubers with the likes of old baylage or thick straw, and leave them in the ground. However, if frost penetrates the ground too deeply in your region, or you experience heavy winter rainfall as well as heavy frosts, (and especially if your soils are poorly drained,) the safer bet is to lift your dahlia tubers. In cold, but not icy winter regions, and where soils drain well, tubers can stay in the ground with a mulch over them.
Lifting and dividing
If you are lifting your dahlias because they require dividing, always dig well back from the edge of the clump so as not to damage the tubers. Leaver the clump out gradually. If you are planning to replant the tubers right away, divide them immediately. If you are storing your dahlias out of the ground for winter, divide them in spring. As you divide the tubers, it’s important to make sure at least one on each clump has a growing point. Check out the video below to help you understand how to do this.
Storage
If you’ve decided to store your dahlia tubers out of the ground for winter, let them dry for a few hours. Line a box with damp (not moist) untreated sawdust, pop the tubers on top, and cover with another layer of untreated sawdust. Keep the box in a cool, dry, rodent-free environment until replanting time, occasionally sprinkling over a little water to keep the sawdust damp.
Dahlias bring colour and life to our gardens, and they are well worth the pre-winter work!