There are some benefits to cutting back dahlias. First off, it’s a good idea to deadhead them throughout the season to encourage more flowers and keep them looking tidy. But there are other ways to prune these summer favorites—from pinching and thinning them out to preparing them for winter. This guide covers all three, so you can enjoy your most stunning dahlias yet.
How to pinch dahlias
One of the best ways to encourage better performing dahlias is to remove the limbs as they grow in the spring. This is known as ‘pinching’, and a similar process can be applied to sweet peas and zinnias.
Gardening expert John Negus suggests using a knife to do this, like the Fiskars Garden Hori Knife from Amazon, instead of shears. The stems are less likely to be damaged. As always, make sure your tool is sharp and clean – not doing so is a common pruning mistake and can result in diseases and pests spreading from one plant to another.
Simply remove the growing points when the plants reach about 12 inches in height, John advises. Cut back a healthy set of leaves. “They will respond by producing side shoots, making the plants bushier and resulting in a wealth of flowers,” he says.
Most important tip: If you want your dahlias to produce larger flowers for bringing indoors, remove any small buds that develop behind the main central bud, advises the Royal Horticultural Society (RHS). This process is called “separation”.
John has been a garden journalist for over 50 years and regularly answers readers’ questions Amateur gardening magazine. He has also written four books and given numerous lectures over the years on gardening.
How to thin out dahlias
Some types of dahlias can also benefit from thinning in the spring. This can improve the health and performance of plants if they are prone to crowding. It can also provide a better balance between foliage and flowers.
Simply remove the tallest, most crowded shoots, says the RHS. You can do this by hand. Leave about seven to ten strong stems on the plant — or fewer if you are planting varieties with particularly large flowers.
The new shoots that appear around the base of the plant can be used for propagation. Take 2-3 pieces of it, says John. Make a horizontal cut below the bottom leaf joint and remove the leaves from the bottom half of the cut. Finally, insert the cuttings to half their length into a special seed and cutting mix and root them in a heated propagator or similar location.’
If you notice any dead or diseased stems on your dahlia plant, remove those stems as well, says Diana Cox of TheGardeningTalk.com.
Diana Cox is the proud founder of TheGardeningTalk.com and has over a decade of gardening experience. Through her blog, she aims to inspire and motivate fellow gardeners by sharing practical techniques grounded in personal experience and extensive research.
Pruning dahlias at the end of the season
When the first frost arrives, the dahlia leaves will turn black. At this point, you can prune the plants immediately, ready for winter. Shorten the stems to within 8 inches of the base, John says. Then lift the plant with a shovel.
You can then store the tubers in a dry, frost-free place before replanting them in the spring.
Alternatively, after cutting them, you can place a layer of mulch around the base of the plants to protect them from the cold, says Diana. However, this is only suitable for warmer climates – if in doubt, lift them up for storage.
Trimming dahlias, once you know how, is a simple task that’s definitely worth it. But don’t forget that there are other basic ways to care for these plants to get big, beautiful blooms – including feeding them regularly. It’s also important to water them frequently, especially during dry periods and if you’re growing them in pots.