As the first frost heralds, it is time to figure out how to overwinter dahlias. Dahlias will continue to flower with their beautiful blooms right up until the temperature drops and the frost arrives. At this point, any remaining flowers, buds and foliage will die immediately. So it is time to start the process of protecting your dahlias from the cold winter weather so that they can regrow next year.
Just like when learning how to grow dahlias, figuring out how to overwinter dahlias can seem quite daunting – especially if it's your first time. Should you dig them up or leave them in situ? How should you store the tubers? Will they regrow next year? All these questions are very important and the answers depend on both your area and personal preferences.
There are lots of different methods available for overwintering dahlias, which means there will be a little bit of trial and error as you figure out the best way in your specific part of the world. Why not try several different approaches to see which works best for you and your winter garden ideas?
How to overwinter dahlias: step-by-step
Planning a winter garden does involve preparing more tender plants for dormancy during cold weather. The key thing to remember when learning how to overwinter dahlias is that the tubers should not be left to totally dry out, as you would with bulbs. 'Dahlias require a cool climate, ample air circulation, and being placed in a dark area if they are not being stored in soil media,' says Tammy Sons, owner of Tennessee Online Plant Nursery (opens in new tab).
- The first frost marks the start of the overwintering period. 'Wait until a frost has turned the foliage black, then cut the plant down to about 6 inches,' advises Tamsin Hope Thomson from Amateur Gardening Magazine (opens in new tab), 'Then lift the tubers and use your hands to get rid of the soil.'
- Having lifted your tubers, the next step is to remove any rotten or dead matter. The tuberous roots should feel firm to the touch and not squishy when pressed. If they don't feel firm or if they look rotten, then remove the rotten material with a clean knife until only healthy tuber remains. Dispose of any unhealthy material on your compost heap.
- Some of the tuberous roots may have become detached from the main body – this is okay. The loose tubers can either be disposed of or you can store them with the rest of the dahlia to see if they might make a viable plant the following summer.
- Dead tubers appear withered and wrinkled. These also need removing and can go on the compost heap.
- 'Do not clean the tubers under a tap; to get water on a tuber at this time of year often spells disaster. Turn the tubers upside down in a crate and leave to dry for a couple of weeks,' advises plantswoman and dahlia expert Sarah Raven (opens in new tab) in her blog.
- Next, label your tubers, recommends Megan Foster, category manager of bulbs & perennials at American Meadows (opens in new tab). 'As you build your collection of varieties, you will want to remember which is which come planting time in the spring. Use a marker or tie on a label with old twist ties.'
- Once the tubers have dried out sufficiently, it is time to store them for the winter. There are several methods for storing dahlias. The best method for how to overwinter dahlias primarily depends on where you a storing them. If you are storing them in a slightly moist environment opt for paper bags or cardboard boxes as these will absorb the moisture from the air. If you are storing them in somewhere that is drier, then a plastic box or bin bag is a better option as these can be occasionally spritzed with water.
- Unlike bulbs they also need to be insulated to keep them warm and prevent them from drying out. You can either cover them in loose, dry compost, sand, vermiculite or straw, or you can wrap each tuber in newspaper.
- Place your tubers in a dry and cool place – unheated greenhouses, sheds, porches or the cellar are all great options.
- Check on your tubers throughout winter looking for signs of rot or severe dehydration. If you spot signs of rot, cut away the compromised material until you are back to the healthy tuberous material. It is also worth moving your dahlias to a drier location to prevent further rot from taking hold.
- Dehydrated tubers are the other challenge when learning how to overwinter dahlias. It is quite easy to tell if your dahlias are dehydrated as they will look shrivelled. If this is the case, occasionally spritz the dahlias with water to help them absorb more moisture.
Should I split my dahlia tubers before overwintering?
Whether you split your dahlia all comes down to a matter of confidence and knowledge about how to overwinter dahlias. 'If you’re not sure where the eyes are, don’t break apart your tuber clump in the fall before storage. The tuberous fingers of the dahlia are winter sugar and moisture survival supplies for the eyes which are the source of new growth for next spring. If you disconnect the two accidentally, you’ll be disappointed with lack of growth,' advises Megan Foster, category manager bulbs & perennials at American Meadows (opens in new tab).
However, if you are certain where the eyes are and that they are still connected to plenty of tuberous fingers, then you can separate the two before overwintering. To do this use a clean knife to cut away the eye and tuber. Then overwinter in the same way you would a non-separated dahlia.
Can you leave dahlias in the ground?
If you live in an area that have milder winters and very free-draining soil then you can leave your dahlia tubers in the ground. However, this is only suggested for hardiness zones 7 to 10. If you are leaving your tubers in the ground, then there are some steps you must follow to protect them during the cooler months.
Once your dahlia has succumbed to the frost, leave it for about two weeks to let the sugars go back into the tuber, then cut the stem back to around 6 inches tall. Dispose of the foliage on your compost heap. After this, cover with a mound of deep mulch to insulate the tubers throughout the winter months.
Can you leave dahlias in pots over winter?
Although dahlias aren't candidates for the best winter plants for pots and borders, yes, you can leave dahlias in pots over winter. One of the beauties of growing dahlias in a pot is the flexibility that it offers. When in bloom, you are able to move your plants around your garden so that the most show-stopping displays are always front and center.
This flexibility continues when figuring out how to overwinter dahlias that you have grown in pots. When overwintering pot grown dahlias, it is vital that they are sufficiently protected from the cold weather. 'It is important to create storage conditions that somewhat mimic dahlia tuber’s natural habitat under earth in a drier climate and non freezing latitudes,' advises Megan Foster.
If you can move your pots inside, into a cool but frost-free, dark and dry place – like a garage, unheated greenhouse or shed – then do so. If you do not have a space to move your pots to, then cover with a mound of compost or manure, as you would with dahlias left in the ground. You can then cover your pot with frost-fleece to help protect it and move it to an area where it will be protected from rainfall.
Allow the pot to partially dry out. It is important to not let them dry out completely, but equally don't water regularly as you would during the growing period. Water as frequently as you would a cactus – a cup of water every month or so is sufficient.
How to overwinter dahlias in Saran wrap
Overwintering dahlias in Saran wrap is a fairly new method for overwintering dahlias, however, it has received a great response. In fact, it is now the go-to method for the American Dahlia Society, having moved away from vermiculite and plastic bags.
Start by lifting your dahlias, as you would for any other method, and clear them of soil. Whether you treat your dahlias with an anti-fungal treatment to ward off rot is a personal choice, and should you not wish to, you can simply skip this step. However, it will offer greater protection for your dahlias over the colder months.
'Add a very small amount (around a tablespoon) of powdered sulfur to 3 cups of dry vermiculite in a gallon size ZipLock bag and mix thoroughly' suggests Marian Mandella, Bernard Mandella, and Richard W. Peters, M.D, experts at The American Dahlias Society (opens in new tab) . Then add your tuber and shake lightly so that your dahlia tuber is coated – 'a very, very light coating should be best.'
Having done this, set your dahlias aside in a crate to dry overnight before wrapping. 'Tear off a sheet of plastic wrap about 20 or more inches long and lay it flat on a level surface. Place a tuber on one end and roll the plastic wrap over one complete turn. Lay another along side and roll again. Be certain that no tuber is touching another; plastic wrap must separate all tubers.' Once you've wrapped your dahlias, fasten with masking tape and label with the dahlias's name before storing in a dark and dry place at around 40°F for winter.
What month do you cut back dahlias?
Cut back dahlias in late fall – October and November, however don't cut them back immediately. 'Leave the stems to go black from the first frost for approximately a two weeks to allow the sugars to go back down the stems to feed the tuber. Then cut the stems off just below ground level,' advises PL gardening expert Leigh Clapp.
You need to do this however you are overwintering your dahlia tubers as it will increase their likelihood of survival and the extra energy it provides will help them to restart growing in the following spring.