09/05/2026
Dahlias & end of season cutting back......
To cut back or not cut back? This is a question I am routinely asked. Like most rules when growing dahlias – there are no rules. Each stage of growing, and how it works best, will depend on your climate & growing environment, ground vs raised beds vs pots, growing medium, pests, purpose for growing, plans for storing etc. And each year, many of these factors vary, so just when you think you have figured out the perfect recipe for success, mother nature comes along next season, or part way through your current season, and throws you a curveball or ten.
For me, depending on how soon the first Autumn frosts arrive, I usually dig and place my tubers in storage at the end of May (to dig or not to dig is a whole separate discussion…stay tuned). In the first couple of weeks of May, I usually have part of my patch starting to yellow, droop and die off, with other parts still vibrant green and producing more buds. All of our beds are protected by shade cloth, so even the first couple of light frosts don’t send the plants to their impending death. Many people don’t cut back at all, they just wait until their plants have died off then pull/dig. Cutting back initially creates more work, and also introduces additional risk to be aware of – hollow stems can trap & direct water down to the tuber which can create an increased environment for tuber rot. Despite this, I choose to cut back, for two main reasons…… 1) it reduces pesky powdery mildew which moves in fast at this cooler, damp time of year. The mildew won’t impact the underground tubers, but I like to remove it to reduce the chance of it spreading to other nearby (non-dahlia) plants which are sensitive to mildew. 2) When it comes to the time of digging & diving, I have less messy, droopy damp growth to remove from the plant. What about the hollow stems? The most common methods for protecting tubers is using bluetak stuffed into the holes or covering the top of the cut stems. For my garden bed plants, I do the latter, quickly & easily with a small piece of aluminium kitchen foil scrunched over the top of the stem. It stays in place well during any wind & rain events and is easily pulled off and popped in the bin before I dig the plant up. For my potted dahlias, I simply cut them back and move the pot under cover where they will be protected from any rain.
HOW FAR TO CUT BACK?
Again, there is no ‘rule’ to say you have to cut back at all, but if you do then general advice across growers is to cut back by a third when the plants are starting to yellow & die off. 2-3 weeks later, cut back by another third. I will either do one or two cut backs, depending on when the frost comes. If I have done the first cutback and we get heavier frost, the plants will often then die off very quickly and be ready for digging. The other, and most important contributing factor for me is rainfall. While dahlias still need a little moisture at the very end of the season, we certainly don’t want them sitting in cooler, wet/soggy soil if there is a lot of rainfall. When I say ‘a lot of rain’….a single large amount of rain is very different to ongoing days of heavy rain. I have recently done the first 1/3 cutback of my plants, and we were forecast for a day of 40mm of rain just after I cut them back. That’s OK! My plants are all grown in raised beds and pots with well draining soil. After several dry weeks, they will get some moisture but most will happily drain away. In contrast, last year at cutback time we had several weeks of consistent heavy rain which meant the ground was going to stay very wet at a time when risk of tuber rot was increased. Last year I dug up my tubers a few weeks earlier than usual before the extended rain period hit. I just dig down and monitor the soil. I’m not fussed if early frosts come, the tubers are well protected under the soil….it is consistent rain that I aim to avoid…...but more on this soon when we chat about digging vs not digging at the end of the season.
The other tip I can give if you choose to cut back, to help avoid too many hollow tuber cuts, is to cur laterals and stems above & next to a node. Stems on most varieties become hollow as the plant matures. Most (not all) plants will be solid at the node points, so by trimming off laterals next to the node, and stems just above the node, you might be able to use the node as a natural stopped, meaning no need to plug or cover a hollow stem. I have included a photo to demonstrate this. In the photo, one of the cuts has a pin hole in the centre, but that hole doesn’t go all the way through the node, so there is no need for me to cover it. IN other places, I have cut on the stem and covered it with foil. At the time of first cutback, I leave the healthy green foliage where possible, to assist with photosynthesis encouraging the plant to direct as much energy as possible into end of season tuber health rather than into trying to produce more blooms. At this stage I am also cutting off any new buds on plants that are still eager to grow.
There are a lot of helpful videos online around this topic, I encourage new growers to check them out but remember, the journey is about learning what others do, but then figuring out what works best for you. This is an ongoing process, with new learnings every season. Enjoy the journey, embrace the failures and love the successes.
Happy growing everyone!
