Knowing when to cut back dahlias in winter can feel confusing, especially when advice varies wildly by climate and frost timing. These late-blooming stunners often put on a show into November, meaning they are spared in the fall garden tidy up. When the display is over, fading dahlias make the garden look messy, but cutting them back too soon can lead to problems including tuber rot and weakened plants that produce fewer flowers next year.
Leaving your dahlias too late is less of a problem than cutting too soon, but it makes it difficult if you plan to dig up and store the bulbs. Ideally this should be done before prolonged periods of frost and snow. Personally, I never dig up my dahlias and instead overwinter them in the ground. I mulch them for insulation, but beyond that it’s a case of survival of the fittest. That said, if you live in USDA Zone 6 or colder, storing dahlias indoors over winter is often necessary.
Whether you’re leaving your dahlias in the ground or digging them up for winter, this part of dahlia care often feels the most uncertain. But there is one rule I always follow that removes all the guesswork and ensures I cut them back at just the right time.
My Easy Rule
After 15 years of growing dahlias, I have developed this golden rule: “If dahlias are black, cut them back; if they’re green, leave them be.” Essentially, you shouldn’t cut back your dahlias until the first hard frost has killed the top growth. You’re looking for blackened foliage and flower heads, and stems that are browning.
It’s important to avoid cutting back dahlias before this stage because the plants continue sending energy to the tubers until frost kills the foliage. Frost doesn’t trigger dormancy – shortening days do – but frost does mark the point when the plant stops photosynthesizing and tubers stop taking in energy. Cutting too early interrupts this process and can reduce tuber vigor – or, in the worst case, kill the tubers.
The exception to this rule is if you live in a frost-free climate, in which case you should cut back in late autumn or winter once nights consistently drop down below 40°F (4–5°C).
What to Do When Frost Arrives
Before cutting back your dahlias, confirm the frost damage. As well as foliage going black, other signs of frost kill include stems going limp and darkening; they may also be a bit mushy. If in doubt, wait another day or two to be sure.
When you’re ready to go, take clean, sharp pruners (I love my Felco 2 pruners, available on Amazon) and cut the stems back to 4–6 inches above the soil line. Leaving a little stem makes it easier to lift the tubers or provides a bit of extra height for mulching if you plan to leave them in the ground.
Lifting and Storing
If you plan to lift and store dahlia tubers, then after cutting back you should dig them up immediately – or within the next couple of days. Don’t just leave them sitting there for weeks.
While you want the dahlias to have been blackened by frost, you don’t want to dig them up when the ground is frozen. If the soil is dry or compacted, water lightly the day before to make the job easier.
Rather than using a shovel to lift your tubers – which can cut them – use a digging fork or border fork, like this Spear and Jackson fork on Amazon. Dahlia tubers spread far beyond the visible stems, so dig 8–12 inches (20–30 cm) out from the base in all directions (14–16 inches/35–40 cm for very large plants). Loosen the soil around them with your fork to free the clump, then slide the fork deep underneath at a shallow angle and carefully lift the tubers.
Lift and gently shake them from side to side, letting excess soil fall away naturally. Only rinse lightly if necessary – but preferably not until it's time to divide dahlia tubers. Holding a single tuber often snaps the neck, so instead cradle the crown (the central stem base) to avoid damage.
Let the tuber clumps dry for about a day or two so the cut ends can callus over. Pack them in a barely damp medium like peat moss, vermiculite, or wood shavings, then store them in a ventilated box in a cool area, such as an unheated basement or garage – ideally 40–50°F (4–10°C). Don’t forget to label them before storing so you know which varieties you have. Check monthly for shriveling or rot, adjusting moisture as needed to keep the tubers firm.
Leaving Tubers in the Ground
If you live in USDA Zones 8 to 10, you can comfortably leave tubers in the ground. If you live in Zone 7, you can leave them with some extra care. Mulching is key and should be applied to a depth of 6–12 inches (15–30 cm) in colder areas; in warmer zones, 3–6 inches (8–15cm) is usually enough. Use straw, compost, bark, or fallen leaves turned into leaf mulch, and keep it a few inches away from the stems to prevent rot.
As well as helping to regulate soil temperature, mulching suppresses weeds and keeps the soil evenly moist.
If a particularly cold snap is expected, add horticultural fleece for a short period, like this one from Amazon. Keep it in place with bricks and remove once the worst of the weather has passed as prolonged use reduces breathability and traps moisture.
In early spring, once the danger of hard frost has mostly passed and the soil has begun to warm, you can remove the mulch. Pull it back gradually as temperatures rise. In colder regions, keep a light layer handy in case you need to cover young growth during a late cold snap.
What if Frost Is Late This Year?
Keep the plants growing until the frost eventually arrives. They will continue to store energy up to this point.
Can I Cut Back Early if the Plant Looks Messy?
Yes, you can tidy things up and deadhead dahlias. Remove stems that have flopped over and are smothering other plants. However, you should leave most of the foliage intact until frost.
What About Dahlias Grown as Annuals?
You can leave these plants until you’re ready to tidy up and compost them.
