How to Overwinter Dahlias, Cannas and Tender Plants

There's a particular sort of heartbreak unique to gardeners. You spend the whole summer admiring a border full of dinner-plate dahlias and architectural cannas — and then one clear October night, the temperature dips, the frost arrives, and by breakfast the whole lot has collapsed into a slimy, blackened heap. I've had this happen, and I can tell you it's a thoroughly miserable way to start the day.

The good news is that it's entirely avoidable. Dahlias, cannas, tuberous begonias and gladioli are what we call tender herbaceous perennials — plants that can't survive a cold, wet British winter outdoors in most of the country, but which store their energy in fleshy tubers, rhizomes or corms that can be lifted, dried and tucked away somewhere safe until spring. Do it properly and you'll save yourself a fortune on replacement plants, year after year, whilst building up ever-bigger, ever-more-floriferous clumps.

In this guide I'll walk you through the whole process — when to lift, how to lift without snapping everything, how to cure and store, what temperature your storage spot actually needs to be, and how to nurse everything back into growth in spring. I've made plenty of mistakes over the years (mostly involving mould, mice and impatience), so I'll share what works and, just as importantly, what doesn't.

The Quick Reference Card

Before we get into the detail, here's the at-a-glance version. If you remember nothing else from this article, remember these numbers — they're the foundation of successful overwintering.

Which Plants Actually Need Lifting?

Not everything in the garden needs this treatment, and one of the commonest mistakes I see is people either lifting things they didn't need to, or — more painfully — leaving things in the ground that really should have come up. So let's be clear about the cast of characters.

The tender herbaceous perennials that can be lifted and stored over winter include dahlias, cannas, tuberous begonias and gladioli. These are the plants that store their energy underground in tubers, rhizomes or corms, and which are vulnerable to frost damage either at the top (foliage) or below (the storage organ itself, if the ground freezes).

Timing: Reading the Frost

Getting the timing right is where so much of the success lies, and it's also where gardeners tie themselves in knots. Lift too early and you cut short the period when tubers are bulking up and storing energy for next year. Leave it too late and you risk the foliage being frosted, which — depending on the plant — can either be harmless or genuinely damaging.

The golden rule is this: tender plants need protection at the first warning of frost, which usually falls somewhere between September and November depending on where you garden. In a cold inland valley you might be lifting in late September; on a mild coast you may still be admiring open dahlia flowers in early November.

There are two schools of thought on dahlias specifically, and I want to be honest that both work:

My own approach is a compromise: I wait for that first light frost to knock the dahlia foliage back, then I lift within a week or two — before any really hard, ground-penetrating freeze arrives. Cannas I tend to lift a touch earlier, as soon as the first frost has marked the leaves, because their storage spot wants to be a bit warmer and I'd rather not risk the rhizomes sitting cold and wet.

Lifting Dahlias, Step by Step

This is the bit that makes people nervous, and understandably so — dahlia tubers are extremely brittle, and a careless spade thrust can shear off half your clump in one go. Take it slowly. There's no prize for speed here.

Step 1 — Cut Back the Top Growth

Once frost has blackened the foliage (or once you've decided it's time), cut back all of the stems to within 3–4 inches of the ground. This leaves you a manageable handle to work with and removes the bulk of the soft, frost-damaged growth that would otherwise rot. Use clean, sharp secateurs or loppers — dahlia stems are hollow and can hold water, so I like to angle the cut slightly so rain runs off rather than pooling into the crown.

Step 2 — Dig Wide and Gentle

Here's the crucial bit. Start digging at least a foot away from the stem on all sides. The temptation is to dig in close and lever up quickly, but the tubers spread out further than you'd think, and they snap at the slightest provocation. Work your fork or spade all the way around first, loosening the soil, and only then gently ease the whole root ball upwards. Go slowly, and be gentle — the tubers really are extremely brittle, and a broken tuber near the crown can be lost entirely.

Step 3 — Clean and Inspect

Gently knock or wash off the excess soil. I prefer a gentle hose at low pressure, then a careful inspection of every tuber. You're looking for the growing points (the "eyes") near the crown — these are where next year's shoots emerge, so they must be intact. Discard any tubers that are soft, hollow, damaged or showing signs of rot. Trim back stringy roots.

Step 4 — Dry Them Off

Turn the clumps upside down somewhere dry and airy for a few days so any moisture drains out of the hollow stems and the skins firm up. A frost-free shed, greenhouse bench or porch is ideal. This drying-off (or "curing") stage dramatically reduces the chance of rot in storage.

Lifting and Storing Cannas, Step by Step

Cannas are a slightly different beast. Their storage organ is a chunky rhizome rather than a cluster of brittle tubers, so they're more forgiving to dig — but they want slightly different storage conditions, and getting that wrong is the usual cause of failure.

Step 1 — Trim the Foliage

After the first frost has knocked them back, trim the dead or fading leaves back to about 2 inches above soil level. Cannas grow tall and lush, so there's a lot of top growth to remove — don't be shy with it. As with dahlias, getting rid of soft frosted leaves reduces the rot risk.

Step 2 — Lift the Clumps

Dig around and lift the whole rhizome clump, shaking off loose soil. Canna rhizomes are far tougher than dahlia tubers, so you can be a little more robust here, though there's still no need to be brutal. Keep the clump intact for now — you can divide in spring if you wish.

Step 3 — Cure for 2–3 Days

This is the step people skip and then wonder why their cannas turned to mush. Dry the clumps for 2–3 days in a shaded spot to cure. You want them out of direct sun (which can scorch and dehydrate) but somewhere airy so the cut surfaces seal over and surface moisture evaporates. A shaded greenhouse bench or a corner of the shed is perfect.

Step 4 — Pack and Store

Once cured, place the rhizomes in peat moss or damp sand. The medium should be barely moist — damp to the touch, never wet. The aim is to stop the rhizomes drying out and shrivelling completely, whilst not keeping them so wet that they rot. Cannas like it a little more humid than dahlias, which is why a slightly moist medium suits them.

Step 5 — Find the Right Spot

Cannas should be stored in a cool, dark, humid location, with temperatures between 40 and 60 degrees Fahrenheit. The ideal sits at the lower end of that, around 40–50°F. Too warm and they may try to sprout prematurely; too cold and you risk frost damage to the rhizomes.

The Storage Environment — Getting the Numbers Right

If lifting is where the brittle drama happens, storage is where the slow, silent failures occur. A clump that looked perfect in November can be a fuzzy grey horror by January if the conditions are wrong. The two enemies are damp (which causes rot and mould) and warmth (which either triggers premature sprouting or speeds up drying and shrivelling). Frost, of course, is the third enemy and the one that kills outright.

Here's the temperature picture for the main plants you'll be storing. The overlap is reassuringly wide, which means most frost-free, unheated spaces will do the job.

The phrase I'd underline is "maintain consistent temperatures". It's not just the absolute number that matters — it's the consistency. A spot that swings from 35°F at night to 60°F when the midday sun hits a shed window is far worse than a steady 48°F all winter. Wild swings cause condensation, condensation causes damp, and damp causes rot.

Where to Actually Store Them

Dahlias vs Cannas vs Begonias — Storage Compared

Because the three most popular plants for overwintering each have their own quirks, I find it helpful to lay them side by side. This is the table I wish I'd had pinned to the shed wall when I started.

What jumps out is how much they have in common: a cool, frost-free spot at 40–50°F suits all three. The differences are at the margins — chiefly that cannas want a touch more moisture and are more forgiving to lift, whilst dahlias demand the gentlest handling but reward you with the most spectacular results.

Storage Methods Head to Head

There's no single "correct" way to store tubers, and over the years I've tried most of them. Each has trade-offs, so here's how the popular methods stack up in my experience.

My personal default? Dahlias go into boxes of barely-moist old compost or vermiculite, cannas into damp sand or peat moss, and gladioli corms get the lazy treatment in open mesh trays in the shed. The pot method is wonderful if you grow in containers and have the space — you simply let the pot dry off, cut back, and shuffle the whole thing somewhere frost-free.

Common Mistakes (and How to Dodge Them)

I've made every one of these at least once, so consider this the confessional section. If you can avoid these five pitfalls, you're most of the way to a 100% survival rate.

That last one deserves a word. Mice and voles will happily nibble their way through a winter's worth of stored tubers if you let them, and a frost-free shed is exactly the cosy spot they're looking for too. Store boxes off the floor where you can, keep an eye out for tell-tale droppings, and if you've had problems before, a lidded crate with a few ventilation holes drilled in is a sensible compromise between airflow and rodent-proofing.

Waking Them Up Again in Spring

All this effort pays off in spring, when your carefully stored survivors come roaring back into life. Here's how I bring them out of dormancy.

Starting tubers into growth under cover gives you a head start of several weeks over plants left to come up cold in open ground — which means earlier, longer flowering. It's one of the quiet joys of overwintering: not only do you save the plants, you get bigger, earlier displays as a reward.

Is It Worth the Effort? My Honest Verdict

Let's weigh it up properly. Overwintering takes time, a frost-free space, and a willingness to get your hands muddy on a cold autumn afternoon. Is it worth it? For me, almost always yes — but let me rate the experience honestly across the things that matter.

The cost saving alone makes the case. Named dahlia varieties and statement cannas aren't cheap, and re-buying the same plants every spring is money straight down the drain. Lift and store them well and a single plant can serve you for many years, multiplying as it goes. The effort score is the only real mark against it — there's no pretending an afternoon of careful digging and an unheated shed are effortless — but it's hardly arduous, and the spring payoff is enormous.

Who Should Bother — and Who Shouldn't

Overwintering isn't compulsory, and for some gardeners it genuinely isn't worth it. Here's my honest take on who'll get the most from the effort.

Frequently Asked Questions