Quote from Lee Garden Ninja on 4th October 2025, 9:12 am

Hi @denislaw10

Thanks for your question about your cannas still being in full leaf. You've done absolutely the right thing moving them into the greenhouse to protect them from frost, and I've got some good news for you. You don't need to cut them back just yet, and in fact, waiting until spring might actually be the better approach for your particular situation.

The traditional advice has always been to wait for the first frost to blacken the foliage before cutting cannas back, but since you've moved yours into the greenhouse whilst they're still green and healthy, you've essentially changed the rules of the game.

Your cannas are now in a protected environment where they can carry on ticking over rather than being forced into dormancy by freezing temperatures. This means you have a choice about when to cut them back, and there are good reasons to consider leaving the foliage intact until spring arrives.

Why Leaving the Foliage Can Be Beneficial

When you cut back canna stems immediately, you're creating fresh wounds on the plant that are exposed to moisture and potential bacterial or fungal infections during the damp winter months. By leaving the foliage in place over winter, you're essentially giving the plant a protective layer. The stems and leaves act as a natural barrier for the rhizomes below, and as they gradually die back naturally in the cool greenhouse conditions, the plant can seal off the connection points more effectively. This reduces the risk of rot setting in during storage, which is one of the biggest killers of overwintering cannas.

Many experienced gardeners have found that leaving the cutting back until spring, just before new growth starts to emerge, can actually result in healthier plants.

The old foliage continues to photosynthesise at a reduced rate if there's any winter sun, and the plant can store just a little bit more energy in those rhizomes for the spring push. It's not a dramatic difference, but every little bit helps when you're trying to overwinter tender perennials.

When spring arrives and you see fresh new shoots starting to appear from the base, that's your cue to cut away all the old brown stems and leaves, leaving just the new growth to flourish.

What to Do With Your Greenhouse Cannas Over Winter

Since your cannas are in the greenhouse rather than lifted and stored bare, your overwintering approach is slightly different from the traditional dig-and-store method. The key thing to remember is that cannas cannot be too wet, too dry, or allowed to freeze. 

Your unheated greenhouse should provide the perfect environment as long as you get the watering right. During winter, your cannas will naturally slow right down and may even go dormant depending on how cold your greenhouse gets. This is completely normal and nothing to worry about.

Water very sparingly over the winter months. The soil should be just barely moist, never waterlogged, and never bone dry. A light watering once a month should be plenty, and even then, only if the greenhouse warms up on sunny winter days and the soil has dried out.

If your greenhouse stays consistently cool and the compost remains slightly damp, you might not need to water at all. The trick is to prevent the rhizomes from completely desiccating whilst also avoiding the dreaded root rot that comes from waterlogged winter soil. Check the pots every few weeks and use your finger to test the soil moisture about 5 centimetres down.

If you're the type of gardener who worries about forgetting things, you might prefer to cut the foliage back now to about 5 to 10 centimetres above the soil level. This makes the pots easier to manage in the greenhouse, reduces the risk of botrytis or other fungal diseases developing on dying foliage, and means you won't accidentally snap off emerging shoots when you're pottering about in spring.

It's perfectly fine to do this, especially if the foliage is starting to look tatty or you need the space. Just use clean, sharp secateurs and make your cuts cleanly without tearing the stems. Then follow the watering regime above and your cannas should sail through winter without any drama whatsoever.

Getting Ready for Spring

Come March or early April, before you move your cannas back outside, you'll want to give them a bit of attention. If you left the foliage intact over winter, cut it all back now to just above soil level, removing all the old brown stems and leaves. This is also a brilliant time to divide any congested clumps if they've been in the same pot for a couple of years. Cannas multiply like mad underground, and dividing them not only gives you more plants to enjoy but also reinvigorates the parent plant. Each division should have at least one growing point or eye visible on the rhizome.

Repot into fresh compost if needed, or at the very least give them a good feed with a high nitrogen fertiliser to encourage that lush foliage growth. Water them more regularly as the weather warms and new growth appears, gradually increasing the frequency as spring progresses.

Don't rush to put them back outside though. Wait until all danger of frost has passed, usually after the last frost date in your area, which for most of the UK is around mid to late May. You can harden them off gradually by putting them outside on mild days and bringing them back in at night, or simply wait until the nights are consistently above 10 degrees Celsius before leaving them out permanently.

Further Reading

For more help with your greenhouse and tender plants, have a look at these articles:

Putting Up a Greenhouse: Why It May End Your Marriage

How to Protect Plants in Winter

Winter Flowering Plants: 15 Plants for the Cold Season

8 Winter Gardening Jobs for Beginners

I hope that helps and gives you confidence with your cannas. Do let us know how they get on come spring!

Lee Garden Ninja