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How to make the most of your greenhouse in winter

As temperatures drop, it’s time to bring those precious plants inside but also consider how you can best use your greenhouse all year round

A greenhouse isn’t just for growing. It’s not just a warm and dry place to sow seeds and grow tomatoes. Think of it as a giant

terrarium – a glass display case for tender plants and cuttings and you’ll have so much more fun and success.

Owning or inheriting a glasshouse can be a bane or a joy, depending on your level of confidence as a gardener but if you think of it as a glass box, like a trophy display cabinet, in winter it can hold many of your treasured plants. If, like me, you have a glass greenhouse that you walk past every day, and look at from many windows, why not create a glorious display of your favourite plants, which may then tempt you outside during the colder months to check on them. 

In this case, I am specifically talking about glasshouses, not plastic-covered polytunnels or greenhouses. Although you can also overwinter plants and grow winter crops in these, they do not retain their heat as well as a glasshouse. You would need to provide additional heating and protection.

© Mark Ashbee
Create a display of beautiful plants to enjoy all winter long, alongside winter salads
Sparkling clean

First things first, give it a good clean, both inside and out. Light levels effect the health of your plants, especially in the darker months of the year. Algae, twigs and fallen leaves can quickly build up in the nooks and crannies on the outside of your greenhouse, so clear gutters and wipe all the glass with a soft cloth (and a gentle touch). It’s also a good time to check for any damage or potential issues with the structure that need repair or replacement.

© Mark Ashbee
Clean greenhouse glass inside and out to provide plenty of light for plants
Where plants have been growing near the glass inside your greenhouse, algae may have built up on the internal glass. Wipe with a soft cloth (I used a very diluted solution of washing-up liquid), being careful not to splash any winter crops growing beneath the glass. We use our greenhouse to grow a crop of winter salads, such as rocket and mizuna, and there is simply nothing better than eating fresh greens from the garden in winter.

Sweep the decks

Both the floor and shelving units will have built up debris and

compost from your work over the growing months. Sweep paths and benches, adding the spent compost to borders or the compost heap. Sort through plant labels (making a mental note to give these a good clean as another winter task), tidy up string, wire, hand tools and other gardening paraphernalia and either put away in a shed or tidy into one area of the greenhouse.

© Mark Ashbee
Sweep away the growing season’s debris, preparing benches for plants to display
This is a good opportunity to seek out any hiding slugs and snails, who will happily munch their way through your winter larder if you don’t relocate them. It’s amazing how tiny they are when young, and can squeeze into the smallest of spaces. Check the rails, the window openings and give all pots a thorough inspection before bringing them into the greenhouse. Snails and slugs often like to hide and around the hole in the base of the pot. Also check for the grubs of vine weevil, who feed on plant roots.

Create a display

Now comes the fun bit. Rather than think of your greenhouse as an overwinter store, curate your own mini glasshouse display. Lots of us have been tempted to buy more tender and exotic plants for our gardens, but many of these will not withstand freezing, or even close to freezing, temperatures. They need to be overwintered.

Use the RHS Plant Finder to check the hardiness rating of your plants. If it’s H1 then you need a heated greenhouse, if it’s H2 or 3 the plants can cope with a cool or frost-free greenhouse. Some will be fine left in the garden with some biodegradable or recycled fleece, while others will need to be brought in to a dry, bright space. Others will need additional protection if that area is unheated. Where you live will undoubtedly affect which crops and plants you can overwinter. If you don’t have a greenhouse, a sunny windowsill or conservatory can be used but keep an eye on light and water levels.

Heavy pots will need to stand on the ground, but smaller and plastic pots of plants can be placed on the benches. Try to stand these on small pieces of wood or slate to allow air flow, and give them space from each other so leaves don’t touch. This helps keep the plants healthy in the humid environment, which can cause grey mould (botrytis). Ventilation is essential for greenhouses in winter to prevent mould and dampness, and to maintain a healthy balance of oxygen and carbon dioxide for plants. Check over each plant as you take it into the glasshouse, removing any dead or decaying plant material. 

Plants that need a winter home

This depends on where you live. I am in West Cornwall, which rarely suffers heavy frosts, so I bring my scented pelargoniums into the greenhouse, still in flower, and some pots of borderline tender Osteospermum. Tender succulents such as Aeonium can remain outside in coastal gardens, but even though I only live a little inland from the sea, I’m not taking any chances with my precious new purchases. The ones that are in pots have been moved into the greenhouse, pot and all. I have many planted in the ground too but have a thick layer of recycled fleece (from a furniture delivery) that I’ll tuck these up with if the unpredicatable temperatures drop.

Where you live it might not be suitable to have these plants in an unheated glasshouse, but you could consider having pots of hardy plants such as chysanthemums, cyclamen, violas, pansies and early bulbs as a display. Penstemons and salvias can often cope with an unheated greenhouse as well.

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Movable pots of tender plants can be taken carefully inside using a sack trolley
This brings me to the fact that we can no longer presume our weather will do what we think it will. Where once we had cold, snowy winters, we now have mild wet ones, but a sharp snap of frost is just around the corner. Best policy is to be prepared. Have covers at the ready, and make ‘insurance’ plants - I’ve done this with a gorgeous Bulbine frutescens, a new plant this year that I thoroughly recommend for its easy nature and pretty orange flowers.

© Mark Ashbee
Take cuttings of borderline tender plants as an insurance for any that are killed by frost in the garden
This plant is so easy to take offsets from, simply by pulling off a section and either putting straight into the ground or into pots of gritty, peat-free compost. Mine rooted and even started flowering within a couple of weeks. I have a few of these, as well as baby aeoniums, safely snuggled in my greenhouse, ready to battle it out this winter.

Before you know it, spring will be here and you’ll be able to start the coming seasons seed sowings. May it be a warm and sunny one.

About the author - Natalie Ashbee

Natalie writes for both the RHS Garden magazine and the website. She has worked as a professional gardener, planting designer, horticultural TV producer and now lives and gardens in (damp) but beautiful West Cornwall. 

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