What to grow in your cold greenhouse

Remove all plant material and supporting string/canes, to significantly reduce the chances of harbouring overwintering red spider mites and whitefly. Shred and compost any healthy crop remains, as long as they’re not showing signs of destructive diseases such as late blight.

Once empty, sweep the structure down to eliminate debris, remove any shading to maximise winter light levels, and give all surfaces a thorough clean. If growing in glasshouse borders, it’s also sensible to refresh the soil – I work in some compost to improve soil structure. You could also add comfrey pellets to add nutrients. 

Then, it’s important to rehydrate the soil as this can often be powder-dry following a long summer. Wet it thoroughly, fork it over, and repeat until the soil holds sufficient moisture.

You can then plant directly into the borders. Plant at wider spacings to deter rots that can be troublesome in winter, like grey mould. If you don’t have borders to plant into, re-use pots of compost or used growing bags. Again, fork these over, feed and rehydrate. If summer crops suffer from root pests or diseases, such as vine weevil, carrot fly or verticillium wilt, avoid placing vulnerable crops in them.

There’s an exciting abundance of hardier edibles that will thrive in a cool, unheated winter greenhouse.

If you’re a lover of quick-fixes then micro leaves are a great starting point – and they help use up spare seeds that would otherwise lose their viability. Tiny in size but mighty in flavour, they’re my go-to favourite and just a scattering of leaves is needed to liven up a great plate of food. Health-boosting shoots are also a brilliant – and quick – option (let’s face it, we’d all grow anything that can help fend off winter ailments). Both micro leaves and shoots can easily be ready within three weeks of sowing. 

Larger leaves can be grown as cut-and-come-again crops or left to quietly bulk up in greenhouse borders and/or containers for mature harvests. Potted perennial herbs such as oregano, thyme, Welsh onion and mint can be moved under cover to encourage the continuation of leafy growth. And hearty root veg such as carrots, turnips, daikon radish and beetroot will all thrive. 

For tender overwintering edibles, such as chillies, keep roots just moist but not waterlogged to avoid soggy, cold compost (cold dry roots are more robust). Maximise any natural warmth but have targeted, emergency heat mats in place on a thermostat for cold snaps as an insurance policy – and lend a keen ear to weather forecasts.

The key to growing successful winter edibles in your glasshouse is to give them sufficient time to bulk up, so that they can sustain themselves for cold-season harvests – so get them in as soon as possible, preferably on a warm day. Most of the listed micro leaves will germinate well given temperatures above 8˚C, so source an accurate max/min thermometer. Better still, I often find that many of my edibles have self-sown, such as land cress, lettuce, minutina (a type of plantain), purslane, leaf mallow, tree spinach and herb fennel. Simply pot these up or directly transplant them into greenhouse borders.

Water young plants when they first go in but then gauge how damp the soil is and only irrigate again when soil becomes dry (aim to keep plants just moist, not waterlogged). Keep plants warm but encourage good air movement and remove any yellowing leaves as they appear. 

On frosty nights or during longer periods of sub-zero temperatures, you can cover plants in biodegradable fleece or old blankets - but it’s not essential (especially if growing very hardy crops like kale and land cress). 

Picking by the leaf rather than wholly enables you to stretch the harvest window of your plants. Cut micro leaves after three to four weeks – more sizeable edibles may stand well into late March or early April.

There are plenty of ways to maximise the warmth of your greenhouse, depending on your situation and available services. By far the warmest part of the building is up off the floor, in the centre of the glasshouse, so if you don’t intend to add any artificial heat then position trays or boxes of your edibles here. Insulating materials such as biodegradable fleece or old blankets can be held above plants, and gaps between pots can be packed with hessian, crumpled up newspaper or straw. Hotbeds can be created if you have ample room and access to fresh animal manure. For this, a soil thermometer is very useful. 

If you don’t have a glasshouse then a polytunnel, lean-to frame, mini greenhouse, or coldframe will still offer some protection, especially if insulated/lagged with the materials described above and positioned against a sunny, sheltered fence or wall.