What can I grow in the greenhouse this August?

Greenhouses and cold frames are often associated with spring sowing or winter protection, but August offers its own opportunities for the experienced gardener. With many summer crops still producing, it’s a useful time to carry out maintenance, plan ahead, and make the most of what can still be sown and grown under cover.

Keep Plants Healthy at Peak Production

Tomatoes tend to dominate the greenhouse at this time of year. As they reach peak production, it’s worth removing older leaves from the lower part of the plant. This allows better airflow and helps sunlight reach the ripening fruit. Keep an eye on the main stem and leaf joints to avoid accidentally removing fruit trusses. Cucumbers and aubergines may also need attention. Yellowing or speckled leaves are sometimes a sign of red spider mite, which can become a problem in warm, dry conditions. Increasing humidity by damping down the floor or lightly misting the foliage may help. In more severe cases, beneficial insects can be introduced if your setup allows. Dead or diseased material should be removed promptly. Clean up any fallen leaves or fruit from the greenhouse floor to reduce the risk of mould and pest problems later in the season.

Sow Now for Autumn and Winter Harvests

There is still time to sow several useful crops under cover. Lettuce, especially when grown for cut-and-come-again leaves, does well in trays or shallow containers. A mix of leftover seed packets can give a good range of leaf shape and flavour. Sown thickly and harvested regularly, these can produce for several weeks. Rocket, mustard leaves and mizuna are also reliable at this time of year. They grow quickly, making them suitable to start in the greenhouse or a cold frame. Sow three to five seeds per module if space is limited, or direct into greenhouse borders if available. Water well to avoid bolting, especially during warm spells. Spinach is another useful option. Sown now, it will produce a decent crop in autumn, slow down over winter, then resume growth in early spring. Use a variety suited to overwintering. Protect it from birds and weather using fleece or mesh where needed. Pak choi and winter purslane can also be sown towards the end of August. These crops respond well to the protection of a cold frame or greenhouse border, giving regular harvests into the colder months.

Raise Strong Seedlings for Autumn Planting

Cold frames are a good place to start seedlings for later transplanting. If you’ve cleared early potatoes or other crops, the space can be reused for modules of chard, kale, or spring cabbage. Even if you don’t plan to grow these under cover long term, starting them in a frame reduces slug damage and improves germination rates. Frames also offer a sheltered environment for hardening off late summer sowings or growing on smaller plants that will go into the beds later in autumn.

Refresh Your Borders and Make Every Drop Count

If your greenhouse has soil borders, now is a good time to refresh them for autumn use. Once crops like tomatoes have finished, remove the plants, dig out the rootballs, and add a top-up of compost, or alternatively you can grow around the base of tomato plants. The warmth retained in the soil will help germinate late summer sowings and encourage root development. Beetroot and fennel can both be sown now for smaller autumn harvests, these may not reach full size but can still be useful. Sow thinly and keep moist to get the best from them. Some gardeners grow these in long trays or containers within the greenhouse, which is a good alternative if border space is limited.

With many parts of the UK seeing drier summers, it’s worth reviewing how you use water in the greenhouse. One simple method is to push a plastic plant pot or length of pipe into the soil next to each plant and water into that rather than over the surface. This helps direct moisture to the roots and reduces evaporation. A mulch of compost or well-rotted manure also helps retain moisture. Around courgettes and cucumbers, this can reduce how often you need to water, and if you are collecting rainwater, make sure your water butts are secure and in good working order, especially before autumn rains arrive.

Save Seeds and Plan for Next Season

If you’ve grown open-pollinated tomatoes this year, August is a good time to save seed. Choose fruit from healthy plants that performed well, scoop the seeds into a glass with a little water, and leave for two or three days until a light mould forms. This breaks down the gel coating on the seed. Rinse and dry on a plate, not kitchen paper, as the seeds can stick and become difficult to separate. Once the seeds are completely dry, store them in a labelled envelope. Use the quieter moments to make a note of which crops did well under cover this year. It can be easy to forget by the time seed catalogues arrive, and knowing which varieties cropped heavily or resisted disease makes planning for next year more efficient.

Rotate Crops and Restore Greenhouse Soil

Rotating crops in a greenhouse or cold frame is trickier than in open ground but still worth doing where possible. Try not to grow tomatoes or cucumbers in the same position every year, especially in soil borders, unless you are growing grafted veg plants. Grafted plants cope with soil-borne pathogens better than those from seed alone, meaning you’ll be able to keep growing your favourite tomato in the same place year after year. A simple system of using containers for some crops and borders for others helps reduce the risk of disease build-up if you don’t want to grow grafted plants. Where space is tight, alternating between heavy feeders like tomatoes and leafy greens such as spinach or lettuce gives the soil a break and makes good use of available nutrients. Then in autumn, when I’ve removed my plants, I like to add a good amount of well-rotted manure to work its way into the soil over winter.

Prepare Your Greenhouse for Autumn

By the end of August, it is worth starting to plan for the transition into autumn. As crops finish, remove them promptly and tidy up support structures, old strings, and empty containers. Check greenhouse doors, vents, and glazing. Repairs are easier now than in October when the weather turns. If you use bubble wrap or insulation in winter, get it ready now. Cold frames may need the lids cleaned or the hinges repaired. Making small adjustments now saves time later when the days are shorter and conditions less inviting.

August under glass is about careful maintenance, timely sowing, and forward planning. A well-managed greenhouse or cold frame can continue producing long after the outdoor beds have slowed down. By removing older foliage, sowing fast-growing crops, and managing water sensibly, you can keep things productive into autumn and beyond. Using this period effectively also gives you a head start on the next growing season, whether that’s saving seed, rotating crops, or simply making sure everything is clean and ready for cooler weather.

About the author

Rob is an experienced gardener specialising in growing fruit, vegetables and other edible plants. He writes for several British gardening magazines, appears regularly on television and radio, and shares his passion for growing his own produce on social media at @robsallotment

Rob also works with well-known horticultural companies, including Suttons, Thompson & Morgan, Dobies and The Organic Gardening Catalogue. Together, he develops new and exciting varieties of fruit, vegetables and flowers, which are available in garden centres across the UK and Europe

When he’s not writing or in front of the camera, Rob can usually be found in his kitchen garden – whatever the weather and almost always accompanied by his faithful dogs, Nipper and Reggie.

Photos & Text: Robert Smith / August 2025

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