Liz’s Allotment Life: Knowing when to sow in a heatwave

As early crops finish, it’s tempting to fill every empty space immediately. But with relentless heat and dry soil lingering across much of the UK, deciding what to sow – and when to hold off – requires a more careful approach. Balancing my work in the World Food Garden at RHS Wisley with the realities of my own plot, managing a late-summer heatwave is all about making realistic choices, protecting vulnerable roots and making the most of the summer glut.

A word of caution – planting in extreme heat

All of what follows comes with a major caveat: on my allotment, I have not sown or planted anything for five or six weeks, and don’t intend to until the rain finally comes. The last additions to my beds were celery and celeriac, but during a brief five-day absence in recent hot weather, some of them shrivelled up and died.

Celery showing severe wilting and heat stress after five days without watering during hot, dry weather

Newly germinated seeds and establishing transplants require consistent watering to support early root development, which I cannot maintain on my allotment right now. Fortunately, more mature crops such as Swiss chard, beetroot, courgettes and tomatoes are still producing abundant harvests. There will be plenty of time come September for direct, successive sowing of hardy winter salad crops.

What vegetables you can sow in an August heatwave

If conditions are cooler or you can sustainably provide reliable watering, there are still crops that can be sown in August for autumn and winter harvests. At RHS Wisley, I’m continuing to sow and transplant in the World Food Garden because the crops can be watered daily. On my allotment, however, I’m holding off until the weather breaks.

• Spring cabbages ( Brassica oleracea ): cultivars such as ‘Wintergreen’ do not form the dense, firm heads of other cabbages, but instead produce tender, nutrient-rich leaves that provide a fresh crop when other vegetables are running out.
• Chicory ( Cichorium intybus ): a neglected autumnal crop valued for its distinct bitter leaves. Cultivars such as ‘Rossa di Verona’ offer striking foliage, while ‘Witloof’ produces thick taproots that can be forced in the spring to produce pale, tender shoots called chicons.
• Turnips ( Brassica rapa ): a crop that is best harvested when small for peak sweetness, although seeing how large you can grow them is also a fun challenge. I like to multi-sow turnips – sowing seeds in small clusters so three or four grow together in a clump – to maximise space and yield. The cultivar
  

  Gardeners often use the word variety when referring to a specific plant, but the correct botanical term is 'cultivar'. Whichever word you use, it means a distinctive plant or plants, given a specific cultivar name and usually bred to enhance certain characteristics, such as flower or fruit size, colour, flavour or fragrance, plant size, hardiness, disease resistance, etc. Additionally, it is worth knowing that, botanically, variety has another meaning - it refers to a naturally-occurring distinct plant that only has slight differences in its looks. For example, Malva alcea var. fastigiata differs from typical plants by having an upright habit.

  ‘Oasis’ is great for a white, slightly sweet choice.
• Beetroot ( Beta vulgaris ): it’s nearing the end of its sowing season but in milder areas quick-maturing varieties may succeed. Interestingly, when the soil is too hot, beetroot seeds can remain dormant, delaying germination
  

  Germination is the process by which a seed develops into a new plant. It starts with the seed coat absorbing water, expanding and splitting, which allows oxygen and water into the seed so its food reserves can be broken down and used by the embryo for growth. The first root (called the radicle) pushes into the soil to anchor the new plant in place and start absorbing water and nutrients. The first shoot (called the plumule) then grows upwards, out of the soil, towards the light.

  until conditions are more favourable. Beetroot I sowed back in mid-June didn’t germinate during the drought, but watering the celery planted in the same location cooled the soil and appears to have triggered a few seeds to germinate in the last fortnight. I might still get a late crop from them.

Beetroot growing in dry soil on the allotment, with healthy foliage despite prolonged hot, dry weather

Crops to avoid sowing in high heat

• Heat-sensitive crops: spinach and lettuce struggle in summer heat. High soil temperatures can suppress germination, while heat can cause early bolting
  

  When a vegetable plant starts flowering and forming seeds, often prematurely, making the crop unusable – salad leaves may turn bitter and root crops fail to swell. Adverse weather or changes in day length can cause bolting in a wide range of vegetables, including lettuces, onions, carrots and other root crops.

  , making the leaves bitter before you can get a decent harvest.
• Certain overwintering crops: Leeks, Brussels sprouts and purple sprouting broccoli have a long vegetative growth period, so they need to be started earlier in the year to provide a good crop this autumn.
• Tender crops: warm-season species like squash, French beans and tomatoes are sensitive to cold and take months to set and mature fruit, so they are past the point where you could expect a harvest from a fresh sowing.

Crop of the Week: Courgettes

Courgettes could easily have taken top spot at any point over the last few months, but this week’s bumper harvest sent me searching for creative ways to manage and preserve the late summer glut. Productive courgettes will continue setting new flowers and fruit as long as they are picked regularly.

Freshly harvested courgettes from the allotment, showing the abundance of a productive late-summer crop

To make the most of my plot surplus, I’ve been experimenting with batch-cooking and freezing an allotment-style ratatouille. Along with a glut of courgettes and sun-ripened tomatoes, the dish is a great way to use overgrown spring onions, such as my ‘White Lisbon’ , which have swollen beyond salad size. I’ve also added beetroot, combining the mild, candy-striped ‘Chioggia’ with deep red ‘Detroit 2’ . Sautéed together and portioned into containers, it’s an easy way to lock in summer flavours for winter meal prep.

Allotment surplus ratatouille

Ingredients (can adjust depending on what you have):

• 2–3 medium courgettes, coarsely diced
• 4 tomatoes, roughly chopped
• 2 small beetroots, such as ‘Chioggia’ or ‘Detroit 2’, diced
• 1 bunch overgrown spring onions such as ‘White Lisbon’ or 1 large onion, sliced
• 2 cloves garlic, finely sliced
• Fresh thyme, olive oil, sea salt and black pepper

Method:

1. Heat olive oil in a wide pan over medium heat. Add the spring onion first, then add garlic, sautéing until soft and translucent
2. Add the diced beetroot, cooking for approximately 5 minutes
3. Stir in the diced courgettes and chopped tomatoes
4. Season with salt, pepper and fresh thyme leaves. Cover and simmer gently over a low heat until the vegetables are tender but hold their shape
5. Serve hot with crusty bread, or allow to cool fully before portioning into containers to freeze as a ready-to-reheat winter dish

Coming up on the blog

Last week, I escaped the summer heat for a research trip to Norway – where 14°C and drizzle was wonderful. I was visiting some gardens to learn more about perennial

Perennials are plants that live for multiple years. They come in all shapes and sizes and fill our gardens with colourful flowers and ornamental foliage. Many are hardy and can survive outdoors all year round, while less hardy types need protection over winter. The term herbaceous perennial is used to describe long-lived plants without a permanent woody structure (they die back to ground level each autumn), distinguishing them from trees, shrubs and sub-shrubs.

vegetables and their sustainability potential.

Thanks to their deep, permanent root systems, some perennial crops can offer greater resilience in dry conditions and require fewer inputs than many annual crops. I’m still digesting what I learned, but I’ll be sharing some of the most interesting ideas from the trip in a future update.