August garden tasks UK — harvest, sow and autumn prep

August garden tasks UK — harvest, sow and autumn prep

August is the most rewarding month in the British garden — the harvest cascade from June and July hits its peak, the kitchen fills with produce, and the standing maintenance pace finally eases. But the second job list opens: every fortnight of August delay on autumn sowings costs three to four weeks of cropping later in the year. This guide is the RHS-aligned UK calendar for August, with the Charles Dowding cutoff dates that experienced no-dig growers use, the lavender harvest window, and the prep work for September. It continues the series from the July garden tasks and hands off to the September garden tasks; localise every date with the frost date calculator, and see every month in the garden calendar hub.

Hit the sowing window: Add your postcode to Growli and the app fires the spring cabbage and winter salad reminders against your specific climate — not a generic chart that misses the 25 August cutoff by a week.

August climate snapshot — the UK regions

August daytime highs match July's, but nights cool faster and day length drops by 90 minutes across the month. Growth slows on most crops by the third week.

The reliable pattern: hot dry spells stress newly transplanted brassicas; wet spells trigger blight and grey mould. Water consistently and ventilate greenhouses aggressively.

Sow this month — the autumn cutoff dates

August sowing windows close fast. Charles Dowding's UK-specific guidance is that before mid-month you can still sow lettuce, leaf beet, chard, endive, chicory, salad rocket, oriental leaves, spinach and collards. After mid-August the list narrows to winter salads, spring onions and spring cabbage — and spring cabbage specifically should be sown after 25 August for transplanting in September and cropping next April-May.

Sow before mid-August:

• Autumn lettuce — Winter Density, Marvel of Four Seasons, Arctic King.
• Salad rocket, mizuna, mibuna, mustards — for cut-and-come-again picking through October.
• Oriental greens — pak choi, tatsoi, komatsuna, mibuna.
• Endive and chicory — for autumn salad bowls.
• Swiss chard, perpetual spinach — for autumn and overwintering picking.
• Spinach — autumn varieties (Medania, Toscane) only; spring varieties bolt.
• Spring onions — White Lisbon for overwintering.
• Coriander — bolt-resistant autumn varieties.
• Turnips — Snowball, Atlantic for September pulling.
• Land cress, lamb's lettuce — hardy cut-and-come-again through winter.

Sow after 25 August:

• Spring cabbage — April, Pixie, Greyhound, Wheelers Imperial. Sow into modules now, transplant in late September for April-May cropping next year.
• Winter lettuce — Winter Density, Arctic King.
• Spring onions — overwintering varieties.

Sow green manures on bare beds:

• Crimson clover, Italian ryegrass, field beans, winter tares — sow direct onto cleared beds. Dig in next March to add organic matter and lock in winter nutrients. The RHS specifically recommends this for August.

Plant out — last transplant window

• Winter brassicas — plant out any remaining spring sowings of Brussels sprouts, sprouting broccoli, kale, winter cabbage by early August. Net immediately against cabbage white butterflies and pigeons.
• Strawberry runners — last realistic window for next year's crop. Use either rooted runners pegged in June or fresh plants from Marshalls, Suttons or D.T. Brown.
• Winter-flowering pansies and violas — plant for October-March display.

Tomato care — the ripening phase

By August the tomato crop is set — the work now is ripening, blight defence and water consistency.

• Stop cordon plants if you have not already; pinch the growing tip out so the plant pours energy into existing trusses.
• Strip leaves below ripening trusses progressively. By late August, indoor and outdoor tomatoes should have most leaves below the lowest red truss removed for airflow.
• Feed weekly with high-potassium tomato feed until the top truss is set, then taper off.
• Water consistently — irregular August watering causes a wave of blossom-end rot and split fruit (the RHS specifically warns about this).
• Check for blight daily — see July garden tasks for the BlightSpy workflow. If blight strikes, pick all unaffected fruit and ripen on a south-facing windowsill.
• Save seed from open-pollinated favourites (Gardener's Delight, Brandywine, Moneymaker). Scoop seeds into a jar with water; ferment for three days, rinse, dry on paper.

Lavender and herb harvest

August is the prime UK lavender window. Harvest lavender when about half the flowers on each spike are open — earlier than fully open, when the oil content is highest. Cut whole stems with secateurs, bundle in elastic bands (string slackens as the stems dry), and hang upside-down in a dry airy shed for 2-3 weeks. Trim the plant back lightly after harvest, but never cut into old woody growth — lavender does not regenerate from old wood.

Other August herb jobs:

• Take softwood and semi-ripe cuttings of rosemary, sage, thyme, hyssop, lemon verbena, salvia, mint and lavender for next year's plants. Cut 8-10 cm shoots below a leaf node, strip the lower leaves, dip in rooting hormone (optional), pot into gritty cuttings compost.
• Cut and dry herbs at peak flavour — the RHS recommends harvesting just before flowering for the strongest oils. Bunch and hang in a dry shed or dehydrate at 38°C.
• Pinch basil weekly to keep it productive; flowering basil turns bitter.

Maintain — watering, pruning and feeding

• Water deeply during dry spells — use rainwater or grey water where possible. The RHS specifically recommends this in August.
• Mulch with grass clippings or compost to lock in moisture.
• Deadhead roses, bedding and perennials weekly.
• Trim hedges for the second time of the year (avoid trimming during nesting birds — late August is generally safe).
• Prune summer-flowering shrubs as they finish flowering — buddleia, lavatera, philadelphus, weigela (if missed in June).
• Summer-prune trained apples and pears — shorten new growth to three leaves above the basal cluster (Lorette pruning, continued from July).
• Stop feeding flowering containers by late August — high nitrogen now produces soft growth that will not harden before frost.
• Earth up celery for blanched stems.
• Stop watering onions and shallots if you have not lifted them; finish lifting in early August.

Pest and disease watch — UK August

• Cabbage white butterfly caterpillars — peak hatch on brassicas. Check undersides of leaves twice a week and rub off eggs and caterpillars. Bacillus thuringiensis (Dipel) is an organic spray for heavy infestations.
• Tomato and potato blight — risk continues through August; check BlightSpy.
• Slugs — second peak of the year, particularly on lifted potato beds and seedling brassicas. Lift main-crop potatoes promptly.
• Powdery mildew on courgettes, squash and peas — by August almost universal on dry-stressed plants. Water at the base, ventilate, remove worst-affected leaves. See powdery mildew — UK guide.
• Wasps — peak nest activity. Pick plums and apples promptly; damaged fruit attracts wasps that then sting harvesters.
• Codling moth — second-generation caterpillars in apples. Hang sticky bands on trunks.
• Red spider mite — greenhouse pest in hot dry conditions; damp down twice daily.
• Botrytis grey mould on soft fruit — late raspberries, blackberries, currants. Remove infected fruit promptly.
• Box tree caterpillar (south-east England especially) — check box hedging weekly; pheromone traps reduce numbers.

Harvest now — peak month

August is the UK's biggest harvest month:

• Tomatoes — outdoor and greenhouse at peak.
• Sweetcorn — pick when the silks turn brown and a squeezed kernel runs milky white (not clear, not doughy).
• French and runner beans — pick every 2-3 days to keep production going.
• Courgettes, marrows, summer squash — every other day.
• Cucumbers — daily.
• Aubergines, peppers, chillies — first picking.
• Onions and shallots — finish lifting; cure 2-3 weeks before storing.
• Garlic — finish in Scotland and Northern Ireland.
• Maincrop potatoes — lift early varieties (Kestrel, Charlotte) by early August to avoid slug damage; main-crop varieties (Maris Piper, King Edward, Desiree) from late August onwards.
• Plums — early varieties (Opal, Czar) finish; Victoria from mid-August.
• Blackberries, autumn raspberries — start cropping.
• Apples — early varieties (Discovery, Beauty of Bath).
• Cobnuts and filberts — early picks.
• Herbs at peak flavour — basil, parsley, coriander, dill, tarragon, mint.

Order for next month — September prep

• Autumn garlic — order now from the Isle of Wight Garlic Farm, Marshalls, Suttons or D.T. Brown for October planting. Best varieties (Solent Wight, Lautrec Wight, Carcassonne Wight) sell out by early September. See when to plant garlic in the UK for the regional planting calendar.
• Spring bulbs — daffodils, crocus, alliums, hyacinths from Crocus, Sarah Raven, Peter Nyssen, J. Parkers. Tulips wait until November but other bulbs go in from September.
• Overwintering onion sets — Senshyu, Radar, Electric for September-October planting from Marshalls, Suttons, D.T. Brown.
• Overwintering broad bean seed — Aquadulce Claudia (RHS Award of Garden Merit) for October-November sowing.
• Green manure seed — crimson clover, winter tares, field beans, Italian ryegrass for September sowing on cleared beds.

Quick wins — five-minute August tasks

• Set a phone reminder for 25 August — spring cabbage and overwintering salad sowing day.
• Cut a bunch of lavender for drying when half the flowers on each spike are open.
• Take six rosemary cuttings in a tray of gritty compost; they almost all root.
• Save seed from one tomato and one bean variety you love.
• Empty water-butts to half before forecast heavy rain.
• Check brassica leaf undersides for cabbage white eggs.
• Pre-order autumn garlic if you have not.
• Pick courgettes every other day — non-negotiable.

Related articles

• July garden tasks UK — last month's job list
• September garden tasks UK — what comes next
• When to plant garlic in the UK — for September-October planting
• Powdery mildew — UK guide — for August courgette and squash watch
• How to grow tomatoes — UK guide — full season playbook
• Seed starting indoors UK — for August module sowings

Reviewed and updated by the Growli editorial team. For questions about anything here, open Growli and ask — or email hello@getgrowli.app.