What to Sow, Plant, and Do in August: Your Month-by-Month Guide for a British Garden

The borders look almost too good to last. That particular richness you get in a British August, the dahlias finally hitting their stride, the sweet peas climbing higher than you planned for, the courgettes doing what courgettes always do and producing a fruit the size of a small dirigible the moment your back is turned. It is generous and a little overwhelming, and it can lull you into thinking the season is still young. It is not. Beneath all that abundance, the garden is already tilting. The evenings are drawing in by a few minutes each day, the quality of light is shifting almost imperceptibly, and the decisions you make this month will determine whether your garden moves into autumn with any kind of confidence or simply collapses into it.

August is the month that requires a certain double vision: you must manage what is happening right now while beginning to prepare for what comes next. That is not easy when you are drowning in runner beans and the lawn needs attention, but the gardeners who get it right tend to treat this month less as peak summer and more as the opening act of autumn.

Sow Now for Spring Colour

It feels counterintuitive to be thinking about spring while the garden is in full summer voice, but August is one of the few genuinely useful months for sowing hardy annuals. Cornflowers, ammi, larkspur, calendula, nigella, and the annual scabious Scabiosa atropurpurea can all be sown now directly into a prepared border or into modules for transplanting in October. Plants that overwinter as small rosettes establish a root system that pot-sown spring seedlings simply cannot match, and the flowering period tends to be earlier and longer as a result.

I sow cornflowers every year in early to mid-August and have never once regretted it.

Sow thinly, water in gently, and if you are using modules, keep them somewhere the slugs cannot find them with quite the same ease as they would outdoors. A cold frame is useful here, though a sheltered bench works well enough. The seedlings will be undemanding through winter; they just need to not be eaten, waterlogged, or treated as a nursery for vine weevil.

Japanese onion sets can also go in now, or from seed if you prefer to grow from scratch. Varieties such as Radar and Electric are reliably hardy in most UK conditions and will provide a harvest in June or July of the following year, which is genuinely useful when the stored main-crop onions from this year have long since run out. Sow seeds into modules or direct into a well-prepared bed; sets can go in from mid-August onwards. Give them a sunny, well-drained position and they will require almost nothing from you until late winter.

Lift and Divide Bearded Irises

If you have been meaning to divide your bearded irises for a while and have been putting it off, August is the month when it actually needs to happen. The six to eight weeks following flowering is the ideal window, and the warm soil encourages roots to re-establish quickly before the colder months arrive. Left much beyond September and you are asking the plant to settle into cold, wet ground with no real incentive to do so.

Lift the clump carefully with a fork, shake off the excess soil, and cut away any old, exhausted rhizomes from the centre of the clump. These woody sections contribute nothing and can harbour disease. The pieces you want to replant are the fresh outer rhizomes, ideally with a healthy fan of leaves attached. Trim the leaves back to roughly a hand's length to reduce wind rock, and replant with the rhizome sitting at or just above the soil surface. Burying a bearded iris is one of the more reliable ways to prevent it flowering, so resist the instinct to tuck it in too deeply.

Replant into a sunny, well-drained spot and resist the urge to water too heavily. They are more stoic than they look.

What to Harvest, and What to Leave

The harvest list in August is considerable. Sweetcorn is ready when the silks have turned brown and a pierced kernel produces a milky sap rather than a clear one. Maincrop potatoes can be lifted once the haulms have died back, though it is worth leaving them in the ground for a week or two after the foliage dies if conditions allow, to give the skins time to set. Climbing and dwarf French beans, courgettes, cucumbers, and tomatoes all need regular picking not as a courtesy but because unpicked fruit signals to the plant that its reproductive work is done. A plant that believes it has successfully set seed will slow production considerably. Keep picking, and it keeps flowering.

What to leave: pumpkins and squash, which need every remaining week of warmth and sunshine they can get. If you net them off the soil and put something under each fruit to prevent rot, that is sufficient. Garlic, if it is already hanging somewhere dry, requires nothing further from you. Leeks and brassicas planted out earlier in summer are simply growing now; they do not need much beyond keeping weed-free and watching for cabbage white butterfly eggs on the undersides of leaves.

August is also the month to stop feeding tomatoes with a high-nitrogen feed if you have been using one, switching instead to a high-potash feed such as a liquid tomato food. Nitrogen at this stage pushes leafy growth at the expense of fruit ripening, which is the wrong priority with the season shortening.

Lawns: Water Less Than You Think

The lawn almost certainly looks worse than it is. Grass that has turned yellow or straw-coloured in a dry August will recover on its own once the rain returns; it has not died, it has simply gone dormant, which is a reasonable response to drought. Watering a domestic lawn through a British August is generally an inefficient use of time and water, unless you are managing a fine ornamental lawn for a specific occasion.

What the lawn does benefit from this month is aeration if the soil has compacted, and a light trim with the blades set no lower than about four centimetres. Scalping the lawn in dry conditions is, to put it gently, inadvisable. If moss has been a problem this year, now is not the time to treat it; wait until September when conditions are better suited.

Borders: Where to Water and Where to Trust the Soil

Established shrubs and perennials in a decent loam or clay soil should manage August without supplementary watering in most years. Clay holds moisture far longer than its reputation suggests, and plants that have been in the ground for a full season or more will have roots deep enough to find water you cannot easily reach with a hose. Water applied to the soil surface on a hot August day loses a significant proportion to evaporation before it reaches the root zone.

What genuinely needs water: anything planted this spring that has not yet fully established, containers (which require daily attention in warm weather and will not forgive you for forgetting), and vegetable beds carrying crops that are actively forming fruit. Focus your efforts there and trust the rest.

If you have mulched your borders in spring, the soil beneath should still be retaining moisture reasonably well. If you have not, a layer of well-rotted compost applied now will help, though it will work harder still if you water the ground first before laying it down.

A Few Other Things August Asks Of You

Deadhead dahlias, sweet peas, and roses consistently. Not because it tidies the garden, though it does, but because deadheading directly extends the flowering period. A sharp pair of bypass secateurs makes clean cuts that heal quickly; a blunt blade tears rather than cuts, and the difference matters more than gardeners sometimes give it credit for.

Clip formal hedges, including box and yew, for the final time this season. Do this before the end of August in most parts of the UK; clips after this point may encourage soft new growth that will not have time to harden before the first frosts.

Collect seed from any plants you want to save for next year: sweet peas, poppies, aquilegia, and most hardy annuals. Wait until the seedheads are dry and papery before harvesting, then store in labelled paper envelopes somewhere cool and dark. Heritage varieties are particularly worth saving, because the named variety you grew this year may not be available next season, or may vary from a bought packet in ways that matter to you.

Pot up rooted cuttings of tender perennials such as penstemons, salvias, and fuchsias if you took cuttings in July. They will be better overwintered as rooted cuttings in a frost-free place than left outside to take their chances.

The garden in August asks rather a lot of you, but none of it is unreasonable. Check the brassicas for butterfly eggs. Pick the beans. Get the iris rhizomes in. Sow the cornflowers before the month slips past you. This is the work that distinguishes an October garden still flowering and holding itself together from one that simply got tired in September and never quite recovered.