Why Mulch Matters More Than Ever in a British Summer

British summers have a habit of being either too wet or too dry, and often both in the same month. A border that looked lush in June can be flagging by mid-July, with asters dropping their lower leaves and phlox sulking in dry shade. Mulching is the simplest, cheapest insurance against that. A generous layer of organic matter slows evaporation from the soil surface, keeps the root zone cooler during a heatwave, and smothers the annual weeds that would otherwise compete for every drop of water left. It also feeds the soil as it breaks down, improving structure so that over a few seasons your border holds moisture far better than it did before.

If you do nothing else this summer, mulch your borders after the next decent rain. That single afternoon's work will buy you weeks of grace in August.

Which Mulch Suits Your Border Soil

The best mulch is the one you can get hold of locally and afford in bulk. For most herbaceous borders, a well-rotted garden compost or leafmould is hard to beat, because it feeds as well as protects. Composted bark is longer lasting and looks tidy around shrubs. A few points worth remembering:

• Garden compost and leafmould suit heavy clay and most perennials, including asters, which appreciate the extra organic matter.
• Composted bark is excellent for shrub borders and acid-lovers, and knits together well on slopes.
• Spent mushroom compost is alkaline, so keep it away from rhododendrons, camellias and other ericaceous plants.
• Gravel or grit works well around alpines, Mediterranean herbs and silver-leaved plants on free-draining soils.
• Avoid fresh woodchip, thick layers of grass clippings, and unrotted manure, all of which can sour the soil or rob it of nitrogen.

The Golden Rule: Mulch Straight After Rain

Mulch spread on bone-dry soil can do more harm than good. A thick layer over dusty ground simply sheds water at the edges and keeps the dry soil underneath dry. Before you start, push a trowel in. If the top 10cm or so is damp, carry on. If it is powder dry, water the border thoroughly first, let it settle overnight, then mulch the following morning.

This is why rain is the trigger. Straight after a good soaking, the soil is charged with moisture and the mulch locks it in. Two or three centimetres of rain is ideal. If a dry spell is forecast and no rain appears, water deeply once, then mulch the next day rather than waiting indefinitely.

Applying It Without Smothering Your Plants

Depth matters, and so does placement. In an established border, 5–7cm of organic mulch is plenty. Around shrubs and roses you can go to 7–10cm, but around low-growing perennials such as hardy geraniums, heucheras and young asters, keep it to 2–3cm.

The single most important rule is to keep mulch clear of stems and crowns. Leave a gap of roughly 8–10cm around each plant, and never bury the point where stems meet roots. Crown rot is a real risk in a damp British autumn, and asters, penstemons and lupins are especially vulnerable. Push the mulch back with your hands as you work, and never pile it against the trunks of trees or shrubs.

Asters, Copper Foliage and the Summer Border Picture

Mulch is not purely functional. A dark, even layer of composted bark sets off copper-toned foliage beautifully, and gives the border a calm, settled look in the weeks before the asters open. Physocarpus opulifolius 'Diabolo', Cotinus 'Grace', bronze fennel and Heuchera 'Caramel' or 'Peach Flambe' all glow against a rich brown surface. Come September, the purple and lilac of Michaelmas daisies, Symphyotrichum novi-belgii and S. novae-angliae, sits perfectly against that backdrop.

Asters are thirsty plants and prone to powdery mildew when stressed, so mulch generously and water at the base rather than over the foliage. A damp root run and dry leaves is exactly what they want. Where space allows, pair them with sedums, heleniums and ornamental grasses for a border that carries colour from late summer right through to the first frosts.

Seasonal Border Care Through to Autumn

Mulching is not a one-off task. A few simple habits will keep the border in good order for the rest of the season:

• Check the layer every two to three weeks and top it up where it has thinned, especially after heavy rain or a spell of strong sun.
• Watch for slugs and snails, which shelter in damp mulch; deal with them early while plants are young and tender.
• In prolonged drought, water deeply once a week rather than little and often, so moisture reaches the roots beneath the mulch.
• Liquid-feed asters with a potassium-rich feed as buds form, but avoid high-nitrogen feeds that encourage soft growth.
• As autumn arrives, pull mulch back slightly from crowns so winter wet cannot sit against them.

Do this well and your borders will sail through the hottest fortnight the summer can throw at them, with the soil beneath still cool, dark and full of worms.