Why coastal borders need mulch more than most

A garden within earshot of the sea asks a lot of its soil. Salt-laden winds strip moisture from leaves, drying them faster than roots can replace it, and the same winds batter the soil surface, lifting fine sand and blowing away the thin layer of organic matter you have worked so hard to build. On top of that, many seaside plots sit on free-draining sandy or shingly ground that lets water and nutrients disappear almost as soon as they arrive.

Mulching is the simplest, cheapest way to fight back. A good layer on the surface slows evaporation, keeps soil temperatures steadier through frosty nights and baking afternoons, smothers weed seedlings before they get a grip, and stops rain and spray from splashing salt onto low foliage. Organic mulches go further, breaking down slowly to feed the soil and hold onto moisture and nutrients that would otherwise leach straight through sand.

Choosing a mulch that will stay put

The wind that defines a coastal garden will happily redistribute anything light. Weight and texture matter as much as richness, so match the material to the site.

• Composted bark or garden compost: the reliable all-rounder for mixed borders. Choose a well-rotted, dark product rather than fresh chippings, which rob nitrogen as they break down.
• Well-rotted manure: ideal for hungry plants such as roses, hydrangeas and fuchsias. Never use fresh manure – it scorches roots and smells dreadful in a small garden.
• Leafmould: excellent for woodland-style planting, but light enough to blow about on an exposed site. Damp it thoroughly as you spread it.
• Gravel or grit (5–10mm): the best choice for lavender, thyme, sea thrift, sedums and other silver-leaved sun lovers. It reflects heat upwards, keeps the crown dry and does not budge in a gale.
• Seaweed: a traditional coastal mulch, rich in potassium and trace elements. Rinse it well to remove excess salt, spread it thinly, and only collect where it is plentiful and permitted.
• Shingle or pebbles: effective and wonderfully in keeping with a seaside plot, though it can get uncomfortably hot and is awkward to plant through later.

Avoid cocoa shell, which is risky around dogs and blows about, and skip peat-based products entirely.

Depth: the difference between shelter and suffocation

Too little mulch does nothing; too much smothers roots and encourages stems to rot. Aim for 5–7cm of organic material on established borders, and 3–5cm of gravel or grit around alpines and Mediterranean herbs. Around small, low-growing plants, 2–3cm is plenty.

Whatever you use, keep it clear of stems and trunks. Leave a gap of roughly 5–10cm around the base of shrubs and perennials, and never mound mulch up against the crown of lavender, rosemary or thrift – damp trapped against the stem is the quickest way to lose them. Spread mulch onto damp soil, ideally after rain, and water the border beforehand if the ground is dry. Mulching dust-dry soil simply locks the drought in.

Timing through the coastal year

In most seaside gardens, mid to late spring is the key moment. Wait until the soil has warmed and weeds have started moving, then clear them thoroughly before you spread. This gives plants the whole growing season to benefit and keeps moisture locked in for summer.

A second, thinner top-up in late summer helps plants through dry spells and into winter. Be cautious about autumn mulching on heavy or cold clay soils – an insulating layer can keep the ground cold and wet, which suits few coastal plants. After a winter of gales, check the layer: organic mulches thin out, and gravel can shift. A quick rake and top-up in March restores the whole border.

Mulching around the plants that define a seaside garden

• Lavender, rosemary and thyme: gravel, 3–5cm, kept well away from the crown. Never compost or manure, which gives soft, floppy growth and short-lived plants.
• Ornamental grasses: a modest layer of composted bark. Avoid burying the centre of the clump.
• Roses and fuchsias: a generous 7cm of well-rotted manure or compost in spring, ringed around the base.
• Hydrangeas and other thirsty shrubs: compost plus a top-dressing of leafmould to hold moisture through summer.
• Sea thrift, sedums and alpines: a thin collar of grit to keep foliage dry and rot at bay.
• Hedges and windbreak planting: mulch in a wide band either side to conserve water and keep grass competition down.

Small habits that make a big difference

Mulching works best as part of a routine. Weed first – mulch over weeds simply hides them. Water new plantings before and after spreading. Replenish annually, because organic layers shrink by roughly half each year. On the most exposed sites, lay a few evergreen cuttings or a light net over fresh leafmould until it settles.

Do this once or twice a year and you will notice the difference quickly: fewer weeds, healthier growth, and plants that shrug off a salty gale instead of sulking for a fortnight. Few jobs in a seaside garden repay an hour's work so generously.