Feeding and watering clematis: what they need and when

Clematis are not difficult plants, but they are thirsty ones, and the large-flowered hybrids in particular repay a little attention to feeding. Understanding what they need, and when, prevents many of the commonest complaints: poor flowering, mildew and sudden collapse.

Start with the soil

Clematis grow best, as the RHS puts it, in deep, fertile, moist but well-drained soil. Get that right at planting time and much of the feeding takes care of itself. In fertile ground, the RHS says regular feeding isn't needed. Instead, it recommends spreading a 5–7.5cm layer of organic matter, such as leaf mould, garden compost or well-rotted manure, around the base each spring, keeping it clear of the stems to avoid rot. The mulch feeds the soil, holds moisture and keeps the roots cool.

Feeding clematis in the ground

Not every clematis needs the same treatment. The British Clematis Society draws a useful distinction:

• Large-flowered hybrids are heavy feeders. The Society suggests a high-potash fertiliser in spring and again a few months later, which it says helps ripen the stems and improves flower colour.
• Small-flowered types need less. The Society suggests a spring feed of blood, fish and bone, and an autumn feed of bone meal.

Why potash rather than nitrogen

Potassium (potash) supports flowering, while nitrogen pushes leafy growth. The British Clematis Society warns that overfeeding small-flowered clematis produces plenty of foliage but poor flowering. Tomato feeds are a widely available high-potash liquid fertiliser. For granular feeds, check the N:P:K ratio on the pack and choose one where the K figure is relatively high.

If a clematis won't flower

The RHS says the most common reason is unsuitable growing conditions: soil that is too dry or too wet, or the top of the plant in deep shade. Pruning at the wrong time comes next (see our guide to clematis pruning groups). If the site and pruning are right, the RHS suggests applying sulphate of potash in late winter or early spring.

Feeding clematis in containers

A clematis in a pot can only draw on the compost you give it, so feeding becomes essential. The RHS recommends an organic, general-purpose liquid fertiliser through spring and summer, at the dose and frequency on the pack. The British Clematis Society suggests that large-flowered types get a high-potash liquid feed at half strength every fortnight from spring to late summer. Each spring, the RHS advises removing the top 5cm of compost and replacing it with fresh, and in late autumn the Society suggests a little bone meal and a thin mulch. Our guide to clematis in containers covers pots and compost.

Watering: deep rather than often

New plants

Newly planted clematis need watering in dry weather for their first few seasons, says the RHS. The British Clematis Society recommends watering at least once a month in the first year, and more often in hot, dry spells.

Established plants

Established clematis in good soil rarely need regular watering, according to the RHS, except on very free-draining soils or in long, hot, dry spells. Plants against walls and fences are a common exception, because the soil there stays dry; check them more often.

How much

A quick splash wets only the surface and encourages shallow roots. The RHS advises watering deeply, to about 30cm down; in the ground, that may take at least four watering cans per square metre. In a dry spell, one thorough soak is worth far more than several light sprinkles. Water in the early morning or evening, when less is lost to evaporation, and aim at the soil rather than the leaves; splashing water is one way fungal spores spread.

Containers

Pots dry out quickly. The RHS recommends watering container clematis most days from April to October and checking them in dry spells through the rest of the year. The British Clematis Society notes that a sheltered, south-facing pot may need watering twice a day in hot weather, but also that clematis don't tolerate sitting in waterlogged compost, so good drainage is essential.

Why consistency matters

The British Clematis Society points out that alternating between soaking and complete drying of the root area stresses clematis and makes them prone to powdery mildew. It also notes that wilt usually strikes plants already stressed by lack of water at the roots (see clematis wilt). Mulching, shading the base and watering before the soil dries out completely all help to keep conditions steady.

Keeping roots cool

• Shade the base with low-growing plants, a few flat stones or pebbles.
• Mulch each spring, keeping the mulch clear of the stems.
• In pots, top the compost with grit and stand other pots on the sunny side.
• Avoid planting tight against walls, where the soil is driest.

Common mistakes

• Using a high-nitrogen feed, such as a lawn fertiliser, and getting leaves instead of flowers.
• Watering little and often, so only the top few centimetres of soil are ever moist.
• Piling mulch against the stems, which can lead to rot.
• Forgetting plants against walls, where the soil stays dry even after rain.
• Letting pots dry out completely, then flooding them.
• Feeding harder when the real problem is the site or the pruning; fix the cause first.

A simple year

• Late winter and early spring: mulch with organic matter; give large-flowered hybrids a high-potash feed and small-flowered types a light general feed; top-dress containers. If flowering was poor last year, consider sulphate of potash.
• Spring to late summer: water deeply in dry spells; feed container plants regularly; give large-flowered hybrids a second high-potash feed a few months after the first.
• Autumn: bone meal for small-flowered types and container plants, as the British Clematis Society suggests; water pots less as growth slows, but don't let them dry out.
• Winter: check pots during dry spells and keep drainage holes clear.