Clematis

Light and exposure

Clematis wants its head in full sun or bright partial shade, with at least five to six hours of light a day to flower well. A wall that catches the morning and midday sun suits it perfectly, provided you never neglect what happens at ground level.

Its roots, on the other hand, hate direct heat and drying out. Mulch generously at the base or plant a low perennial in front of it, a hardy geranium, a hosta, or simply a few flat stones, to keep the roots cool and shaded while the foliage climbs towards the light.

Watering

The first two years after planting are decisive. Clematis builds a deep root system and needs soil that stays consistently moist without ever being waterlogged. Water generously once or twice a week in summer rather than a little every day, the water needs to reach deep to encourage the roots downward.

Once well established, clematis copes better with dry spells, but a plant in a pot or against a wall sheltered from rain stays vulnerable throughout its life. A thick mulch at the base clearly reduces watering needs and protects the shallow roots.

Soil and repotting

Clematis likes soil that is fresh, rich and well drained, ideally neutral to slightly alkaline. Soil that is too acidic slows its growth, a yearly dose of well rotted compost and, if needed, a little lime or wood ash are usually enough to correct that.

Avoid heavy soil that stays waterlogged in winter at all costs, roots rot quickly in airless ground. In clay or clay-limestone soil, which many gardens have, plant with the rootball slightly raised and work in compost and some coarse sand to open up the structure.

Temperature and humidity

Most large-flowered clematis are perfectly hardy, down to around -25 °C, which means USDA zones 4 to 9, and sail through the winter outdoors without any protection once mature. Clematis montana does not go quite so low, more like zones 6 to 9, and evergreens of the Clematis armandii type want a mild winter. First year plants, however, benefit from a good mulch at the base over winter while the roots settle in.

The real risk is not winter cold but late frosts, which can scorch tender young shoots once growth has restarted. Keep a fleece within reach until your local frost season is properly over.

Pruning and upkeep

Pruning is what puts off most beginners, even though the rule is simple once you know your group. Before the secateurs come out, work out which group your clematis belongs to, the plant label usually says so, otherwise you can work it out from the flowering time and the species.

Group one flowers in spring on the previous year’s wood, this is Clematis montana and the small flowered spring clematis. Prune these lightly, just to tidy the shape, straight after flowering, never during winter dormancy or you cut off next year’s flowers. Group two covers the large flowered types that bloom in late spring and early summer and often flower again more modestly in late summer, classic hybrids such as Nelly Moser. Prune these lightly in late winter, before the buds swell, keeping the framework and removing only dead wood and stray stems. Group three flowers in summer and autumn on the current year’s growth, viticella and jackmanii are the typical examples. These can be cut back hard in late winter, to around thirty centimetres from the ground, without any worry, they come back vigorously and will still flower the following summer.

Pests and diseases

The great scourge of clematis is wilt, caused by a fungus in the genus Calophoma, once known as Ascochyta clematidina. A whole stem collapses suddenly, sometimes within days, while the rest of the plant looks perfectly healthy. It looks dramatic and it worries people, but it is almost never fatal.

Cut the affected stem right down at ground level, or a little below, and bin that wood rather than composting it. The plant very often comes back from the crown or the roots within a few weeks. Planting the rootball a few centimetres deeper than it sat in its pot, burying a node or two, helps a great deal with recovery after an attack, since the plant then has backup buds underground. Slugs also happily attack tender young shoots in spring, protect them until the growth has hardened off.

Propagation

Layering is the simplest and most reliable way to multiply a clematis that is already established. Bend a flexible stem down into a shallow trench in the soil, leaving the tip poking out, hold it down with a stone or a peg, and cover it with soil at a node. The layer roots within a season, and can then be separated from the parent plant.

Cuttings also work but need more care, take a semi ripe cutting at a node in high summer, with a single pair of leaves cut in half, and keep it in a humid, enclosed spot until it roots. Sowing is only useful for species clematis, hybrids never come true from seed.

Toxicity

The fresh sap is irritating to skin and mucous membranes on contact, and the whole plant is toxic if eaten, causing digestive upset such as mouth irritation, nausea and vomiting. Wear gloves when pruning and keep children and pets away from the fresh sap.

Month-by-month care calendar

Green Hub tips

• Always work out your clematis’s pruning group before reaching for the secateurs, group one, two or three. Getting it wrong is the number one cause of clematis that never flower.
• Follow the golden rule: head in the sun, feet in the shade. Mulch the base or plant something low in front of it to keep the roots cool.
• Always plant the rootball a few centimetres deeper than it sat in its pot, it helps recovery after wilt and encourages new shoots from the base.
• If a stem suddenly collapses in summer, do not panic, that is clematis wilt. Cut it back to ground level, the plant almost always regrows from the crown.