Clematis wilt: causes, symptoms and what to do

Few garden sights are as dispiriting as a clematis in full bud that collapses within a day or two, leaves hanging limp as if it had never been watered. The usual verdict is "clematis wilt", but this fungal disease is only one of several reasons a clematis droops, and the right response depends on which one you are dealing with.

What clematis wilt is

True clematis wilt is caused by the fungus Calophoma clematidina, previously known as Phoma clematidina and Ascochyta clematidina. According to the RHS, it survives in the soil on dead plant material, and its spores are splashed or carried onto leaves and young stems, where infection begins. An article on the Clematis on the Web site by Paul Margot adds that the spores spread by rain splash rather than wind, that moist conditions are needed for infection, and that the disease is often introduced on nursery plants that show no symptoms.

Symptoms to look for

• Sudden wilting of shoots, often just before flowering.
• Leaf spots, with leaves shrivelling and leaf stalks turning black.
• A blackened area inside the stem when you split it lengthwise. Paul Margot describes such lesions as often around 10cm long, at or just below soil level, though they can occur higher up.
• Regrowth: the RHS notes that the roots are often not killed, and healthy new shoots may appear from the base, sometimes from below ground.

Which clematis are at risk

The large-flowered hybrids are the main victims. The RHS names 'Henryi', 'Vyvyan Pennell', 'Mrs N. Thompson', 'Duchess of Edinburgh', 'William Kennett', 'Marie Boisselot' and 'Ernest Markham' as particularly susceptible, with 'Comtesse de Bouchaud', 'Jackmanii' and 'Nelly Moser' also susceptible.

By contrast, the RHS lists C. alpina, C. montana, C. orientalis, C. integrifolia, C. macropetala and C. tangutica as resistant, along with cultivars such as 'Avant Garde', 'Bonanza' and 'Confetti'. C. viticella is described as tolerant rather than resistant. If you have lost plants to wilt before, these are a sensible next choice; our guide to combining clematis with roses suggests several viticellas.

Is it really wilt?

The RHS stresses that wilting in resistant species and hybrids is very unlikely to be caused by this fungus. Before you cut anything, rule out the other common causes:

• Drought: the British Clematis Society suggests a simple test. Soak the plant thoroughly and check after an hour. If the stems are still drooping, wilt is likely.
• Slugs and snails: they can graze through young stems at the base.
• Physical damage: stems twisted by wind, snapped by a hose or caught by a strimmer collapse above the break.
• Root problems: Phytophthora root rot, honey fungus, waterlogging and, in pots, vine weevil grubs can all stop roots working.
• Stress: shallow, dry soil and hot, exposed sites. The British Clematis Society notes that wilt usually strikes plants already stressed, for example by lack of water at the roots.

Other problems that can look similar

Two other conditions are sometimes mistaken for wilt. The RHS describes clematis slime flux, in which a foul-smelling ooze comes from a dead or dying stem; it is caused by bacteria entering through damaged tissue. Powdery mildew shows as a greyish-white coating on the leaves and doesn't cause sudden collapse. Timing is a useful clue too: true wilt typically strikes in late spring and early summer, while young stems are soft and flower buds are swelling, and moist conditions are needed for infection, so it often follows a wet spell even if the symptoms appear in dry weather.

What to do if your clematis has wilt

• Cut out all wilted stems back to healthy tissue that isn't stained black, even if that means cutting to ground level.
• Dispose of the material promptly rather than composting it, so it doesn't contaminate the soil.
• Clear fallen leaves and plant debris from around the base.
• Disinfect your secateurs after use, and avoid moving infected plants or soil to other parts of the garden.
• Feed and water well through spring and early summer to support regrowth; the British Clematis Society suggests a clematis or rose fertiliser.
• Check the plant regularly and remove any new symptomatic stems promptly.

The RHS states that no chemicals are available to treat clematis wilt, so the response is cultural: hygiene, cutting back and good growing conditions.

Will it recover?

Usually, yes. Plants are not killed outright by wilt, and new shoots generally arise from the base, although the plant may take a season to rebuild and you may lose that year's flowers. Be patient: new shoots can take several weeks to appear, so don't dig a plant up in a hurry. If a very susceptible cultivar collapses year after year just before flowering, it may be time to dig it out. The British Clematis Society says the soil is not affected and another clematis can be planted in the same place; Paul Margot is more cautious, advising against replanting with another large-flowered hybrid. Choosing a resistant species or a viticella for the spot satisfies both views.

Why deep planting helps

Large-flowered hybrids are planted deeper than they sat in their pot: the RHS suggests 5–7.5cm below soil level, and the British Clematis Society 10–15cm. Buried stems carry dormant buds, so if wilt kills the top growth, the plant has a reserve from which to regrow. Paul Margot notes a downside: on old, deeply planted plants, lesions may lie well below ground and act as a continuing source of infection. Deep planting is about recovery, not immunity. Our guide to planting a clematis gives the depth for each type.

Prevention

• Buy healthy, vigorous plants from a reputable source, and look at the base of the stems before you buy.
• Choose resistant species or small-flowered types for difficult or exposed sites.
• Improve the soil with deep cultivation and organic matter, and mulch to keep roots cool and moist; the RHS recommends this to reduce root stress.
• Water deeply in dry spells, especially in the first few years (see feeding and watering clematis).
• Protect young stems from slugs and physical damage, and tie them in so wind can't twist them.