Clematis can be affected by many pests, and diseases but should not be regarded as difficult plants. Careful attention to planting and subsequent cultivation will produce vigorous plants capable of surviving most pest and disease attacks and avoiding disorders.

Most of the following extracts are taken from the BCS Fact Sheet on this subject and cover common problems. Consult the leaflet for a more comprehensive coverage or login to the members section.

Pests

Aphids

Various greenfly and blackfly attack clematis, especially during spring and early summer. Heavy infestations at the shoot tips can stunt growth and soil foliage with a sugary, sticky excrement known as 'honeydew'. Black, sooty moulds can develop in this substance. Check new growth for aphids and, if necessary, spray with an appropriate pesticide or alternatively attract aphid predators such as ladybirds.

Caterpillars

Most caterpillar damage occurs on new growth in the spring. The culprits are usually caterpillars of the angleshades moth or the lesser yellow underwing. Both emerge to feed at night, eating leaves down to stalks and boring into developing flower buds. Hand picking by torchlight on mild evenings can reduce infestations, otherwise spray with an appropriate pesticide.

Earwigs

Earwigs hide by day and emerge at dusk to eat ragged holes in the leaves and flower petals. Plants grown against fences or walls are at increased risk because the supporting structure provides many daytime hiding places. Some earwigs can be trapped by placing flowerpots loosely stuffed with hay on the end of bamboo canes near the clematis - they are then removed and destroyed during the day. Heavy infestations may need spraying at dusk with an appropriate pesticide.

Leaf Miner

The larvae of the leaf miner fly, tunnel between the layers of the leaf causing a whiteish brown tunnel that winds its way around the leaf usually ending in a larger blotch. They do not usually cause any stunting to the plant although if left to get out of hand can make the plant look a bit unsightly. In the early stages you can pinch the larger brown blotches (usually at the end of the trail) and kill the larvae or alternatively cut off the leaf and burn it. If getting out of hand I believe there are insecticides on the market that can be used to control the spread of these creatures.

Rabbits and Mice

Rabbits will bite off young shoots of herbaceous clematis or sever the stems of taller plants. Mice climb up the stems to eat the shoot tips and flower buds. In rabbit-infested gardens it may be necessary to put wire netting aroung the plants. Mice are more difficult to combat, but setting mouse traps under the corner of logs or bricks may help.

Red Spider Mite

These usually attack plants inside the greenhouse although they can be found outside in dry hot weather. The mites are very small and are not usually noticed until their fine webbing becomes obvious. They eventually cause the leaves to become mottled before becoming brittle and dropping off. They can be controlled by weekly sprays of insecticide, the introduction of a predatory mite (Phytoseiulus persimilis) or alternatively increase the humidity in the surrounding area.

Scale

Scale have been a relatively dormant pest over the last decade, but as the climate warms up and the summers get hotter so these insects are making a comeback. They infest clematis planted in hot sunny positions where they attach themselves to the stems or the undersides of leaves. From these positions they can feed on the rising sap within the plant.
Like aphids they cannot process all the goodness within the sap and the excess is passed out as honeydew, which again, like aphids, attract ants. These insects have a somewhat shiny protective coating which can help protect it from externally sprayed insecticides but it cannot cope with systemic types.

Slugs and snails

Slugs and snails are nocturnal (most active at night) creatures and this is when they do the most damage. A variety of methods can be used to combat slugs and snails, some of which are: surrounding the base of plants with fine grit or broken egg shells; smearing vaseline around the rims of flowerpots; using plastic 'collars' made from 2 litre soft drink bottles as a barrier. Probably most effective is a midnight troll through the garden with a flashlight, collecting and disposing of all slugs and snails that you find.

Vine Weevil

These insects in their adult form look like medium sized black beetles and can normally be found residing on or under leaves which they feed on leaving what looks like an irregular bite. This probably will do little harm to the plant but it is the eggs and associated larvae that they lay in the soil by the roots that do the damage. The larvae have a voracious appetite for roots and you will not know your plant has a problem until it wilts and dies. They can be a real problem if you grow plants in containers where they are most likliest to be found. There is now some control products available on the market including nematodes. The adult weevil cannot fly so if you have a greenhouse then it is a good idea to grease the pillars as this stops the weevils climbing onto any raised benches.

Whitefly

These insects are only usually a pest to plants grown in greenhouses although they do occasionally attack outside plants. They generally do little damage as they usually arrive too late in the season to do any great harm. They can be controlled by insecticides but due to their short life cycle you must spray at least three times in periods of less than one week.

Diseases

Slime flux

When woody stems are injured, bacteria which normally live on the surface of the bark can gain entry to the deeper wood. In spring, when temperatures and root pressure increase, the bacteria multiply rapidly and are forced out of the wounds with the rising sap as a nasty-smelling, slimy ooze. The condition is often fatal above the entry wound but if the plant is cut off below the affected area it will sometimes regrow. There is no chemical treatment.

Powdery mildew

Clematis are very prone to infection by powdery mildews, especially under conditions of water stress. The disease only affects leaves and young stems where it is unsightly but not fatal. Avoid water stress and treat infections with an appropriate fungicide.

Clematis wilt

This is the most important disease of clematis. Large-flowered varieties vary in their susceptibility whilst small-flowered forms are mostly resistant. Clematis wilt is caused by infection with a fungus which results in the sudden wilting and collapse of the upper parts of the plant. The plant can sometimes be saved by cutting back below the level of infection - even if this means cutting it down to ground level. The infected material should be carefully removed because the fungus can live in it for many months. There is no chemical treatment available to amateur gardeners.

Wilt is usually blamed for any and all collapsed clematis - experts at the Royal Horticultural Society note that it is actually relatively rare. There are many other causes of collapsed plants.

Adapted and abridged from BCS Fact Sheet No. 6 Pests, Diseases and Disorders of Clematis prepared for the Society by members of the RHS Advisory Service