C is for: An essential guide to cabbage root fly, caterpillars and cats

Our green-fingered correspondent Anna Pavord continues her series of pests and problems standing between you and your perfect garden.

Town gardeners tend to dislike the local moggy tramping through their patch (Getty Images)

Cabbage root fly

The damage is done, not by the fly, but by its grubby larvae, which nibble on the roots of the cabbage family. Attacks are worst in late spring and early summer. Try interplanting cabbages with flowers. This confuses the cabbage root fly because it identifies suitable hosts by feeling their leaves with its feet. Being cautious, it tests out more than one plant before it settles to lay eggs. If after a cabbage, it finds several marigolds, or verbenas, it may fly on elsewhere.

The cabbage root fly lays its eggs in the earth close to the stem of the host plant. Collars (at least 10cm across), cut from old carpet underfelt and slotted round the stems of brassicas, make effective barriers.

Canker

Canker, like blight, is a fungal disease, and there are different kinds. Most common is the canker that attacks apple and pear trees. The disease is spread by wind and rain and spores get into the bark through cracks, leaf scars or where pruning cuts have been made. 'Cox's Orange Pippin', 'Elstar' and 'Gala' apples are particularly prone.

Canker attacks trees that are already struggling against adverse conditions, particularly poor drainage. Your best defence is to plant trees on well-prepared sites. Among cooking apples, 'Bramley's Seedling', 'Lane's Prince Albert' and 'Newton Wonder' all have good resistance to canker. The eating apples 'Egremont Russet' and 'Falstaff' are equally robust.

Carrot fly

The damage is caused by larvae, yellowish maggots that bore into the roots. They often stare at you, squirming horribly, when you clean freshly-lifted carrots for supper. If you sow seed after 1 June, your carrot seedlings will miss the first generation of maggots, the one that causes most damage. Harvest carrots before late summer to miss the final generation. Use physical barriers such as fleece to stop insects getting close enough to lay their eggs. Barriers must be at least 75cm/30in high.

Cat

For a really virulent display of spleen, listen to a town gardener sounding off on the subject of cats. Catapults, water pistols, pepper and holly leaves are all recommended. A spirited debate in The Independent's letters page implied that the best results came from lion's dung, freshly applied after rain. First, get your lion's dung… Most repellents are based on pepper dust, naphthalene, citronella oil or ammonium sulphate, but the effects wear off quickly.

Caterpillar

Every caterpillar should carry a banner saying, 'Think of me as a butterfly'. A certain amount of caterpillar damage is the price we pay for the beauty of butterflies and moths. The worst are the progeny of the cabbage white butterfly and the similarly-coloured caterpillars of the mullein moth, at their busiest in June and July. Pick them off by hand.

Chainsaw

Men think this a macho accessory, but there are few gardens large enough to warrant owning one. Better to hire someone who has the necessary expertise (and the goggles, the ballistic trousers and the steel-capped boots). Chainsaws cut with a linked, toothed chain which whizzes around at enormous speed. They don't stop immediately when they are switched off; they kick back suddenly if the chain touches an obstacle; they cannot distinguish between a tree limb and a human one. Electric chainsaws are lighter and quieter than petrol-driven ones, but not capable of such heavy work.

Chlorosis

When leaves that should be bright, pulsating green turn a pallid, sickly yellow, they are said to be chlorotic. It happens when the plant cannot absorb the minerals it needs from the soil. Minerals make chlorophyll. Chlorophyll makes green. The deficiencies may be of nitrogen, magnesium, manganese, but aref most commonly of iron. A dose of chelated iron (ask your ironmonger) often helps. Chlorotic-looking leaves may also be caused by waterlogged soil, low temperatures or weedkiller poisoning.

Clematis wilt

Wilt is more likely to affect large-flowered cultivars of clematis than small ones. Shoot tips turn black and dark blotches show up on leaves and stems; the whole plant quickly collapses. It seems to be caused by the fungus Ascochyta clematidina, which gets into the stems through small wounds. Cut back the clematis hard; plants with wilt quite frequently recover, sprouting new stems from the base. Deep planting helps ward off the disease.

Codling moth

You may even at this moment be eyeballing the maggot of a codling moth (Cydia pomonella), which starts life in the centre of an apple and then eats its way out. You meet it halfway. Recognise it by its small brown head and its squirming white body. Apples are more likely victims than pears. Up to a third of a potential apple crop can be spoilt by codling moth maggots. Pheromone traps may help. They lure male moths to their death by mimicking the scent of female moths. The females lay the eggs, but without the males, they won't be fertile. Use the traps from May onwards.

Colour

Whether colours 'go' will always depend on individual taste and style. A garden has only to seem beautiful to its creator. What the rest of the world makes of it does not matter a jot. Thinking only of colour can blind you to the rest of a plant's attributes – or its faults. If you just swoop on a plant because it is blue or white or yellow, you may overlook the fact that its leaves are as inviting as last week's salad and that it holds itself with the grace of a sailor on a spree.

Complaints

It is fair to point out to a bulb supplier that the purple tulips you ordered came up bright yellow. Tulip bulbs all look much the same at planting time, but a thoughtless substitution (which nobody has told you about) can cause havoc in a planting scheme. Unfortunately, we now live in a culture of complaint, where few want to accept that the thing they are complaining about is actually their own fault. With plants, this is particularly tricky.

A nurseryman parts with a well-grown plant, but a few months later, is confronted by an aggrieved customer saying it has died. It turns out that the plant has been set in entirely the wrong place and never fed or watered. What does the nurseryman do then? Most swallow hard and offer a replacement, but I think that's unfair.

Yes, there are 'rights' as laid down in the Sale and Supply of Goods Act (1994). But there are also responsibilities and unless we the customers act responsibly we will find ourselves without anyone willing to offer goods or services at all.