One of the fortunate aspects of rock gardening is that you don't have to wage the constant battle against pests and diseases, which bedevils the vegetable and fruit grower. The range of insects and fungi which can cause a serious problem is limited and the likelihood of an attack by more than one or two of them is not common. Even so, you should do all the cultural things which are necessary to prevent problems from arising.
The rules for pest and disease prevention are the same as for the rest of the garden. Poor drainage is the main cause of root rots. Make sure the plants you buy are free of insects and leaf diseases and follow the correct planting procedure. Be careful not to overfeed and always clear away rubbish and dead plants promptly. Slugs are attracted by damaged rather than healthy leaves. Finally keep watch for trouble and if pests appear remove them or the badly affected leaves by hand picking if practical. If not it may be necessary to spray where there is a large infestation always read the instructions and precautions on the pack before use.
The problems described below are the ones which are most likely to call for action. Other troubles occasionally occur, mice may dig up bulbs, and roots may be damaged or disturbed by ants, root aphids or cutworms. In the alpine house the plants may be afflicted by red spider mite, mildews and whitefly. The use of proper cultural techniques is the way to keep down pests and diseases under glass as it is outdoors. One final point don't assume that all leaf discoloration is due to disease. Yellow leaves on lime hating plants indicate iron or manganese deficiency. Spray with sequestrene.
Slugs and Snails
A serious pest of many rock garden plants in late winter or early spring, the result of an attack on young plants can be devastating. Irregular holes are formed and telltale slime trails can be seen. Damage is worst when the site is poorly drained and the weather is unsettled. Keep the area clear of rubbish and scatter Slug Pellets around plants. Follow the precautions.
Greenfly and blackfly may appear in large numbers when the weather is warm and settled. Young growth is distorted and weakened leaves are covered with sticky honeydew which later becomes covered with sooty mould. They also transmit virus diseases. Keep plants watered in dry weather. Spray if necessary with s systemic insecticide or horticultural soap.
Many different leaf-eating caterpillars may attack rockery perennials. Some are uncommon, but angle shades moth and cabbage white butterfly can be a nuisance. Pick off the caterpillars if this is practical where damage is widespread it will be necessary to spray with an insecticide. Ready to use Pest Pistols are available.
Several birds such as sparrows blackbirds and bullfinches can be troublesome every spring in your rockery and yet your neighbour's rock garden may be spared. Primulas and Crocuses are favourite targets but all types of cushion forming plants may be disturbed in the hunt for grubs. Control is difficult - repellents are rarely effective and black cotton or netting is unsightly.
These wrinkled white grubs are the larvae of the adult dark brown vine weevil. They are extremely destructive underground eating the roots of many rockery perennials. If a plant suddenly dies, look in the compost for the rolled up grub. Difficult to control spray with HCH. When grubs become invasive you may need to call upon professionals to tackle the problem. They are extremely destructive and can overtake areas rapidly.
Botrytis or grey mould is a destructive disease in a wet season. The fluffy mould appears on the leaves and stems, flower petals become spotted and then rotten. Pick off and destroy mouldy leaves and flowers. Improve ventilation if it is an alpine house problem. Spray infected and surrounding plants with a systemic fungicide.
Cats can be pests of new rock gardens and newly planted specimens. Their scratching disturbs the new plants and the resulting root damage can lead to the death of the recently introduced plants. Pepper Dust applied liberally around the plants may help, but the real answer is to cover all bare soil with one inch of grit mulch.
Perhaps the worst headache of all - an invasion by moles can cause havoc. The hills thrown up by their tunnelling are unsightly and cause severe root damage. Eradication is not easy; smokes and sonic deterrents should be tried first. It may be necessary to set traps or to gas them. This work is best carried out by a professional exterminator.