Anna Pavord's A to Z of pests and problems: A is for algae, allergies, ants and aphids

This week, our green-fingered correspondent kicks off her handy A-Z of everything that can come between you and a beautiful, satisfying garden

Autumn can be a deliciously mournful season: mud, piles of dead leaves, rotting Michaelmas daisies. To increase your paranoia, I'm starting an A-Z of Problems and Pests. Black spot on the roses? Wisteria needing a haircut? Fence wars? Read all about it.

A is for…

Acid or Alkaline

Soil is acid, alkaline or, if you are lucky, neutral. Much of the acid soil in Britain lies along the west coast, where in the early-20th century, English gardeners dreamt of the Himalayas and planted vast woodland gardens of rhododendrons, magnolias and camellias. Native plants provide quick signposts to the nature of your soil. Oak favours clay. Beech likes chalk. Rowan prefers acid land. But if you want more precise guidelines, buy a kit (it's like a toy chemistry set) to tell you what you have got. Measurement is by pH (potential of hydrogen). The scale runs from 1-14; neutral is around 7. Above that, soils are said to be alkaline, below it, acid. Most vegetables grow best in a slightly alkaline soil. Rhododendrons need acid soil, between 4.5 and 6 on the pH scale. Avoid problems by working with what you have got and growing plants that like your soil.

Algae

This describes anything green and vaguely slimy: on fences, on the glass of greenhouses, on paths, paving, or in ponds. Nobody ever talks about one alga. They only work in the plural, being microscopic organisms, and not in themselves harmful. On fences, they flourish in shade and damp and add what I think of as texture in a garden. Soapy water and a scrubbing brush will get rid of them, if you think it necessary. Damp shade encourages algae growth on paths and paving, too. It can make them slippery, but a watering can of Jeyes Fluid provides a cure.

Algae are only likely to be troublesome in ponds. These water-borne algaef depend on sunlight, carbon dioxide and mineral salts to survive. Cut down the light available to them by growing water lilies or other floating plants over at least half the surface of your pond. Add oxygenating plants which will absorb much of the available carbon dioxide. Get rid of decaying leaves and other vegetation as quickly as you can.

Allergies

Those who've got them won't need to read any more about them, since they will already have an exhaustive list. Allergy to pollen is perhaps the commonest manifestation, but new gardeners need to be aware that specific plants may also cause allergic reactions. The beautiful blue-leaved shrub rue (Ruta graveolens) can cause skin blisters, as can the handsome family of spurges (species of Euphorbia). Many gardeners also have allergic reactions to the leaves of chrysanthemum and Primula obconica. Wear gloves if necessary.

Ants

Few of us could put a proper name to an ant, though we quickly recognise that the red ones (species of Myrmica) are the ones that bite hardest. Who can blame them? They work harder than anything else on earth to make their complicated homes underground. In the process, they throw up little heaps of fine soil, which drives lawn fanatics frantic. I find them a problem only in pots, when their underground tunnels cut down the contact that plant roots must have with surrounding soil. The plants wilt, then die. Being extraordinarily clever, they have developed a useful symbiosis with aphids. You often see ants running up and down the stems of aphid-infested plants to collect the sugary honeydew excreted by aphids. In paving, a kettle of boiling water dribbled along the cracks, despatches enough to make a difference. Goldfish are very partial to ant eggs and woodpeckers do a good job of hoovering up ants on a lawn.

Anxiety

Gardeners – especially new ones – worry too much. Relax. Gardening is there to be enjoyed, not to keep you awake at night. Plants are programmed to survive and will always do their best to overcome whatever crass things you do to them. It's the way of experts to make things sound more complicated than they need be. Whole books used to be written about the correct pruning of roses. Now they've discovered that a quick once-over with a hedge-cutter is as good a treatment as any. The simplest way to happiness is to mark down what does well in your garden and then grow a lot of it. Don't be bullied by the style-mongers. Your garden needs to please only you.

Aphids

The rag-bag term covers a bundle of creatures, none of them good. Aphids can be green, yellow, pink, black, grey or brown and feed on the sap of a wide variety of plants. The honeydew they secrete attracts further problems such as sooty mould, and aphids are also prime vectors of various viruses, which they introduce into plants as they feed. If you grow vegetables, you will be no stranger to the black bean aphid (Aphis fabae) which gathers in dense clusters on the growing shoots of broad beans. The same aphid attacks globe artichokes, cardoons, nasturtiums, dahlias and poppies.

Aphids give birth to more live aphids when they are themselves only a week old, so colonies build up with alarming speed. Fortunately, there are plenty of natural predators – beetles, birds, earwigs, hoverflies, lacewings, ladybirds, parasitic wasps and spiders. But their numbers only build up when there is plenty for them to eat, so there is always a time lag while you wait for the cavalry to charge to your defence. Learn to be patient. Avoid using nitrogen-rich feeds, which encourage the soft, sappy growth that aphids like best.

Biological control agents can help. But many need high temperatures (21C/70F) and humidity (80 per cent) to work effectively. If you use an insecticide, make sure that it is specific only to aphids, otherwise you will wipe out your friends as well as your foes.

Attitude

If you look for problems, you will find them, and the smaller the garden, the more intently will your gaze be focused on each petal or blade of grass. To expect perfection is a sure recipe for disaster. Cultivate tunnel vision. Look mostly at the things that are doing well. Do not expect miracles your plants cannot perform. Many problems occur because we have put plants in the wrong places. If we use our eyes, we start to learn their language, to understand the linked chain of events that has led to disaster. Of course, we would much prefer to feel that the fault is not ours, but it often is. Be prepared to accept that.