Anna Pavord's A to Z of pests and problems: W is for whitefly and wind

Our green-fingered correspondent warns that you can never know enough when it comes to gardening...

W is for...

Whitefly

This is a menace in greenhouses and conservatories. It can ruin tomatoes, cucumbers and aubergines as well as ornamental plants such as geranium and fuchsia. The whitefly's young offspring suck in sap at one end and excrete a sticky syrup (honeydew) at the other. The honeydew in turn attracts sooty mould which stops the leaves from photosynthesising – converting light into energy.

Only a day or two after maturing, the adult whitefly starts to lay an irresponsible number of eggs. Having hatched, the larva wanders about until it finds a tempting feeding station. When it has plugged into a leaf vein, it stays at the same trough until it pupates and itself becomes a fly. Then the whole ghastly cycle starts again, the plant meanwhile looking distinctly sick.

The whitefly's most effective enemy is a minute wasp, called encarsia, which commercial growers have used for years to protect their crops. Encarsia's method of attack is grisly. If you are squeamish, skip this bit.

The adult wasp lays its eggs inside the larva of the whitefly and, like revivified mummies in a horror film, the young eat their way out from inside, emerging after about three weeks as fully-fledged new wasps.

To operate productively, the wasp has to have comfortable living conditions. It will not work while night temperatures are below 10C/50F and is most energetic in day temperatures of 18C/64F and above. If the temperature is too low, the wasp does not breed as fast as the whitefly and so it cannot hope to win the battle.

Encarsia is a well-behaved guest. If you leave the doors and windows of your greenhouse or conservatory open, it will not try to bolt. While there are whitefly to attack, it will remain at its post.

Sticky traps hung above affected plants work well. So does a portable vacuum cleaner. You just shake the plant gently to get the whitefly on the wing and then suck them up with the nozzle. Solutions of soft soap or washing-up liquid also work.

Wind

Wind is a more unpredictable, nerve-wracking enemy than drought or frost and there is less you can do about it. Victims are not always the plants that look the most frail. Many clematis, for instance, stand up remarkably well to gales, unless they happen to be growing through a shrub that, itself, keels over.

A clematis leaf stalk, like a baby's fist, grips what it touches and hangs on with a determination that belies its fragile appearance. But when the first fresh shoots of clematis appear, you need to spend time pointing them in the right direction, and tieing them in to their supports where necessary. After that they can mostly be left to their own devices. If you miss out on the initial training, shoots tangle into each other, stand away from whatever they are growing against and so are more prone to wind damage.

The prevailing wind in the UK is the southwesterly. The most cutting is an easterly which can bring snow all the way from Siberia, with devastating effect in a garden. Whichever direction it comes from, winds are getting worse, by which I mean stronger. The last big > blow here in the West Country came at the end of October, taking out both power and phone lines. BT, preoccupied with their lucrative Superfast Broadband, took 18 days to restore land lines to our valley. Since we have no mobile phone reception here, that mattered.

Winds rarely affect the whole country with equal force. The 1987 gale hit London full-strength, which is why the rest of us heard so much about it. Up in Fraserburgh, on the east coast of Scotland, inhabitants are regularly battered by winds of more than 100mph, and the fishermen wondered what the 1987 fuss was all about. But you don't see many lush gardens in Fraserburgh.

To my mind, wind in April or May is worse than those at any other time. It tears blossom off fruit trees and cherries. It seizes the lush new leaves from beech trees and piles them up in corners like sodden heaps of confetti. Fleshy soft shoots of climbing roses are at their most vulnerable. Herbaceous plants that should be neat mounds of foliage sprawl flat on their backs. Foxgloves and delphiniums are cast to the ground.

Staking and tieing provide only a partial answer to problems of wind. Sometimes a plant is blown right out of the ground, stake and all. Often stems snap and what's left is not worth having.

Plants, even those that seem to have the same habit of growth, show different tolerances of wind. In our garden, Rubus tridel 'Benenden' with long arching whippy growths – covered, in May, with flat, papery white flowers – rides out storms untouched. Verbena bonariensis, which has immensely tall, thin stems, topped with flat heads of purple flowers, never needs staking and never suffers. Agapanthus and sea holly, likewise.

Much will depend on the situation. All gardens have spots that are more sheltered than others and it's worth saving these for the plants that most need them. Or you may consider putting in a hedge, particularly to shelter a garden on its eastern boundary. A hedge filters wind, creating less turbulence than a solid boundary, but it needs to be reasonably tall. Blackthorn, sea buckthorn and hawthorn will all stand up to harsh conditions. On exposed sites, conifers may suffer from windburn.

XYZ is for...

Now we have come to those impossible letters X, Y and Z. And a problem with lists themselves, which is that having got to the end of them, you think you've learnt something. With gardening, learning never stops. The more you know, the more you realise you don't know.

If you let it, your garden will teach you more than you can ever teach it. Use your eyes. Watch where plants are self-seeding – often a good indication of where you should have put them in the first place.

Do not be bossy with your plants. Give all things in the garden the benefit of the doubt. And look forward to spring when you can scarce help seeing the possibilities rather than the problems in your garden. Good luck.

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