Anna Pavord's A to Z of pests and problems: G and H are for greenfly and honey fungus

Our green-fingered correspondent continues her essential A-Z of the pests and problems standing between you and an almost perfect garden...

Genetic modification

This is only a problem if you are well-fed. I understand the case against, but could not argue it in front of a crowd of starving children. If GM presently offers the best chance of feeding an over-populated world, can we object? Much rests on the 'if'. I first wrote about GM in 1989, when ICI was busy buying up seed companies – a preliminary to the patenting of genes. At the time, that seemed to me an evil move and it still does. But genetic manipulation can make plants resistant to virus, insect and fungal attacks. Selective breeding has the same aim, but the work is slower and less certain in its results. So, is it, in the widest environmental sense, better to accept GM than to use pesticides? And the answer to that depends entirely on who is doing the sums.

Germination

Hang on to the fact that seeds are programmed to survive, and will desperately try to do the right thing even if you are doing wrong. There are five variables: soil (or compost), water, temperature, light and air. Though there are optimum conditions for each type of seed you sow, there is plenty of built-in elastic. First – don't be in too much of a hurry, especially if you are sowing direct into soil outside. A cold bed is as off-putting for a seed as it is for us. So is a wet one. If you are sowing in a pot of compost, water the pot before you sow and allow it to drain. Sow seed thinly and cover with vermiculite, which is easier for a seedling to push through than clods of compost. The smaller the seed, the less covering it needs.

Some gardeners swear by heated propagators to germinate seed. I don't. The heat may make the seed germinate faster, but at some stage it has to take on the real world. The tougher plants have been raised, the better they will be able to cope.

For sowing, you need fresh compost and clean pots, which protects against the dreaded 'damping off', a total collapse of seedlings attacked by soil-borne fungi such as Pythium. It most commonly happens with flowers such as snapdragons and asters, stocks and African marigolds. It can also attack brassicas. There is no cure.

Remember that some seeds, such as the fashionable cone flower (Echinacea) are by nature erratic germinators. That may be irritating to a gardener, but it's a clever safety net in the woodland and out on the gravelly hillsides of North America, their natural habitat. It spreads the options. Habitat is often a useful guide to germination, as it is to the way you grow a plant in your garden.

Greenfly

They come with the roses, clustering sometimes so thickly around the buds, that you think the bud is another leaf. Together with the slug, these are the most commonly complained about garden pest. The greenfly is one of the 550 different kinds of aphid that thrive in Britain. They breed prodigiously because for the whole of the summer, all aphids are female and their young grow up in a week, and then start to give birth themselves. Don't even think about it. It is too frightening. Greenfly are sap suckers and virus spreaders. Their enemies are ladybirds and hoverflies. If you spray greenfly, only use an insecticide that is specific to this particular pest. If you use an all-purpose killer, you will wipe out your friends as well as your foes.

Ground elder

If it is growing in a place where none of your garden plants are, then you have a chance of getting rid of it. Its favourite thing is to spread stealthily through a border of herbaceous plants, getting close up and personal with clumps of Michaelmas daisy and day lily. If you don't want to dig up the whole border, all you can do is to weaken the weed slightly by pulling up any bits you see. You can, over about three years, clear it from an open piece of waste ground by using glyphosate. If I was taking on an allotment infested with ground elder, I would unhesitatingly use herbicide to clear it. After that, you can be as organic as you like, but you need to start on clean ground and neither carpets nor black plastic will completely kill perennial weeds.

When we came here, I took everything out of the borders by the house to 'clear' them of ground elder and bindweed. We weedkilled for three years. Then I replanted. But from their hiding places under the wall of the house on one side and under the stone terrace on the other, the bullies crept out again into the light. They are great survivors.

H is for…

Herbicide

The essence of gardening is maintaining a balance between the things you want in your garden and those you don't. Weeding is extremely therapeutic and there is no better way of learning about your plants than being constantly among them, refereeing between bindweed and clematis, ground elder and hostas. But herbicides (weedkillers) have their place, too.

The most important distinction between the different sorts is this: some linger for a long time in the ground and some don't. Where possible, prefer the latter to the former. Long lasting, residual weedkillers stop weeds in their tracks but they also prevent anything else growing in the soil.

Glyphosate (as in Monsanto's Roundup) is one of the most useful of the non-residual weedkillers. It kills top growth relatively slowly but it works through the leaves of weeds down to the roots, which it also kills. You can use it any time that weeds are growing, though it is most effective when there is the maximum amount of leaf for it to cover. Docks and nettles are best sprayed just as they are coming into flower.

Honey fungus

Your neighbour will probably tell you you've got it as soon as you move into your new house. It is a soil-borne parasite that attacks the roots of trees and shrubs, causing slow decay and eventual death. Apple trees, privet, rhododendron and roses are particularly susceptible.

It is very difficult to treat. Take heart from the fact that, like pneumonia, it only finishes off things that are already weak because of some other cause – often drought or old age.

The fungus appears above ground as a honey-coloured toadstool but the damage is done below ground by black bootlace rhizomorphs which invade the roots of the victim. You can best avoid getting it by digging out all old roots of trees or shrubs that you cut down. If you suspect you already have it, concentrate on looking after any new shrub or tree you plant very well indeed. A shrub that is growing lustily with plenty to eat and drink is unlikely to succumb to honey fungus.