Anna Pavord's A to Z of pests and problems: D is for damping off, drought and drainage

Our green-fingered correspondent continues her handy A-Z of the pitfalls and pleasures of gardening

Damping off

It sounds like what happens at the end of a love affair, but it's easier to sort out. If a trayful of newly emerging seedlings keels over and dies in clumps, that's damping off. It's caused by various soil and waterborne fungi such as phytophthora and pythium and spreads rapidly. Sometimes the infection works faster than the germination rate and the seedlings may not emerge at all.

It's more likely to happen when compost is too wet, temperatures too high, light levels too low and seed sown too thickly. So if you get all those things right, you may never have to cope with the dreary consequences of damping off. Hygiene is important, too. Use fresh compost, tap water rather than water from a rain butt, and clean pots. I wash plastic pots in the dishwasher, a load at a time. Painlessly, it cleans and sterilises them.

As with all diseases, prevention is better than cure. Bordeaux Mixture (based on copper sulphate) has been the traditional treatment, but is to be withdrawn from sale at the end of February this year. Fruit and Vegetable Disease Control (made by Bayer Garden and based on copper oxychloride) is effective, but don't use these compounds as a matter of course. Save them for seedlings that are particularly prone to collapse: antirrhinum, lobelia, petunia, salvia, stock, zinnia. Among vegetables, lettuce and tomato are the most likely victims.

Danger

The biggest dangers are likely to come from machinery, which is a good reason for having as little as possible. Beware taking over a greenhouse wired up by an amateur electrician. Learn about trip switches and power breakers. Watch out for the sharp end of canes. Use slug pellets cautiously if theref are dogs and children in the house. Use poisonous chemicals as sparingly as possible.

Deer

Traditional deer barriers are single fences, extraordinarily high. This is what makes them so expensive and such an eyesore. Barriers can be kept relatively low, say 150cm/5ft, if there is room to create two parallel lines of them, 120cm/4ft apart. This does not give the deer enough room to land and take off again, but is wide enough to deter it from trying to clear both barriers in a single leap. Even so, it's not a simple solution.

Some gardeners hang hanks of human hair along their boundaries, slightly spooky, but, I'm told, effective. Or you can accept their presence and grow things they don't like. That means doing without roses and planting choisya, cistus, honeysuckle, magnolia and peonies instead. There's a full list in Graham Stuart Thomas's book, Ornamental Shrubs.

Dormancy

Why have plants been given this gift and not us? I can't think of anything better than sliding gently underground in November and emerging in the sparkly light of spring, having missed all the dross of winter. Dormancy is only a problem when gardeners try to persuade a seed to overcome its natural reluctance to sprout in conditions which are not propitious.

Dormancy can be caused by a hard seed coat, an immature embryo in the seed, which needs time to mature, or a chemical inhibitor which in the wild allows seeds to germinate over a long period of time. That means they don't compete with each other.

Dormancy can be shallow, intermediate or deep-seated, depending on how difficult it is to break. Hard seed coats can be rubbed with sandpaper, or chipped with a knife. Some seeds (primula, for instance) is best sown before it is dry and has developed a hard coat.

Slightly more complicated is the business of stratifying seed – making it think it has been through a winter which hasn't actually happened. Seed of many alpine plants needs it and you do it by putting the seed in the fridge. Seeds with shallow dormancy may need only 3-4 weeks, deep-seated dormancy can take 20 weeks to break.

In the natural order of things, chemical inhibitors are broken down in the gut of the bird or animal that eats the seed. Magnolias, roses and mountain ash are all geared to this system and need to be washed before sowing.

Drainage

Poorly drained soil tends not to be wet all the time – just when there is a lot of rain. So turning a badly drained patch into a bog garden may not work, unless you can find a way of keeping it damp all the time.

Poorly drained soil tends to be infertile because there is too little air in it and not enough bacterial activity. That slows down the rate at which humus decomposes and releases nutrients. Willows, alders, poplars, sycamores, dogwoods, elders (all the fast growing, slightly coarse trees) will cope with poor drainage, but generally, it's best to try to improve drainage before planting.

In the long term, mulching helps, bringing bulk to soils that drain too fast and opening up the texture of soils (typically clay) that don't drain fast enough. You can never mulch too much. On a small scale, you can help plants (such as tulips) that depend on a well-drained position, by adding grit when you plant, but it is not a long-term solution. In containers, you need drainage holes and a layer 5cm/2in deep of crocks, gravel or coarse compost at the bottom of the pot.

On completely intractable ground – compacted, low lying, or where new building has messed up the sub-soil – land drains or a soakaway may be the only answer. But that's an engineering problem, not a gardening one.

Drought

The worst drought I can remember was in 1976, which ended in a majestic thunderstorm on August Bank Holiday weekend. I danced in the rain. Perhaps on average, one summer in 10 is unusually dry, and it's more likely to be a problem in the eastern counties, where gardeners can expect about 50cm/20in of rain a year, a quarter of what falls on the gardens of north-west Scotland.

Mulching helps conserve moisture in the soil, but put it on when the soil is damp. Seep hoses I loathe. They don't stay buried and you wander round a garden feeling that the whole thing is like a patient wired up to drips. And they rot bulbs such as tulips which depend on a dry rest in summer.

Remember, though, that plenty of plants are adapted to survive dry weather: ceanothus, cytisus, phlomis, rosemary, dianthus, euphorbia, rudbeckia, poppies, pelargoniums. And before you complain that your garden is suffering from drought, think of Africa. They really know what the word means. We don't.

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