GARDENING / How to make the most of your liquid assets: Feeling guilty about watering the plants? They don't need as much as you may think. Michael Leapman charts a path to righteousness

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MIDWAY through a third successive dry summer, gardeners in the eastern half of England are again under pressure to use less water. For quenching the thirst of our parched plants, we are made to feel as environmentally sinful as those who use peat-based compost or apply chemical insecticides with an aerosol.

Three principal questions arise. Is the water position truly as bad as it appears? How can we make the best possible use of what we are allowed? And how little watering can we actually get away with?

Figures from the Royal Horticultural Society's garden at Wisley, in Surrey, tell the first part of the story. Annual rainfall there was 542mm in 1990 and 521mm in 1991, compared with an average of 654mm for the 29 years between 1951 and 1980. The total for the first five months of this year was 183mm, against the 1951- 1980 average of 243mm. Wisley, which gets its water from the river Wey, is now rationed to only 7.5m gallons a year from that source.

'We need two or three times that,' says Bill Simpson, the RHS's director of horticulture, 'so we have just spent pounds 71,000 on sinking a borehole to tap into an artesian well. That will give us another seven million gallons, but if it gets really bad we may water only where the higher- value trees and shrubs are and accept that we'll lose some of the rest. But we obviously don't want to do that if we can help it.'

For those of us not about to plumb artesian wells under our back lawn, the secret lies in efficient management of the water we have, starting with proper assessment of need. There is no point in watering plants when they do not require it, nor in offering them more moisture than they can use. Once the soil's water level is topped up to capacity, the surplus drains off wastefully.

Any plant's need, as Bill Simpson explains, depends on its individual characteristics, the type of soil, and rainfall and temperature patterns. 'In recent summers we've had a combination of high temperatures and low rainfall. High temperatures mean loss of moisture through evaporation from the soil and transpiration through the plants' leaves.'

Assuming that you start the summer with fully moist soil after the winter rains, the amount of water you have to put in amounts to the difference between that loss of moisture and the rainfall. Professionals have gauges for measuring all that, but the rest of us can only keep our eyes open and use our judgement. The type of soil also has to be considered. A sandy, fast-draining loam will dry out far more quickly than clay, and will thus need watering earlier. But if you are on clay, be vigilant: when it does get too dry it can set brick-hard. Mulching at the beginning of the summer helps to restrict evaporation. Bark, straw or black polythene (if you can bear the sight of it) are suitable materials, as is farmyard manure, especially in a hard-water area where it can counteract the water's alkaline content.

Not everything in the garden needs water at the same intervals. The basic rule is that shallow-rooted and young plants want watering first and most often, and those with long roots may not need any water at all, except in unusually dry conditions.

The society this year has revised its advisory leaflet Watering During Dry Periods, available free to members. It employs complex mathematical formulae to show that scarcely anything in the garden needs watering more than once a week, and for a shorter time than you may think.

If you have a hose and sprinkler system of average capacity (200 gallons an hour), a small garden of 500sq ft should not need the hose for more than an hour a week to bring it up to its full moisture level in the driest months of the summer.

During a hosepipe ban, though, giving the garden enough water becomes a chore: 200 gallons means at least 100 trips with the watering can. (Plants in containers naturally need watering much more often.)

The solution here is to be selective. The Water Services Association's leaflet in its 'Waterwise' series, called Your Gardening Guide to Water (available free from local water companies) lists some deeprooted plants that need little watering once they are mature: they include lavender, grey-leafed plants (some sedums, for instance), hebes, oriental poppies and geraniums. It might have added buddleia, sempervivum, nicotiana and mesembryanthemum. Among the blacklisted 'water guzzlers' are azalea, hostas, primulas, viburnum and clematis.

Even garden vegetables, with their high water content, need watering only once a week for leafy crops, and once a fortnight for roots and tubers. Some of them, however, have to be kept moist when, and just after, they produce their flowers: these include peas, beans, sweetcorn, marrows and tomatoes. Soft fruit wants a lot of water when the fruit is just beginning to swell.

When you do water, Bill Simpson advocates a thorough soaking rather than the 'little and often' technique sometimes recommended. 'Some people will give a little dribble each night, and all you're doing is wetting the top surface. On a hot, sunny day that will evaporate, and you've been wasting your time. A good soaking once a week or fortnight is better, because it will get down to the roots, and will be held there if you apply a mulch afterwards.'

For the same reason he is against using a sprinkler: 'On a very hot day, the water evaporates as it's being spun around in the air. It's better to use a seep hose (with holes in it at intervals) which applies water directly into the soil where it's needed.'

A useful four-page article on water conservation in this month's issue of the Consumers' Association magazine Gardening from Which? offers some other hints and devices. If you sink a drainpipe with a few holes in its side near the roots of trees, it will continually send water where it is needed. Pot plants will require less watering if they are placed in a large container half-filled with moistureretaining compost, or on a bed of gravel lined with polythene.

Diverting rainwater from the roof is an efficient means of irrigation: a water butt, though scarcely an elegant piece of garden furniture, is a sure badge of environmental good behaviour. (Prices at garden centres range from pounds 20 for a 25-gallon butt to pounds 45 for a 50-gallon model.)

The use of waste water from the kitchen, or even the bath, is sometimes recommended. Some people have rigged elaborate systems to siphon water from the bathroom to the garden, but if they involve hosepipes, they are technically illegal during a hosepipe ban. In any case, Bill Simpson advises caution. 'Be careful that the water doesn't have additives like bleach, bath salts and oils.' Small quantities of such substances will not affect most plants, but the alkaline content will upset acid-loving ones. (Gardening from Which? recommends applying leftover tea and old tea-bags to these.)

Lawns are a contentious area, and Bill Simpson has firm views about them. 'I believe that people who water their lawns in a drought are being irresponsible at a time of water shortage. The grass may get to look brown and dead, but it is tough and will always recover after rain. It's true that not watering the lawn may encourage weeds, but I think we just have to live with that.'

What he advocates, therefore, is a strategy of survival under which some elements which make our gardens pleasing have, temporarily, to be sacrificed while we keep our most precious stock alive. If we are to remain good citizens, we have to tolerate some parts of the garden looking less than perfect, in the knowledge that, when the rain does come, they will quickly return to their best. And we shall not have had to suffer floods of ecological guilt.-