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Using and managing water in your garden and on your allotment is essential to grow healthy plants that better withstand the attacks of pest, disease and periods of drought. A better understanding of how and why plants need water will help you water at optimum times and use your water more efficiently.

How plants take up water from the soil

The soil can be thought of like a sponge, containing numerous pore spaces that can hold water. This acts as a reservoir of water that is exploited by the plant roots. The ability of plants to exploit this reservoir differs greatly between plant types: some plants have much deeper rooting systems than others so can use water from greater depths. Also some plants have a much greater ability to suck any remaining water out of dry soil.

Different soil types hold different amounts of water, with sandy soils holding the least, and heavy clay soils the most. The more water a soil can hold the less often it will need watering.

Sand	Holds least water - needs frequent watering
Sandy Loam
Loam
Light Clay Loam
Clay Loam
Heavy Clay Loam	Holds most water - needs less frequent watering

How soils gain and lose water

The reservoir of water in the soil can lose water or gain water in different ways. The ways in which water is gained are obvious: Rainfall, irrigation (eg. sprinkler, watering can etc), and movement of water upwards from the water table.

The ways in which water are lost from the soil can either be beneficial to the plant or wasteful.

Transpiration by the plant

Water that is used (transpired) by the plant, ie taken up by the roots, into the plants then released through pores in the leaves, is beneficial. It is essential for many processes in the plant such as taking up soil nutrients and plant growth and development.

Evaporation from the soil surface

Water that evaporates from the bare surface of the soil is a waste of water and can be reduced in many ways (see next section on how to use water more efficiently in the garden)

Drainage from the soil

When the soil reservoir can hold no more water, ie the pore spaces are all full, water will drain out of the soil to lower depths. This is wasteful and can wash soluble nutrients, especially nitrogen, to lower depths where the roots cannot reach it. Water can also drain out of the soil through cracks if the soil is very dry.

Runoff from the soil surface

When the water cannot be adsorbed by the soil surface, it will run off into another location and cannot be used by the plant. This occurs when the rate that water applied is too much for the soil to adsorb it. Very dry soil, or a cap or crust on the soil surface caused by poor cultivations can reduce the ability of the soil to adsorb water. If areas in the garden are identified as particularly prone to run off, this can be reduced by establishing areas of permanent vegetation.

How to use water more efficiently in the garden

The key to using water more efficiently in the garden is to target the water to when the plant needs it and minimise the wasteful processes such as evaporation from bare soil, drainage and run off.

Water your soil properly

Many people water the very surface of the soil which will then evaporate rapidly without ever reaching the plant roots. Check with a trowel that you are actually soaking the soil beneath. It is better to give the soil a good soaking every few days rather than just wetting the surface regularly.

Target the water at the soil rather than wetting the foliage. Water on the foliage will just evaporate or remain on the leaves encouraging fungal disease. Drip irrigation systems are by far the most effective way of delivering water to plants, as water is targeted to the plant roots rather than wetting the soil surface. They take a lot of setting up, but once in place, watering takes very little effort.

Mulching

Mulching with compost or straw has a huge effect on reducing the amount of water that evaporates from the soil surface. It reduces the amount of watering needed and will also suppress weed growth. There are many different materials that are suitable for mulching from newspaper and cardboard, hay and straw to grass cuttings and leaf mould. All are excellent at retaining moisture in the soil and reasonably cheap. Gravel and grit on pots are also useful but may not be quarried in a sustainable way.

Water the plant when it most needs it

There are critical stages when it is most important to water plants. For directly sown plants the soil should be kept moist otherwise the seeds won't germinate. Likewise after transplanting, plants only have poorly developed roots so will frequent watering. After this critical period, the water requirement of plants differs.

As a very general rule, more leafy vegetables (eg spinach) will require more continuous water to allow leaf expansion. For plants that produce fruits (eg tomatoes, beans) watering is most critical from fruit or pod set onwards. If you want potatoes to be free from scab then you need to avoid letting the soil dry out for about six weeks after tuber initiation. This is approximately when the plants are 15 cm high, but you may wish to dig up a few to check.

Improve the moisture holding capacity of your soil

If you have a very light sandy soil, water will drain out quickly. Improving the organic matter content by regular additions of compost, garden waste, manure and crop residues will gradually, over time improve the ability of the soil to retain moisture.

Harvesting water

Your next line of defence against drought should be to harvest as much rainwater as possible. Most water authorities now offer water butts at subsidized prices. You can also buy some rather smart wooden affairs from garden centres and online suppliers, or re-use any suitable container you can get your hands on. Make sure all water butts are covered - this prevents any nasty accidents to children, wildlife and pets, it stops the water becoming a mosquito breeding ground in the height of summer, it keeps out leaves and other debris and excluding the light prevents the water from turning green. Try to fit butts to every available down pipe, and fit guttering connected to butts onto sheds and greenhouses. Most butts only hold around 200 litres so you may need to connect several together to see your garden through a severe drought. If rain is forecast in a dry spell, pop outside and fill up as many watering cans as possible from your butts - this leaves spare capacity in the butt for more water.

More financially advantaged readers may like to consider installing underground storage tanks taking rainfall directly from their roofs to an evaporation-proof container.

Greenhouses

These can be one of the most water-hungry areas in the garden. Seedlings are very vulnerable to 'damping-off disease' - a catch-all term for a variety of fungal infections that can kill an entire tray of seedlings in hours. Because of this danger, seedlings should only be watered using tap water, which has been treated by the water company to be free of any pathogens. Once your seedlings are past the first potting-up stage, harvested rainwater can safely be used. Never place pots directly onto your greenhouse staging as much of the water applied to them will simply drain off.

Invest in capillary matting or gravel trays for greenhouse staging to prevent runoff when watering and to enable plants to access this water when needed. There are many automatic watering systems on the market suitable for use in a greenhouse from inexpensive 'watering spikes' to entire systems with porous hose and timers. Some can use harvested rainwater or be connected to water butts and some need connecting to the mains - this would be illegal during a hosepipe ban so think carefully before deciding which system to buy. Simple measures such as putting plants in a spot that is shady at midday, and making full use of gravel trays and capillary matting will reduce the need for watering.

Reusing water

If all your beds are enriched with organic matter and mulched, as much of the rain falling on your property as possible is being harvested and stored, only then should you consider using greywater. Each summer the Garden Organic Information team receives lots of enquiries on the best way to use bath and shower water on the garden - and our advice is always to address the issues of storing water in the soil and collecting rainwater first.

A bath full of clean looking water looks like a wasted resource, and maybe you did only use environmentally-sound detergents, but storing water previously used for washing poses a real risk of incubating legionnaires disease (Legionella spp.) or other diseases and parasites. If you really must use it, do not store the water for any longer than it takes to cool. Then, only water ornamental areas and top fruit with it. Avoid using on any food eaten raw, such as salad crops and soft fruit, at all costs. To be able to store it or use it on more of your food crops, it will need to be cleaned via a reedbed - and that is a topic for a whole other article. If you want more information on small-scale reedbeds, The Centre for Alternative Technology have an excellent tipsheet available from their website www.cat.org.uk or call 01654-705950.