Gardening in a changing climate
Watch your water
Increasingly, gardeners have either too much or too little water - it's either droughts and hosepipe bans or flooding or waterlogging. But, as Leigh Hunt explains, we can learn to adapt the way we garden. Leigh is a Horticultural Advisor based at RHS Garden Wisley.
More information on water use and saving
Water is essential for life. From discovering a dried-up pot plant, to spotting one that has turned yellow from sitting in a saucer of water, gardeners recognise that the consequences of too much water, or too little, are life-threatening for plants.
While it should be possible to control or anticipate the amount of water plants receive, in the past two years alone we have had to cope with hosepipe bans in 2006 and summer floods in 2007. It is not surprising many of us are confused by what to do. We are told to grow drought-tolerant plants one year, and how to deal with waterlogging the next!
Changes in climate are likely to make this situation worse. Although overall predictions are for warmer, wetter winters and hotter, drier summers, the overriding factor is the increasingly erratic nature of the weather. It may be fairly easy to adjust to the warmer conditions expected over the next 20 years, but it will be more difficult to maintain gardens when water is scarce, or a deluge saturates them for weeks at a time. It is becoming clear that careful management of water through the seasons will become even more important. This includes collection and storage of rainwater for dry spells, gardening in such a way as to make sure every drop counts, and that excess water can drain away.
Thinking laterally
What can be done in periods of excessive rain, particularly in urban areas where there is often nowhere to drain the water except into the garden? Try to reduce runoff from hard surfaces such as driveways and patios by using permeable paving that allows water to soak in where it falls. Place green roofs on sheds and garages (right) to slow and reduce runoff. Gardeners could start creating seasonal pools or ‘swales’ that fill in winter and slowly dry to a bog garden in summer: a lining of gravel or cobbles helps prevent them becoming unattractive, dried, cracked depressions in a drought.
A changing climate will inevitably bring changes to gardens. Thankfully, we gardeners have long been good at adapting to conditions - whatever the weather throws at us. It is a case of building on our greenfingered skills and learning how we can manage water even more efficiently through the seasons.
Some of the key water-management issues to consider
Assessing water need
Clever gadgets aside, gardeners still need to understand how much water plants actually need, and when and how to water for the best results.
Water should be directed underneath the foliage. There should be enough to wet the top 30cm (12in) of soil, where most plants’ roots are - or should be. Too little water just wets the soil surface and either does not reach the roots or encourages them upwards where they tend to dry out. An excess will drain out of reach of the roots and be wasted.
Watering leafy plants from above with a watering can may be wasteful; better to water directly onto the soil beneath.
Harvesting the rain
Traditionally, for many gardeners, the collection of rain has been limited to a water butt or two. A typical water butt holds about 200-litres (44 gallons), just enough to keep one medium-sized container of bedding blossoming all summer.
Even with regular rain topping them up, it is clear that as many as half a dozen water butts are unlikely to meet all of a gardener’s watering needs during dry spells and hot summers.
One solution is a domestic rainwater-harvesting system, which can easily fill an underground tank holding 6,500 litres (more than 30 ordinary water butts) from the roof of most homes.
A specialist system of this size costs about £2,000, plus about £500 to install. Needless to say, this is not a quick saving on water bills at current prices, but it can be seen as insurance against hosepipe bans, or simply an environmentally responsible act.
It would be better to fit such systems when houses are built. There is some encouragement in the UK Government’s voluntary Code for Sustainable Homes, which awards a star rating for environmental credentials. However, there is little evidence of builders taking up this challenge, yet installing a system while building would only save a few hundred pounds compared with the price of fitting one later.
Saving water
Many other measures will help to conserve water during summer. Preparing the soil in autumn or winter reduces moisture loss compared to summer cultivation; applying a mulch in late winter will help lock in water.
Digging in organic matter such as well-rotted garden compost retains extra moisture and can provide plants with the equivalent of an additional 5cm (2in) of rain: about 20 days’ supply for many common garden plants.
Deep, double-digging or using raised beds also increases the volume of soil into which plant roots can spread and draw moisture.
Many measures are common sense: put saucers under containers to catch runoff, remove all weeds (they use water too) and use ‘grey water’ where possible (see below).
Increasingly sophisticated automatic irrigation controls are available to the home gardener, with sensors that turn off the water when it is raining, or when the soil is too moist, though the accuracy of soil-moisture meters will depend on the soil type and on careful positioning of the probe.
Using grey water
Grey water should be used with care, but can be useful in times of water shortages.
Plants can be watered with shower, bath, kitchen and washing machine water (from rinse cycles), collectively referred to as ‘grey’ water. It varies in quality and may contain contaminants such as soap and detergent. Fortunately, soil and potting composts are effective at filtering them out, and the residues can sometimes act as a mild fertiliser.
To minimise bacterial growth, grey water should only be saved for 24 hours, unless filtered through a reedbed or professionally-designed system. It is best applied by watering can; grease and fibres can clog irrigation systems.
There should be no problem with small-scale, short-term use of grey water to tide plants over in summer drought. An exception is on edible crops, due to the risk of contamination from pathogens in the water.
Long-term, extensive use, or permanent altering of indoor plumbing should not be attempted without expert advice.
Softened tapwater and dishwasher water are less useful. Salts used in them can damage soil structure, particularly if rich in clay. This said, short-term use of softened water should not cause serious damage and may be worth considering in an emergency.
Improving drainage
While overwatering can be correctable, waterlogging and flooding are more of a problem. If they happen in winter, when most plants are dormant, the damage can be easily missed, but with summer floods, symptoms such as wilting, yellowing and browning of the leaves usually appear rapidly. Either way the roots suffocate, drown and rot, and the conditions are right for diseases such as Phytophthora.
Where waterlogging is known to be a problem, there are ways to improve the growing conditions. Using raised beds and planting trees and shrubs on mounds helps to lift roots above high water tables, and provides some protection against flooding.
Poor drainage on clay soils can be improved by cultivation. Similarly, water lying on lawns can be encouraged to soak in by spiking and topdressing with sand to create mini-drainage channels.
In the worst cases, perforated plastic drainage pipes can be laid in the ground to move water to soakaways and ditches.
Baskets and containers
Baskets and containers are notoriously thirsty, but careful watering will save water and allow plants to flourish.
Scientist Tijana Blanusa and a team at the School of Biological Sciences, University of Reading, with funding from the RHS, set up experiments using petunias and busy lizzies to monitor amounts of water take-up by the plants.
There were three major conclusions. First, there is no need for hanging baskets and containers to drip after watering: bedding plants performed well when watered little, but daily. It took 160ml (about a teacup) of water each day to saturate the compost supporting each petunia, but only 80ml was needed to grow a good plant. In neither case did compost drip after watering.
The second surprise result is that irrigation applied 5cm (2in) below the soil surface, through porous hose systems, increased plant quality even though the upper soil was dust-dry.
The third outcome confirms what gardeners have long suspected: overwatering leads to poor-quality plants.