How to water
Plants can only effectively use water through their roots, taking water from the surrounding soil or compost. So water needs to get to where it’s needed, at the tip of the roots and not the leaves. Wet or humid foliage will encourage fungal problems and evaporation from the surfaces.
Watering more thoroughly, but less frequently helps get the water down to the deeper root tips. It is better to water the garden before drought really sets in, to keep the soil moisture levels even and avoid the soil being continuously dry. But, equally important, the soil doesn’t have to be really wet all the time because plants roots need air as well as water to grow well. Consider the way we drink a glass of water, it doesn’t have to be full all the time, but we might prefer it was topped up half way rather than it becoming completely empty.
Where plant roots are restricted for example in containers or growing next to a wall or fence post, more frequent watering may be needed as the roots are extracting water from a smaller volume of soil than if they were growing freely in the border soil.
Once drought has set in to border, it is futile to try and remedy this by light watering over a wide area. Light watering may encourage surface rather than deep roots, leaving plants more susceptible to drought. Instead, make a single thorough watering of the plants that are suffering. Try to water in the cool of the evening or the very early morning, so that less water is lost immediately to evaporation.
Watering effectively where drainage is poor is very difficult. It’s better to improve the drainage or choose plants that are appropriate for the conditions such as those suited to both wet and dry conditions. Roots are very susceptible to airless conditions, particularly when the soil is warm in summer.
How much water to apply
How much water is needed will depend largely on the water requirements of that particular plant and how actively it is growing. The type of soil or growing media as well as weather are also important considerations.
Light sandy or chalk soils need watering more frequently than heavy clay soils, but less water can be applied at each watering, as the excess will drain away easily. Heavier, clay-based soils can be watered less frequently, but need heavier applications of water because they hold more water within their structure. Adding organic matter increases the water holding capacity of most soils.
When watering containers, try adding 10% of the volume of the container at each watering. So for a 10 litre patio pot, add 1 litre of water. Pour it on slowly, aiming to keep it in the pot and not allow it to drain out of the bottom. A saucer placed under the pot will catch any excess and allow it to be re-absorbed. For small containers, gently lift the pot after watering to see if it feels heavy, and if not, add a little more water. You will soon gauge how light the pots are when they are in need of water.
If plants have wilted between waterings, you may need to water more often, but slowly and thoroughly so that the water reaches the root zone. Avoid disturbing the soil surface if you can, by slowing the flow. Pop a rose attachment on your watering can, a variable nozel on your hose or simply reduce the pressure at the tap. This will allow water to infiltrate into the soil more slowly.
Sources of water
Rainwater is an excellent choice for plants and the first ‘go to’ for gardeners as it is freely available if you are able to store it. Tap water requires treatment and energy to deliver it to our homes and can contain more minerals than many plants need, especially ericaceous plants. Grey water from our homes can also be used in very dry spells.
Methods of watering
Watering cans: Most garden watering can be aimed specifically at the stem bases beneath the foliage canopy using a watering can, leaving the surrounding soil dry. This helps to limit weed problems and ensures all the water goes where it is needed, to the roots.
Self watering pots and containers: These have an in-built reservoir that stores water away from the root zone but is connected by a capillary system or wick. The plants will draw water into the root zone as they need it, prolonging the time between watering and collecting and storing rain water. These containers are best watered directly into the reservoir rather than from the surface of the soil so that fines of compost and nutrients are not washed into the reservoir where it can make the water anaerobic and smelly.
Seep hoses: These hoses or pipes with holes in them deliver water accurately to established plants and plants in rows. They can be hidden beneath soil or mulch, which also avoids evaporation losses. Water doesn’t move much sideways from seep hoses. Therefore the lengths of hose need to be positioned across plant root systems, such as going under a shrub, or the lines placed 30-45cm (12-18in) apart in denser plantings. They work best on heavy soil where the water spreads further sideways than on lighter soils.
Automated irrigation systems: To save time and labour on bigger or more water-demanding areas such as fruit and veg plots, install a drip or trickle irrigation system. Only the root zone should be wetted - water that penetrates deeper will be inaccessible to most plant roots, and leach nutrients into the deeper layers of soil. Suppliers can advise on installation of these systems. They can be operated on timers or with moisture or rain sensors but still need checking especially when setting up if wastage is to be avoided. They can be particularly useful if you go away on holiday. Solar powered pumped systems can make use of stored rainwater whereas most drip irrigation needs mains pressure to work well.
Sprinklers: These have only limited use in gardens, as they need mains pressure to work and can use as much water in an hour as a family of four people would normally use in two days.