Down Garden Services

When and How to Water

"Gardening requires lots of water - most of it in the form of perspiration." - Lou Erickson, cartoonist and illustrator

Water is important to the gardener as the hotter drier summers now being experienced in places are leading to water shortages and hosepipe bans. Not all plants require watering and develop deep roots to reach well below the water table. Grass has relatively shallow roots so is usually the first to show signs of drought, but it usually recovers so should not be watered - indeed too much water encourages the roots to stay near the surface and not grow deeper so leaving the grass more vulnerable to drought conditions.
Plants in containers or planters are the most vulnerable and require the most attention, so use larger ones if possible. The foliage directs water to the outside of the canopy and if planted in the open the finer roots would be beyond this to gather the moisture, but in a pot they are prevented from reaching it. Palm trees are better suited for container growing as their foliage directs any rainfall towards the trunk, so it runs down to the roots.

The climate experts predict that in the British Isles we will have hotter, drier summers and milder winters which may be wetter in some areas. This will affect the range of plants that can be grown. In summer more drought-tolerant plants will do well, but these may not survive the winter wet when the soil may become waterlogged - in the British climate it is wet, cold conditions that kill plants rather than cold, dry conditions, eg. alpine plants experience the latter in their natural habitat. The condition of the soil will have greater importance; it will have to retain moisture in the summer, yet have better drainage for the winter. Trees will be the most vulnerable as they require most water and most have relatively shallow root systems, eg. Beech trees are already showing signs of stress in the summer and some have already been lost in Southern England. In these areas Hornbeam is a better choice and arboriculturalists have already decided to replace drought-damaged trees with species such as Walnut which are better adapted to drier conditions.
Digging-in plenty of compost or other organic matter would be the best remedy as it retains moisture and opens up wetter soils to improve drainage. When planting trees which could be damaged by waterlogged soil, plant them on a mound of soil which has a better chance of draining water from the root ball. If drought could be a problem a thick mulch of organic matter over exposed soil will help to keep in the moisture as well as reducing weed seed germination. To water trees which are sitting a bit proud of the surrounding soil it will be necessary to build up a circular dam about 100cm in diameter and 30cm wide at its base, tapering to about 10cm at the top. This should hold the water until it seeps down close to where it is needed.
In containers the inclusion of water-retaining gels hold the water for longer and reduce the amount lost by evaporation.
The best time to water plants is after sunset as there will be less evaporation and it has all night to percolate down to the roots. So-called 'grey' water can be used, this is water that has already been used for washing or water which has been collected from the roof. Any soaps and detergents must be biodegradable to avoid a build-up of toxins in the soil. Also it would be best to avoid watering containers with repeated applications of washing water as any toxins will tend to build up.
Irrigation systems which deliver the water at soil level are best; these can be dribbling nozzles or seep hoses which are made from reconstituted rubber tyres or perforated tubing, and are laid around the garden close to the plants. They can be attached to a watering timer to operate during the night, or the countdown egg-timer-like ones that cut off the supply after the set time.
If planting a large tree or shrub it will assist in watering if a piece of perforated drainage pipe or a pierced plastic bottle is buried beside it to direct some water deeper down. Retain the top of the bottle to keep out debris, but it will have to be pierced as well to let in air allowing the water to flow out through the hole(s) in the bottom.

Using a spray over everything is the least efficient method, some say that if done while the sun is out it can scorch the foliage, but others have shown that the water would evaporate before this could occur and this also has a cooling effect (latent heat of vaporization). Also the plants may not receive enough water if the leaves direct it away from the base of the stems. It is suitable for more open ground such as a new lawn or seedbed. Placing a straight-sided container within the area to catch some water will indicate when enough has been supplied. About 2.5cm should percolate down to about 20cm in moist soil so if the time is noted to reach this level, this can be used for future waterings. A sample digging with a trowel should reveal how far down the water has gone as this can vary with different soil types and the degree of moisture in the soil at the time. However, it is important not to water too often as the plants can develop shallower roots, becoming less drought tolerant by being unable to find water lower down in the soil. So a good watering once a week is better than a dribble every day.

If a lack of water is to be a problem in your area, changing the planting in the garden might be the best solution. Some plants can tolerate such conditions better than others. Eryngiums, Sedums and woody herbs like Thyme and Rosemary are well suited as is Lavender.

Overwatering can be just as harmful to plants as a drought. In fact more indoor plants are thought to be killed by too much water than by too little. The symptoms of overwatering are similar to those of a lack of water, the leaves wilt and turn yellow before being shed. The tips and edges are usually brown. Seeing the wilting can be taken as a lack of water and more water is applied, so check the compost first to see if it is wet - a wilting plant where there is plenty of moisture can also occur if the roots are being eaten as in a Vine Weevil grub attack. The roots also suffer and begin to rot as there is no air left in the waterlogged soil - this is probably what is causing the wilting as there are insufficient roots functioning to supply the plant. Usually the watering is too frequent or the drainage holes in a pot are blocked - in open ground the drainage needs to be improved by digging in plenty of grit and organic matter.

Irregular watering can damage a plant by causing it to split. This can be the bark of trees and shrubs or it is more common with developing fruit and root crops. The split itself is not usually fatal, but the wound can allow in diseases such as botrytis. Try to keep watering regular and in containers with smaller amounts of growing compost such as hanging baskets, use water retaining gel to hold moisture for longer periods and prevent drying out. Irregular watering can cause a condition called blossom-end rot in tomatoes, aubergines and peppers. A blackened spot appears at the end of the fruit due to a lack of calcium.
With house plants feel the weight of the container after watering and check regularly by picking up the pot. After a while you will know by the weight when to add water. With heavier pots use a finger to test the planting medium for moisture, when it feels dry it is time to water.

Where irrigation is used on outdoor crops the salts washed down into the subsoil, can be brought up again by over-watering which raises the water table and the salinity rises at root level. This is a problem for commercial growing in drier regions where there is a risk of desertificaton due to irrigation salinity which poisons the soil and no plants will grow. The irrigation water also contains dissolved salts which can build up in the soil as the water evaporates and is used by the plants. If the levels become greater than those within the plants the concentration gradient is reversed and the plants cannot take up any water.
The overuse of fertilizers can produce the same effect as the levels of dissolved salts increase beyond where the plants can cope.
On a smaller scale the overuse of fertilizer in containers can reverse the osmotic balance so the plant is unable to take in water.

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