The Efficient use of Watering
By Jonathan Diaper
All that rain! The unpredictability of the weather, though, reminds me of the importance of water in our gardens. It has been the driest winter and early spring I can recall. February saw me giving first aid, with a can full of water each, to some lacklustre laurels.
Plants lose most moisture through the leaves by the process of transpiration, the process that makes woodland a cool haven on a hot day. Transpiration increases if the weather is breezy, therefore it does not have to be sunny for a plant to be suffering from drought.
The amount of watering needed will vary depending on many factors including the soil type, soil structure, the amount of organic matter present, whether the area is close to a building (in a ‘rain shadow’) and whether a mulch is applied. Lighter soils need watering more frequently than heavier, clay soils. However, when dry, clay soils need heavier applications of water because they hold more water within their structure. The whole subject can quickly become complicated and veer out of the scope of a blog altogether!
The rain is a gift to newly planted trees and shrubs but these should be watched closely as the weather turns warmer and drier. Overwatering is as detrimental to a plant as under-watering. To be sure of the soil conditions at the roots dig a hole to a spade’s depth and squeeze a little soil in your hand. If it binds together easily no water is needed. If it binds, but falls apart when tapped, then water. If it does not bind at all then saturate. As a general rule the amount of water you give should equate to the volume of compost the plant was planted in. For example, if a tree came in a 35 litre pot then it will need 35 litres of water. In hot weather this is likely to be once a week – although there is no hard and fast rule.
Baskets and containers will demand time for watering, heating up in the sunlight and often basking on a patio. Most bedding plants are sun lovers, so will hate sodden compost. I have habitually watered until the water runs out of the bottom of the pot, but have come to the opinion that this may not be the best idea.
An RHS trial on Petunias and Begonias concluded that the plants performed well when watered little but daily. Only 80ml of water was needed per day to grow a good plant and in neither case did the basket or container drip after watering. They found that overwatering leads to poor quality plants.
The most efficient use of water in the garden, therefore, is the midpoint between two extremes, the arid and the sodden. Mulching will save countless gallons of water, but the mulch needs to be deep enough to be effective (a minimum of 10 centimetres). Adding organic matter to the soil, in the form of compost or manure, creates the happy medium gardeners dream of. Roots are better able to access the available water more easily – and therefore nutrients – held between the soil particles.
The best time to water is in the early morning or evening. In hot weather, watering in the evening avoids the humidity which could encourage fungal disorders. A hose directed at the base of a plant is a more efficient method of watering than using a sprinkler.
Sprinklers will soak the whole area, encouraging weed seeds to germinate which will in turn compete with the plants for water. Irrigation pipes beneath the soil surface deliver water to the roots and may be controlled by an automatic timer. It can be easy to take the system for granted, however, and supplementary watering may be needed at times of drought.
Plants will display their stress to the observant gardener. They may have a hang-dog look, or the leaves may be at a slightly different angle than usual. The colour may be darker, the surface not as shiny as usual.
Planting for the conditions will save a lot of headaches. Many plants, after they have been established, will thrive in drier conditions. Lychnis coronaria will add pinpoints of cerise pink to a border, or grasses like Stipa tenuissima will add an airy touch to a sun baked spot.