How and when to water your lawn
This month we wanted to give you a bit of background to watering summer lawns. We do appreciate that for some of you are looking for the opposite as your lawns resemble paddy fields!
After heavy summer rainfall, you can expect a huge growth spurt once the weather turns warm again. Check that your lawn can breathe, and fork or spike any low areas where any water has been lain the longest.
Summer lawn watering is all to do with water balance:
Imagine your soil is a sponge and your grass is a suction pump.
When rain falls, it fills the sponge until it can take no more. At this point any more rain that falls drains out the bottom. After the current rain, most of you will be at close to this point. This is referred to as the “Field Capacity” of your soil. You can’t add any more water to this.
What happens next is that your grass starts the evapotransipration process. This is when the plant draws water up the plant, moving plant food, and then plays its part in photosynthesis, turning sunlight and carbon di-oxide into sugar. Much of the water then passes out the leaves and is evaporated and lost to your plant and soil.
What you must do is balance the amount of water leaving your soil, through the plant, with the amount that enters it from rain or irrigation.
For the purposes of lawns you should consider your grass to use up approximately one inch (25mm) depth of water every week. This increases if it is hot, dry and windy and will decrease if wet and cold.
That means your grass should receive around 1”(25mm) every week from rain or irrigation.
How do you measure this?
You need to get a rain gauge and record your rainfall each week. Follow this link to see the type of gauge you need:
Subtract the amount of rain you have received, in a week, from 1”(25mm). The result is how much you should add by hose.
eg: 25mm required – 5mm rainfall = 20mm to be added by irrigation
When you are adding your irrigation water, use your rain gauge again to measure what your sprinkler is adding. Check in three spots for evenness of application and time how long it takes to add what is required.
Sounds easy doesn’t it!
Here are some complications you need to consider:
- Don’t irrigate every week. Add up your deficits (the amount you should be adding) and water deeper every fortnight
- From an environmental point of view, allow an increasing deficit over the season. i.e. if you should apply 20mm to keep balance, only apply 15mm and start an “overdraft” on your account. All soils are different, but you can allow a total deficit (overdraft) of up to about 1”(25mm) to 2”(50mm)
This principle is called irrigation scheduling and is used extensively in the farming world to better utilise water as scarce resource.
If you want to increase your accuracy, you can find the actual evapotranspiration rates for the week. This can be found from a variety of sources including the burgeoning live weather station feeds on the internet.
Follow this link to a good example from Sutton Coldfield:
As we go to press, this station is showing over 4.5” of rainfall in the last month! It is also showing daily evapotranspiration rates of just about 1mm. This means that as opposed to using 1”(25mm) a week, the grass is currently only using about 0.35” (7mm) per week.
If my explanation is still a bit unclear to you, the following article may help. It is referring to the American situation but much of it is relevant to the UK:
Remember to add more water to the dry areas of your garden, especially under large trees and shrubs. Much of the “shade” problems in gardens are caused by drought owing to the huge quantities of water that trees can consume (they can be use up to 200 gallons per week for 1 tree!!)
When it gets hotter and drier, remember to avoid watering in the heat of the day as you will lose more water to evaporation. Watering your grass early in the morning best.
Next month we will discuss some troublesome weeds and summer maintenance of heavily used parts of your lawn.
Talk to you then.