Watering on London clay: why we programme every irrigation system with cycle-and-soak

Most gardens in Teddington, Twickenham, Richmond and Kingston sit on London clay. It holds nutrients well, it cracks in August, and it takes water slowly. That last point is the one most irrigation systems ignore.

The numbers

A clay soil absorbs water at roughly 5 to 8 millimetres an hour. A typical pop-up spray nozzle applies 80 to 130 millimetres an hour. A rotor on a half-circle arc applies around 10 to 12. A rotary nozzle applies about 16. Everything except the drip line is putting water down faster than the ground can take it.

What happens to the rest is easy to see after a run: puddles on the lawn, water running off the beds onto the path, a wet surface and a dry root zone underneath. The controller says the lawn had twenty minutes. The roots got five.

What cycle-and-soak does

Instead of one run of twenty minutes, the controller runs the zone three or four times for five to seven minutes each, with a pause of about thirty minutes between them. During each pause the water that has landed moves down into the soil, so the next cycle lands on ground that can accept it.

The total run time is the same. The amount of water that reaches the roots is not.

How we set it up

Rain Bird’s ESP-TM2 and the newer smart controllers have cycle-and-soak built in: you set a maximum cycle length and a soak time per zone, and the controller splits the programme for you. On a clay lawn we usually start at three cycles of six minutes with a thirty-minute soak, then adjust after watching the first few runs.

Where a controller does not support it, the same result comes from writing three short start times an hour apart. It is less tidy, but it works.

Why it matters more on a pumped system

If your garden runs from a break tank and a pump, cycle-and-soak also spreads the draw on the tank across the programme. A tank that empties halfway through one long run leaves the last zones dry; short cycles let it refill.

Drip lines are the exception

Pressure-compensating dripline puts down about 7 litres an hour per metre, spread along its length, which is slow enough for clay to keep up with. That is one reason we use it for borders and hedges rather than sprays. It still benefits from a rest between runs in the heaviest soil, but it does not need the same splitting.

A note on mowing height

Irrigation and mowing height work together on clay. A lawn cut at 65 to 75 millimetres in summer shades its own crown, loses less water and needs less irrigation than one cut short. We raise the cut on every maintenance lawn from June and lower it again in September.

If you would like us to look at how your garden is watered, book a free visit or read about our irrigation systems.