How much water a garden and a lawn need
Litres a week, where they come from, and why most weeks need no watering at all.
One millimetre of rain is one litre of water on a square metre. The whole arithmetic of watering follows from that sentence: how many millimetres a week a plant takes, how many the rain gave, and the difference is what you carry.
What a dry summer week takes
| What | Per week | How often |
|---|---|---|
| lawn | 25 L/m² | 2 waterings |
| bed of low-water plants | 10 L/m² | every 7 days |
| mixed bed | 18 L/m² | every 4 days |
| bed of thirsty plants | 28 L/m² | every 2 days |
For 100 m² of lawn that is 2,500 litres in a dry week, two and a half cubic metres. It is why the rain tank in a Verdlo plan is sized for three dry weeks rather than for one watering.
Why a real week is different
Those figures are a dry summer week, and a week like that happens a few times a season. Verdlo works the week out from evaporation: the forecast says how much water the sun, wind, heat and humidity will take (ET0, FAO-56), multiplies it by the plant's crop coefficient and subtracts the rain that fell and the rain that is coming. In a September week with 24 mm of rain, the answer is no watering at all.
Rules that change more than the number
- Water seldom and deeply rather than a little every day: shallow watering keeps roots just under the surface.
- Early in the morning, at the base of the plant, not over the leaves.
- A 5 cm mulch layer cuts the need by roughly a third.
- Double these amounts in the first season after planting, while the roots are still shallow.
- Below 10 degrees do not water: the plants are barely drinking and wet cold ground does more harm than a dry week.
