The formula
minutes = area × depth ÷ efficiency ÷ flow × 60
Where it comes from
Watering is thought about in litres per square metre, the same way rain is measured in millimetres, and they are in fact the same figure: one litre per square metre is exactly one millimetre. But it is carried out in minutes of running water, and the bridge between the two is the flow of the system. Efficiency comes in because not everything that comes out reaches the root: drip loses little, sprinklers lose to evaporation and wind, and a hose by hand runs off a fair amount.
How to work it out by hand
- Multiply the area by the depth you want to apply
- Divide by the efficiency to get what has to be put out
- Divide by the flow of the system: that gives hours
- Multiply by sixty for minutes
What is worth knowing
Watering a little and often is the most widespread mistake: it keeps the top couple of centimetres damp, the roots never go down looking for water, and the plant is at the mercy of any hot day. Longer, more spaced waterings force roots deeper and give a far more resilient plant. Time of day counts too: at dawn far less is lost to evaporation than at midday, and the leaf does not sit wet all night, which is how fungal disease starts. And one check worth more than any calculation: push a screwdriver into the soil a while after watering. If it goes in easily to fifteen centimetres, the watering was enough.