How much water does a garden actually need in a dry spell?

Watering advice is almost always given in vague terms — a good soak, water thoroughly, keep it moist — because the honest answer depends on your soil, your plants and your weather. But the arithmetic that connects rainfall to watering cans is simple, and doing it once permanently changes how much water people apply.

Step by step

1. Learn the one conversion.One millimetre of rain falling on one square metre delivers one litre. That single relationship connects every rainfall figure you will ever read to the watering can in your hand.
2. Measure the area you are watering.Not the garden — the bed, or the root spread of the plant. A border two metres by four is eight square metres, so a ten-millimetre equivalent soak for it is eighty litres, which is eight full cans.
3. Decide what you are replacing.In a dry summer week, a growing garden loses a meaningful depth of water to evaporation and transpiration. You are not trying to replace all of it everywhere — you are trying to keep the plants that matter above the point where they suffer.
4. Prioritise rather than spreading it thin.New plantings in their first two years, containers, anything cropping, and anything you cannot replace. Established trees, shrubs and lawns can be left; lawns recover from dormancy and established woody plants have deep roots.
5. Deliver it in one place at a time.Eighty litres spread evenly over a border and eighty litres delivered to the six plants that need it are completely different interventions. The second one works.
6. Adjust for your soil.Sandy soil holds little water and needs smaller amounts more often. Clay holds a great deal but accepts it slowly, so it needs applying slowly and then leaving for longer. Loam sits between.
7. Check it landed where you wanted.Cut a spade slice an hour later and read the wetted depth. That converts your calculation into a measurement of your own ground.

Tips

• A watering can is a measuring instrument. Knowing that yours holds nine or ten litres turns 'a good soak' into a countable number of trips.
• A rain gauge costs very little and tells you how much of the rain you keep hearing about actually fell on your garden. Five millimetres feels like proper rain and is half a can per square metre.
• Water going onto paths, onto bare soil between plants and onto foliage is water not going to roots. Aiming it costs nothing and roughly doubles the effect.
• Mulching after watering cuts evaporation losses substantially, so the same volume lasts longer. It is the cheapest way to reduce the number you have to deliver.
• The real constraint is often what you can carry rather than what the garden needs. Sizing the job honestly leads to better decisions about what gets watered.

Common mistakes

• Watering the whole garden a little — It is the least efficient possible allocation. Most of it evaporates, none of it reaches root depth, and the plants that genuinely needed it got a fraction of what they needed.
• Assuming a hose delivers more than it does — A hose on for five minutes delivers a substantial volume, but spread over a lawn or a large border it is a very small depth. Timing a hose into a bucket once tells you its actual flow and is worth doing.
• Watering established lawns in a drought — Grass goes dormant and browns off, and recovers when rain returns. It is the single largest water demand in most gardens and almost the least necessary.

Questions people ask