The one-millimetre rule
Here is the most useful fact in garden watering: one millimetre of water spread over one square metre is exactly one litre. So when guidance says a vegetable bed needs around 25 mm a week, a 6 m² bed needs about 150 litres. That turns vague advice into something you can measure with a watering can or a rain gauge.
How the estimate is calculated
- Crop demand: a weekly depth in millimetres based on how thirsty your plants are.
- Weather factor: hot, sunny and windy weeks increase evaporation, cool cloudy weeks reduce it.
- Mulch: a mulched bed loses less water from the surface, so the estimate is reduced by a fifth.
- Rainfall: subtracted directly, because each millimetre of rain is a litre per square metre you do not have to carry.
- Method efficiency: some water always misses the roots. Sprinklers lose the most to evaporation and paths; drip lines lose the least. The planner scales the total up to allow for that.
The factors are deliberately simple rules of thumb, not measurements of your garden. A trowel test tells you far more than any formula, so use the number as a starting budget and adjust it.
Soil type changes how often, not how much
Sandy soil drains quickly and holds little, so the same weekly amount is best split into smaller, more frequent doses. Clay holds water well but absorbs it slowly; water less often and more gently so it soaks in rather than running off. Loam sits between the two. That is why the soil choice changes the number of sessions rather than the weekly total.
Watering methods compared
| Method | Water reaching roots | Setup effort | Best for |
|---|---|---|---|
| Watering can | Good when aimed at the base | None | Small beds, pots and new plantings |
| Hose with spray gun | Good to fair | Low | Medium gardens and quick rounds |
| Sprinkler | Lowest | Low | Lawns and large, dense plantings |
| Soaker hose | High | Moderate | Rows and raised beds |
| Drip irrigation | Highest | Higher | Permanent beds, greenhouses, timers |
Hand watering
- No equipment to buy
- You spot pests and problems early
- Time-consuming in heatwaves
Automatic drip
- Consistent and efficient
- Works while you are away
- Needs setup, checks and cleaning
When and how to water
Early morning is the best time: the soil is cool, less water evaporates, and leaves dry quickly as the day warms, which helps limit fungal disease. Evening is the next best choice. Aim water at the soil rather than the leaves, and give a thorough soak rather than a daily sprinkle. Deep, less frequent watering encourages roots to grow down to where the soil stays moist.
Check before you water. Push a trowel or finger 5–10 cm into the soil beside a plant; if it is still damp at that depth, wait a day. Pots, hanging baskets and new plantings dry out much faster than open ground and often need checking daily in summer.
Collecting and saving water
- Fit a water butt to a downpipe from a roof, shed or greenhouse.
- Mulch bare soil to slow evaporation.
- Group thirsty crops together so you water one zone, not the whole plot.
- Use hedges or fences to shelter beds from drying winds.
Signs you have got it wrong
Leaves that droop in the afternoon heat but recover by evening are often just coping with the sun. Plants that are still wilted in the cool of the morning are usually short of water. Yellowing lower leaves in soil that stays soggy point to overwatering, which starves roots of air. Adjust the planner inputs and your routine until the soil stays evenly moist without puddling.