“Water regularly” is the most common and least useful plant advice. Plants do not need a schedule; they need enough water to replace what they lose to the air, and that changes with plant size, weather and soil.
The rule of thumb: 1 inch (25 mm) a week
Most vegetable gardens and lawns need about 25 mm (1 in) of water a week in the growing season, from rain and watering combined. Over one square metre that is 25 litres; over 100 square feet it is about 62 gallons. In hot, dry, windy weather the figure can double.
Typical needs by plant
| Plant | Typical summer water |
|---|---|
| Established lawn | 25 mm (1 in) a week |
| Vegetable beds | 25–38 mm (1–1.5 in) a week |
| Tomato plant in fruit | 2–4 L (0.5–1 gal) a day |
| Large patio pot | Daily in heat, every 2–3 days otherwise |
| Newly planted shrub | 10–20 L (3–5 gal) twice a week for the first summer |
| Young tree | 40–60 L (10–15 gal) a week in dry spells |
Why it changes through the season
Water use follows evapotranspiration: how fast the sun, heat, wind and dry air pull water from soil and leaves. A full-canopy tomato in July uses several times more than the same plant in May. Agronomists multiply a reference figure (ET₀) by a crop coefficient; our crop water requirement calculator does this for common vegetables.
How to measure what you apply
- Sprinklers: set out straight-sided cups or tins, run for 15 minutes, measure the depth and multiply by four for the hourly rate.
- Hoses: time how long it takes to fill a bucket to find the flow, then use our watering schedule calculator to convert the weekly need into minutes.
- Rain: a simple rain gauge tells you how much nature has already provided.
Deep and infrequent beats little and often
Watering deeply two or three times a week sends roots down where the soil stays moist. Daily sprinkles keep roots shallow and most of the water evaporates. Sandy soils need smaller, more frequent waterings; clay soils hold more and need watering less often. The irrigation scheduling calculator works out the ideal interval from your soil and root depth.
Ways to need less water
- Mulch 5–8 cm (2–3 in) deep to cut evaporation.
- Add compost: every 1% more organic matter helps soil hold more water.
- Water at the base early in the morning.
- Collect rainwater: the rain barrel calculator shows how much a roof can catch in one storm.
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