Interactive lesson

Garden care, step by step

Interactive lesson: how much water and when, watering at the roots, hoeing, mulch, canes and pinching out tomatoes through the summer.

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The whole lesson as text

1. How much water it needs

In summer a garden needs roughly 20–30 litres of water per m² a week, rain included.

20–30 L a week — In July and August in lowland England a garden needs roughly 3–5 litres per m² a day, or 20–30 litres per m² a week.
Rain counts — 1 mm of rain is 1 litre per m². If 15 mm has fallen in a week, you add another 5–15 L per m².
A rain gauge in the garden — Any straight-sided container, a tin or a jar, in the open. One centimetre of water in it is 10 mm of rain, or 10 L per m².
Root depth — Lettuce and cucumber drink shallow, 20–30 cm. Peppers and cabbage medium, 40–60 cm. Tomatoes and courgettes deep, 60 cm and more, so they cope with a short dry spell.

Why: Plants take up water through their roots and lose it through their leaves. On a hot day a tomato plant can lose 1–2 litres of water. How you water decides where the roots grow.

In the wetter west (Ireland, Wales, western Scotland) the need is roughly 30 % lower; in hot, dry summers (much of the US, an English heatwave) about 40 % higher.
Every soil holds water better when it is rich in humus. That makes compost the best protection against drought.

2. When and how to water

Water seldom and deeply, early in the morning, at the roots and not over the leaves.

Early morning — The water reaches the roots before the heat, and the leaves dry by midday. At midday don't spray the leaves with the hose: half the water evaporates.
Finger test — Push a finger or a screwdriver 5–10 cm (2–4 in) into the soil beside the plant. If it is moist at that depth, don't water, even if the surface is dry.
Seldom and deeply — 10–15 litres per m² at a time, once or twice a week. On slopes and clay, water in two goes with half an hour in between.
Blossom end rot — A dark, sunken patch at the bottom of tomato and pepper fruit is not a disease. The answer is not spraying but even moisture and mulch.

Why: Little and often keeps the roots near the surface. Seldom and plenty draws them down, where they find water even when the top layer dries out.

In the evening, water only at the roots. Leaves wet overnight are an invitation to blight and slugs.
Young transplants and freshly sown beds need a little water, but often, for the first 1–2 weeks.