Watering Plants the Right Way: How, When and How Much

Watering plants the right way means knowing how, when and how much, because depth and timing matter more than frequency.

Water deeply and less often, at soil level, early in the day, and check the soil 5 to 10 cm down before you pour. A garden bed generally needs about 25 mm of water a week in mild weather, which is 25 litres per square metre, and more in heat and wind. Deep soakings push roots down, while daily sprinkles keep them near the surface where they dry out first.

This guide is for anyone watering beds, borders, vegetables and containers outdoors, and it covers quantities, soil differences, methods, timing, critical growth stages and diagnosis. It’s part of the Plant Growing Guides guides. For indoor plants, see How to Water Houseplants Correctly instead, as the rules differ.

Garden watering at a glance

How much is 25 mm?

One millimetre of water over one square metre is one litre. So 25 mm is 25 litres per square metre, which is two and a half standard 10 litre watering cans on a bed of one square metre. To measure what your sprinkler or hose delivers, place a few straight-sided tins around the bed, run the water for 30 minutes and measure the depth in the tins. If you collect 8 mm, you need about 90 minutes for 25 mm. A rain gauge or the same tin tells you how much nature supplied, and you only top up the difference.

Soil type changes how deep the water goes

The same 25 mm does very different things in different soils, so the best schedule depends on what is under your feet. Figures below are rough, and local variation is large.

On clay, water runs off the surface faster than it soaks in. Water for 10 minutes, rest for an hour, and repeat. On sandy soil, water drains past the roots, so shallow-rooted crops such as lettuce and onions benefit from smaller, more frequent doses. Not sure which you have? See Understanding Your Soil Type.

Check the soil, not the calendar

1. Dig a small hole 10 cm deep beside a plant, away from the stem.
2. Squeeze a pinch of soil. If it holds together as a damp ball, wait. If it crumbles dry, water.
3. Check again an hour after watering. The soil should be damp at least 15 cm down. If not, water more slowly or for longer next time.
4. Adjust the interval. If the soil was still damp on watering day, wait longer. If plants wilted before you got there, shorten the gap.

Midday wilting alone isn’t proof of thirst. Large-leaved plants such as courgettes, hydrangeas and tomatoes droop in strong sun even when the soil is moist, and recover by evening. Check the soil first. If it’s moist and the plant is limp in the morning, root problems or overwatering are more likely than dryness.

Choose the right tool

When to water

Early morning is best. Water soaks in before the heat, leaves dry within an hour or two and slugs are less active. Evening watering is acceptable in dry climates, provided water goes on the soil. In humid conditions, wet leaves overnight encourage mildew, blight and leaf spot, so avoid evening overhead watering. Midday watering isn’t harmful to leaves, which is an old myth, but a good share evaporates before it reaches the roots.

Stages when water matters most

• Germination: the seedbed must never dry out (see How to Plant Seeds Step by Step).
• First 2 to 3 weeks after planting: roots are small, so water every 2 to 4 days (see How to Transplant Seedlings).
• Flowering and fruit set: beans, peas, tomatoes, peppers, cucumbers and sweet corn lose flowers or yield when stressed.
• Fruit swell: uneven watering splits tomatoes and carrots, and causes blossom end rot, a calcium transport problem linked to irregular moisture.
• Leafy crops: lettuce and spinach bolt or turn bitter if the soil dries repeatedly.

Containers, baskets and new plantings

Pots dry far faster than beds. A 30 cm tub of tomatoes may need 2 to 4 litres on a hot day. Water until it drains, and choose pots to suit your habits (see Choosing Pots and Potting Mix). If the mix has shrunk from the sides, water runs straight through. Soak the pot in a tub for 20 minutes to rewet it.

New shrubs and perennials need a deep soak, around 10 to 20 litres per shrub depending on size, once or twice a week through their first growing season, and in dry spells for a second. Trees need a slower approach, covered in Tree Watering and Mulching.

Cut water use without cutting plants

• Mulch 5 to 7 cm deep to slow evaporation (see Mulching Mastery). Keep it clear of stems.
• Group plants by thirst, with tomatoes and courgettes together, lavender and rosemary apart.
• Collect rainwater. A roof of 20 square metres sheds about 20 litres per millimetre of rain. Use covered butts to deter mosquitoes.
• Check local restrictions. Many areas limit hose and sprinkler use in drought, and rules on greywater vary.

Climate adjustments

Too dry or too wet: telling them apart outdoors

A worked example: a 4 square metre vegetable bed in summer

A bed of 1 by 4 metres in loam, with no rain, needs about 100 litres a week at 25 mm, or about 140 to 160 litres in a heatwave. Split into two sessions of 50 to 80 litres, that’s five to eight 10 litre cans each time. Water slowly, check the soil 10 cm down the next morning, and if it’s dry at that depth the next session should be longer, not more frequent. If you have a drip line with 2 litre per hour emitters spaced every 30 cm, a 4 metre line delivers about 27 litres an hour, so two runs of 2 to 3 hours covers the week. Emitter rates vary, so measure yours.

If the soil is dry and hard and the water runs off or pools, the soil has become hydrophobic. Poke holes with a fork, use a soil wetting agent sold for the purpose, following its label, and water in several short stages. Top-dress with compost afterwards so it stays absorbent.

How water moves through your bed

Clay holds water for days, while sand drains within hours, so tomato and bean plants in sandy beds need smaller, more frequent drinks. Compost improves both soils because humus holds moisture in sand and opens clay to air. Mulch reduces evaporation and cools the roots, so a layer of straw or leaf mould around courgette and cabbage protects them in a heatwave.

A soaker hose or drip line feeds water to the roots and prevents the leaf wetness that spreads blight and powdery mildew on tomato and squash. Deep watering encourages lettuce and carrot roots to chase moisture downward, and that makes them tolerate a dry week. Uneven watering causes blossom end rot in tomato, because calcium moves poorly when the soil swings between drought and soaking. You’ll also find that a raised bed or terracotta pot dries faster than flat ground, so check them first.

How watering plants the right way depends on depth, timing and soil

Clay holds water and causes root rot, while sand drains fast and dries roots, so loam and compost balance both and mulch protects soil and reduces evaporation by up to 50%. Tomato and pepper need steady water because swings cause blossom end rot and split fruit, whereas lavender and rosemary tolerate drought and suit gravel mulch.

Water deeply to 15 cm once or twice a week, because shallow water encourages weak roots. Water early in the morning and you’ll reduce blight and powdery mildew on squash and rose, and water at the base of hosta and fern rather than over the leaves. Lawn grass needs 25 mm a week in summer, overwatering kills succulent and cactus, and aloe needs a dry spell between drinks. Drip lines of 2 litres per hour suit beds, your extension service or the rhs gives local rules, and the drip irrigation guide shows the setup.

You can test with a finger 5 cm down, and you’ll water only when it feels dry.

Mistakes to avoid

• Watering lightly every day so roots stay shallow
• Watering by the calendar when it rained yesterday
• Wetting leaves of tomatoes, roses and squash in humid weather
• Leaving hoses on without a timer
• Ignoring hydrophobic soil, where dry compost or peat sheds water

A water butt supplies free water for deep watering, described in Rainwater Harvesting for Gardens.

What to read next

Learn to read the signs of too much or too little in Reading Plant Signals, plan sowing and planting with the Seasonal Planting Calendar, and see how to lay out a garden that’s easy to water in The First Garden Plan. Back to the Plant Growing Guides.