Watering Tomato Plants Consistently for Better Harvests

Why Consistency Matters More Than the Amount

Tomatoes are fussy about water in a way that few other allotment crops are. A courgette will forgive you a dry week; a tomato plant will sulk, drop its flowers and reward you with a handful of cracked, tasteless fruit. The reason is that tomatoes evolved in seasonally arid conditions and cope with dry spells by shutting down. What they cannot cope with is swings — bone dry on Monday, drenched on Friday.

When the soil moisture lurches about, the plant's uptake of calcium becomes erratic. Calcium is carried through the plant in the water stream, and it cannot move backwards through the stems. If water is short during a flush of growth, the far end of the fruit — the blossom end — is the first place to be starved. The result is the sunken, leathery brown patch of blossom end rot, which ruins more greenhouse tomatoes in the UK than any pest.

The fix is not a calcium feed. It is a steady, reliable supply of water at the roots, week in, week out, from the moment the first truss sets until the last fruit is picked.

How Much, and How Often

As a working rule for a British summer, give each outdoor tomato plant around two litres every two or three days in cool, dull weather, rising to two litres daily in a hot spell. In a greenhouse, where temperatures climb far higher and air movement is limited, expect to water every single day, and twice on the hottest days of July and August.

Plants in containers need more attention still, because their roots cannot reach deeper moisture:

• Growbags: two to three litres per bag per day once the plants are established, split between the stems. Water slowly so it soaks in rather than running off the plastic.
• Large pots (30cm or more): check daily by pushing a finger two inches into the compost. Water until it runs from the drainage holes.
• Ring culture or bottomless pots sunk into a bed: water into the ring; the roots below will find the moisture.

Aim for moist but never waterlogged. Roots sitting in cold, saturated compost sulk and yellow, and you will have swapped one problem for another.

Water the Roots, Not the Leaves

Tomato foliage is best kept dry. Wet leaves, especially in the still, humid air of a closed greenhouse, are an open invitation to blight and grey mould. Direct the water at the base of the stem, or better still, into a sunken pot or a length of pipe pushed alongside the plant so the water lands deep where the roots actually are.

Water in the morning if you can. The plant then has the whole day to take up what it needs, and any splashes on the leaves dry off quickly. Evening watering leaves foliage damp overnight, which is exactly the condition fungal spores are waiting for. If evening is the only time you have, water at the roots and leave the greenhouse door and vents open until dusk.

Mulch, Drip Lines and Other Time-Savers

Most watering problems on an allotment are really time problems. If you can only get to the plot every other day, build in some insurance:

• Mulch generously. A 5cm layer of garden compost, grass clippings or well-rotted manure over damp soil cuts evaporation dramatically and keeps the moisture level far steadier.
• Install a soaker hose or drip line on a timer. It delivers water slowly and evenly, and pays for itself in one dry August.
• Use saucers under pots and stand growbags on a gravel tray so run-off is not wasted.
• Shade the greenhouse with a wash-on shading product or a piece of fleece over the roof in midsummer, which reduces water demand at a stroke.

Reading the Plant's Signals

Your tomatoes will tell you how you are doing. Learn the signs and adjust before damage is done.

• Split fruit after a dry spell followed by heavy rain or a soaking: classic erratic watering. Keep it steady and pick fruit before a storm if it is forecast.
• Dark, sunken patch at the blossom end: blossom end rot, the calcium-uptake problem described above.
• Rolled, leathery leaves in the afternoon that recover by evening: normal heat response, not drought. If they stay curled all morning, water.
• Wilting that persists into the cool of the evening: the compost is dry well below the surface. Soak thoroughly rather than sprinkling.

Adjusting Through the Season

Young plants recently moved to their final position need less water than a mature cordon carrying five trusses, so start moderate and increase as the plant grows. Once the fruit begins to ripen, ease back very slightly — a small reduction in water at this stage concentrates flavour and sugars — but do not let plants flag, or you will pay for it in splitting.

Keep the routine identical whether it rains or not. A rain shower rarely penetrates a leafy canopy to the roots, so a dry-looking surface under a tomato plant tells you very little. Push a finger in, or use a cheap moisture meter, and water on what you find. Do that, and you will be picking heavy, glossy, well-flavoured fruit right through to the first frosts.