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Water Requirements for Tomatoes

More people have problems due to irregular watering with tomatoes than any other cultivation issue. Allowing tomatoes to dry out and then deluging them with water causes the fruit to suddenly swell, cracking the skin of the tomato and this allows fungal growth to get a hold, completely ruining the fruit.

Watering Tomatoes

Watering Tomatoes

Calcium Balance

Worse still it alters the internal calcium balance of the plant and this causes blossom end rot where the base or blossom end of the tomato develops a brown patch. Poor watering practice also contributes to blossom drop, this is where the flowers fall off and no fruit develops at all.

Keeping the compost too wet can be as bad, it can cause the roots to rot and help rot to develop at the base of stem.

Too much water sometimes results in drowning the roots but usually it just deprives the plant roots of oxygen which slows growth and reduces vigour.

What Affects The Water Requirement of Tomatoes

The experienced grower develops a feel for how much water he should give to the tomatoes. It will vary according to how developed and large the plant is, the more leaf area the more a plant will transpire, a fancy word for sweat, and lose water to the air.

The transpiration rate of tomatoes is obviously affected by the weather. Just as we sweat on a hot, sunny day but not on a cold, cloudy day , tomatoes transpiration rate will vary.

The problem for the new gardener is developing that judgement, how much water to use. The table below should help give some guidance. It assumes you have a full sized tomato plant in a greenhouse. Adjust for the size of the plant and if the compost feels too wet (using your water meter mark one finger) or dry, change as necessary.

Growing Tomatoes in Growbags or Pots

If you are using growbags, then putting your finger into the compost is the only way to be sure there is enough water. With pots, the compost may be dry on the surface but wet underneath.

Growing Tomatoes in Greenhouse Border

If you are growing your tomatoes in the greenhouse border then there is probably a reservoir below surface level but you still need to add water to keep it topped up.

One good method is to cut the base off plastic bottles and insert them, neck end down, by each plant. This delivers the water to the root area and doesn’t encourage the roots to grow across the surface as just watering direct can.

It also keeps the soil surface dry, which prevents the greenhouse becoming too humid in poor weather encouraging fungal disease. It also helps prevent weeds from germinating in the border, keeping your work down.

Another is to lay white plastic over the border. You can either buy the plastic or cut things like fertiliser bags to form sheets. The white colour has the benefit of reflecting light up onto the plants, helping fruits to ripen.

Do not use black or clear plastic as this can over-heat and cook the plant roots, killing your crop. You can water through bottles as mentioned above or use a drip hose under the plastic to water.

Water Requirements of Tomatoes
Weather Conditions Litres per Day Pints per Day
Very dull, cloudy or dull for most of the day 0.14 -0.28 0.25 – 0.50
Fairly dull, overcast for most of the day 0.28 -0.42 0.50 -0.75
Fairly sunny, cloudy at points with bright periods 0.71 – 0.85 1.25 – 1.50
Sunny, just occasional cloud cover 1.10 -1.20 2.00 -2.25
Very sunny, clear blue skies all day. 1.50 -1.80 2.75 -3.25

Further Information: Tomato Growing Guides