When and How to Water Vegetables

Well-Fed and Watered

Plants are like babies — liquid continually going in at one end and coming out at the other, and a lot of work involved in the process. Only with plants the intake is at the bottom end, through the roots, and the moisture is lost at the top, through transpiration from the leaves.

Very little of this water is actually absorbed into the plant, but the constant ‘throughput’ is essential to keep the plant cells operating efficiently. Only then can the roots exert enough pressure to absorb nutrients from the soil, can the stomata (openings) on the leaves function so that respiration and photosynthesis (the conversion of energy from sunlight) take place, and nutrients move around the plant. As soon as there is a water shortage the plant becomes less efficient, until it eventually wilts and dies. 

clay-flower-pot-buried-near-a-tomato-plant-makes-an-economical-watering-system_thumb Watering is an energy and time-consuming chore, and since water is frequently in short supply in summer it makes sense both to conserve what water there is in the soil, and to take measures to lessen the need for watering.

Here, very briefly, are some suggestions:

  • Improve the soil structure by working in as much organic matter as possible.
  • Keep the soil mulched in summer to cut down evaporation, the principal means by which water is lost from the soil.
  • Work the soil deeply every few years. This encourages roots to grow deeper, so drawing water from deeper reserves.
  • On steeply sloping land, cultivate across the slope; this minimizes loss of water through. ‘run off’ down the slope.
  • In hot weather carry out as little surface cultivation as possible, pulling up weeds by hand, or hoeing only very shallowly. Once the top few inches of soil have dried out evaporation is slowed down, but it will increase if fresh moist soil is brought up to the surface.
  • Remove weeds, which not only compete with vegetables for water, but increase the loss of water through transpiration.
  • Where space is unlimited adopt wider spacing, so that plant roots can draw water from a larger volume of soil.
  • In exposed gardens in particular, erect artificial windbreaks around vegetables to cut down the force of the wind, which increases evaporation from the soil and transpiration from the leaves.

A postscript to these ‘conservation measures’ could be: ‘Don’t water when it isn’t necessary’. Excessive watering may wash nitrogenous fertilizers below the root zone; it can discourage root growth, so making plants more susceptible to drought; it may increase the growth of the plant as a whole without necessarily increasing the edible part; worst of all, it may reduce the flavour.

However, there comes a time in most summers when vegetables need watering. The most important point to remember is that if watering is to be any good and reach the root zone of the plant, it must be thorough. Light sprinkling on the surface is useless. Soil becomes wet layer by layer, and until the top layer of the soil is saturated, the soil beneath remains completely dry.

Thorough wetting takes more water than you may realize. Poke a finger into the soil after what seemed like a good watering. More often than not only the top inch or so is wet! In hot weather it is not worth giving less than 9 litres/m2 (2 galls/sq yd) at any one time — unless, of course, you are watering seedbeds, seedlings or young transplants.

Watering should, however, be as gentle as possible. Large droplets damage the soil surface, splash mud up onto plants, and can even destroy very young seedlings by knocking them over. These should always be watered with a fine rose on the can, turned upwards if necessary to soften the impact even more. If watering is automated, use a fine gentle sprinkler, or the perforated ‘lay flat’ polythene tubing which is laid on the ground between rows, and is very gentle in action.

The most economical way of watering large individual plants like marrows and pumpkins is to bury a clay pot in the ground near the roots, and confine watering to the pot (see image above). Water will slowly seep through to the plant. The best time to water is in the evening. This prevents water being lost through evaporation.

Much work has been done on how and when to water vegetables. Occasional good waterings can in many instances accomplish very nearly as much as frequent light waterings. This is most true of leafy vegetables such as spinach and cabbage, and has been demonstrated experimentally with summer cabbage. Yields were increased 100 per cent by watering eleven times between planting and harvesting; but by watering only twice, yields were still increased by 80 per cent, and even a single watering, two weeks before harvest, gave increased yields of as much as 65 per cent.

This illustrates the point that many plants have critical periods in their development when shortage of water is particularly damaging, and the application of water particularly beneficial. Watering at these so-called ‘moisture-sensitive’ periods gives the highest returns.

The level of response depends on the type of vegetable. For example, most leafy vegetables (lettuce, celery, spinach, cauliflower, kale, calabrese) respond like the summer cabbage in the experiments quoted. With these vegetables the highest yields are obtained by frequent watering, which encourages the growth of leaves and shoots. They would benefit from 9-14 litres/m2 (2-3 galls/sq yd) each week. Failing this, the best method is a single heavy watering, say 18 litres/m2 (4 galls/sq yd), given ten to twenty days before the vegetable is ready for cutting. That is their critical period.

With vegetables grown for their fruits — for example tomatoes, marrows, cucumbers, sweet-corn, peas and beans (all ‘fruits’ in the botanical sense) — the critical period is when the fruits are setting and swelling. Generous watering at this stage pays dividends. With peas and beans, watering when the plants start to flower, at the rate of 4.5-9 litres/m2 (1-2 galls/sq yd), results in much higher yields. The underlying reason is that once plants start to flower and fruit their energies are concentrated on flowering and fruiting, and root growth is restricted as a result. So unless there is water within reach of the roots, the plant will suffer. The roots will not grow out in search of water as they do when the plant is younger.

watering-continues-in-your-absence-by-conveying-water-from-bucket-to-pots-along-improvised-wicks With root crops, watering tends to encourage the growth of lush foliage at the expense of the roots, so it should be aimed at maintaining steady growth by preventing the soil from drying out. Potatoes are an exception, virtually always responding to watering, which can be timed to promote earlier cropping or higher yields. It must be pointed out, however, that different varieties of potato respond differently.


Moisture-sensitive periods apart, all vegetables are particularly vulnerable to water shortage when being sown and transplanted. So make sure they are not neglected at this stage. If sowing in dry conditions, water the seedbed several days in advance, or water the drills before sowing. If conditions are dry when transplanting — always a traumatic time for a plant — you can save water by watering just a small area, about 15cm (6in) in diameter, around the plant. This is one case where small daily waterings, no more than 0.1 litre (1/4pt) each time, is the best policy until the plant shows signs of recovery. The roots, inevitably damaged by transplanting, can’t cope with more.

Finally, a tip on emergency watering for individual vegetables, or plants in pots or growing bags, when you are away on holiday. Make wicks about 1cm (1/2in) thick with wool, soft string, or glass-fibre lagging. Place one end near the plant’s roots or on the pot or bag, and dangle the other in a bucket of water. Water will seep along the wick to the plant. The same technique, using a mug instead of a bucket, can be used to keep mustard and cress moist on a window-sill.

25. April 2011 by admin
Categories: Garden Irrigation, Gardening Ideas, Vegetable Gardening | Tags: , , | Comments Off on When and How to Water Vegetables


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