Good water supply

Professor Carrot Says

Professor Carrot Says

Never let your plants dry out

Water is held in the soil in spaces and in pores – as the water is removed it gets held more tightly by surface tension.  So the plant has a harder time extracting it.

When the surface tension is too great for the plant to pull the water off the soil particles then the wilt point is reached.

If the soil gets compacted the situation gets worse  – if you add more organic matter as compost or manure, then the soil structure is improved and the plants can get at the water better. 5.5 kg compost per sq m is recommended in Know and Grow Vegetables.  Best to distribute through top 12 inches of soil when digging though those who believe in no-digging principles  believe earthworms will drag it down.

Research shows that this will increase available water in a sandy loam soil by 25%

How much water does the soil hold?

It depends both on the crop and on the nature of the soil. Some soils hold more than others.  Here’s a ready reckoner that tells you how much you will need to top the soil up with under different conditions.  It’s based on the idea that the average root depth is 0.5 meters, that water evaporates at 3.5mm a day (which is obviously going to vary with the time of year and how much the soil is mulched) and that if one quarter of the available water is lost, then the plant is adversely affected. Table from Science and the Garden by Ingram, Vince Prue and Gregory.

Soil moisture content and replacement needs

Soil moisture content and replacement needs

How do you know how much watering to do?

This will depend on the plant and its stage in the life cycle – and the nature of the soil itself.

In early growth,  plant roots continue to look in new areas for water and nutrients.  This continues until limits, often caused by fruiting and flowering are reached.  Or because the soil nutrients are being  fully competed for  by other plants.   So if the plants are spaced widely they can grow with less water because there’s less competition – if they are close together to control size they need more water.  So cauliflowers at 60cm spacing do better with more water but at 86 cm spacing it makes no difference.  Weeds of course compete strongly for water

Root growth reduces at flowering and fruiting times. So then the plants are totally dependent on water – and start to dry out 24 hours after heavy rainfall

The effect of watering plants depends on what you want to achieve. It affects the growth of different plant organs in different ways.

Watering generally results in a rapid increase in leaf and shoot growth.

So with summer cabbage, the size of the head at harvest time is roughly proportional to the amount of water given. This is good because it’s the cabbage head is what we want to eat. By contrast, carrots and red beet will produce more top – but this is not what we want.  It doesn’t go to swell the root in the same way

Too much water available to peas and beans early on gives excess vegetative growth without benefiting the eventual crop. However once they start to flower and produce pods it’s very beneficial.  This will increase the number of pods and the number of peas and their size.

Watering can also affect the time of maturity.  It can bring forward early potatoes but delay maturity of onions.

Watering can also affect the quality and flavour of vegetables.  More water gives more bulk but less flavour.  Organic produce appears to be have more density and flavour.

You can reduce the need for water by

  1. Improving the soil structure with organic material
  2. Get more roots – deeper cultivation and wider spacing of crops
  3. Conserve water by mulching and removing weeds
  4. Water more effectively
    1. Confine to moisture effective stages of growth
    2. Limited the frequency of water applied
    3. Limit the amount of water applied

Advice for individual plants.

If you are going to just give a single watering for leaf crops like celery then do it 10-20 days before harvest – with potatoes – do it at the marble stage

Transplanting  is a real traumatic shock for the plant so try and give them enough moisture in advance to allow them to penetrate the soil surface before capping can occur

Leafy vegetable crops should be watered frequently for maximum results.  Succulence depends on plants never being checked by lack of water or nutrients (especially nitrogen.)  With Celery daily waterings can be useful.  Cauliflower should be watered over the transplant period.

If dry, 15 l / sq m / week is enough to keep them growing well.  Otherwise wait until 2 weeks before harvest and give 22 l per square metre.  Giving more water than this to cabbage and lettuce will result in burst heads.

In fact  with summer cabbages grown on sandy loam – watering the crop 11 times during its growing period  gave a 100% increase in head weight while watering it twice gave an 80% uplift and one watering 10 days before harvest gave a 65% uplift.  So if you are growing outside you could manage perfectly well with a couple of waterings only.

The spacing of Sprouts means that once planted they shouldn’t need watering unless its really dry.

Once peas and beans are up don’t water till they start to flower – otherwise they’ll make too much stem

Tomatoes, marrows, squash, cucumbers, sweet corn also need a lot of water while they’re developing.

Root vegetables – water every 2-3 weeks – 16-22l per sq m. If they dry out and then you water them they’ll split


Earlies – for a high yield  water throughout – earliest then don’t water until marble stage.

Maincrop – don’t water until the tubers start to form

Onions – generally grown at wide spacing and with a deep root system its not necessary to water too much. Wet conditions at the end delay finishing off. Over wintered Japanese onions will be lifted in June and won’t keep more more than a couple of months.  They need to be sown in august and given enough water to germinate

Seed producing plants need to be watered well

Acknowledgement – most of this information comes from J K Bleasdale’s Know and Grow Vegetables series.

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