Watering regimes examined

Although it is quite possible to garden successfully in many ways without hardly ever doing any watering it is also one of the most productive and effective labours we can undertake if we wish to improve most of our kitchen garden crops. Indeed I suspect many of us do not realise just what good results can be obtained by assiduous watering compared to many other gardening chores we consider equally beneficial.

Even in the wetter regions such as the west of the UK there are often many weeks during the growing season that have less than the optimum amount of rain. On average a vegetable bed full of crops needs approximately an inch of rain every week, more if it is sunny and windy and slightly less if it is colder than usual or later when the crops are mature. In the drier south and east more of the growing season produces much less than an inch per week and watering is even more important. Ministry figures suggest that for farmer's field crops extra water is required nine years out of ten if respectable yields are to be obtained.

Obviously not all crops require as much water as others and indeed deep rooting perennials such as fruit trees can tap reserves of water in the deeper soil strata and so need less attention. However as we now grow more and more of our fruit as trained and dwarf forms on constrictive root stocks their ability to find deeper water becomes considerably reduced and so these too will benefit from extra water whereas an orchard of large old standards nearby may happily get by with no extra water at all.

The crops that need most water are those that are very succulent such as the saladings, leaf vegetables and those that are high performers such as sweet corn. (If you think about the amount of material produced in a season by one sweet corn plant you can soon see why -not only the green plant but the immense amount of sugar and carbohydrate manufactured and turned into kernels.) But of course all plants also need terrific amounts of water just to grow; and this water is only passed through them and not retained as material. In order to suck up soil water containing nutrients the plant has to transpire from the leaves and although invisible this can be extraordinarily large. A fruit tree can be transpiring hundreds of gallons of water per day and of course if there comes any shortage then all the processes have to slow down or even stop entirely.

Wilting is the most apparent symptom of gross water shortage and if it ever occurs then much damage has been done and a considerable proportion of the crop is likely to be lost. And the check in growth, even if soon overcome, has other deleterious effects. Potato tubers develop Spraing which is a cracking and hollowing inside the tuber. Fruits cease swelling temporarily and the skin hardens and then cracks when growth picks up again. And worst of all; water stress makes the plants much more prone to pests and many diseases.

For example it is generally held that aphids suck sap, this is not true, they have no mechanism to suck, they merely pierce the cell walls and the sap pressure forces it through their bodies and out the back end where it emerges as honeydew. The aphids remove proteins and mineral nutrients but have little use for all the sugars so these are passed through. It is their presence that makes honeydew a problem as bacteria and fungi use them as a resource turning the residue black and this coating on the leaves then prevents them working efficiently.

As aphids do not suck they prefer plants with a high sap pressure to a low one, plants have higher sap pressures when water is in short supply or when very high levels of fertiliser exist in the soil water. The reason for this is because of the way water moves from dilute solutions into more concentrated ones striving to equalise their strength. If the soil solution is very strong then the sap needs to be even more concentrated in order to 'suck' water into the plant (bringing with it some of the dissolved components that can pass the selectively permeable membranes of the plant's roots.) Thus the drier the conditions the more likely aphid attacks become.

Similarly many diseases 'prefer' plants that are under water stress, in particular mildews which are almost always initiated by conditions of damp air and dry roots. The plant is desperately trying to work uphill transpiring into damp humid air which does not receive more water vapour very easily, and which the roots are unable to replenish quickly enough. The plants have their stomata wide open and the damp leaf surfaces are ideal for the mildews to 'germinate'. Indeed almost all mildew attacks are prevented or reduced by careful attention to soil and air moisture levels.

And of course we must not overlook the fact that almost every crop is three quarters, or more, water anyway. (and some such as cucumbers seem to be about 150% water) And not only the succulent ones but even apparently hard material such as carrot or potato is still nearly all water. Even the hard dry starch found in the sweet corn kernels mentioned above is still mostly water, combined with a tad of carbon dioxide and hardly anything else.

Now of course at the end of a wet winter there is usually ample soil moisture but this can soon disappear, especially if there are strong winds which can quickly dry out the soil. Thus although most of us pay attention to watering when we sow or transplant we are not so careful when the seedlings emerge or the transplant starts to grow. But actually this is a most critical time as the strong winds of late spring can soon desiccate the top layer of soil into which the roots have only just encroached and before they have had time to delve deeper. So many crops get checked when they are tiny as their root systems are themselves so tiny. Later on when the root system has enlarged the likelihood of serious checking is much reduced as the roots find deeper reserves.

Indeed the earlier a plant gets a check from insufficient water the worse the damage and many crops will at least get to harvest if they are but started off well enough. The root crops in particular are the most enduring and I have found that early sown carrots did remarkably well in a bad drought as they followed the water down as the top soil dried out, whereas later sowings could never catch up even though given copious amounts of water from on top.

It has been shown that for almost every crop that flowers; sweet corn, beans, peas, potatoes and all the cucurbits such as courgettes, pumpkins and cucumbers, then one substantial watering when the flowers appear will double yields except during the very wettest years. Of course using mulches and close planting to conserve water helps but nothing can supply the sheer quantity of water the plants will utilise at this time in their growth other than pouring it on.

The problem is how to apply the water most efficiently, and least expensively. [Those of you on a meter may well find, depending on your local authority, that you pay much more for your water in the garden than you first suspect. Although water is sold to us by the cubic metre at not a totally unreasonable charge you will find that in many regions your sewage charge is based on your water consumption! Thus if you use equal amounts in your house for domestic purposes and in your garden you will be paying twice as much for your sewage as you ought as obviously half the water consumed and paid for is never going out again as sewage -but you will still be charged for it anyway unless you can make the authorities agree otherwise!]

And of course there is the other problem of a hose pipe ban just when you need to use it most. The perfidy of our system that allows car washes and commercial manufacturers to carry on p****** away our drinking water as much as they please while we are prevented from using it on our food crops is truly disgusting and corrupt, to say nothing of allowing them to continue their profligate waste when our house supplies are cut off and we are forced to use standpipes!!! Incidentally a hosepipe ban is usually controlled by the rivers authorities not the water companies themselves and is apparently a ban on hosepipes not just on using them for tap water. I gather it is equally a crime to use a hosepipe during a ban for siphoning water from your bath or even a water butt! Bureaucracy being the usual thoughtful, sympathetic and humane b**** it usually is. Fortunately they rarely dare prosecute such patently obvious minor misdemeanours but never the less we must obey such stupid laws.

There are however certain little publicised exceptions to hose pipe bans. For example I have had it confirmed that during a ban I may use a hosepipe for hygiene purposes such as cleaning down my cider press, cleansing a hen house or a dog kennel. (I got in hot water for a tongue in cheek remark on the radio that I had imagined putting a kennel on wheels and towing it alongside a row of trained fruit whilst I methodically gave it a thorough cleansing, admittedly I don't actually have a dog, but I might be about to get one and I'd never want to put a new pet in an old fouled kennel would I?)

I believe there is also an interesting and little known allowance of 12,000 gallons per day of water extracted from your own well as long as it is used for private purposes only. (Though the law is very unclear I suspect you may still not use a hosepipe to deliver this. Certainly the restrictions on hosepipe use seem unsustainable if tested in the courts as manifestly the ban ought in justice only be on using them for mains water and not for siphoning rain or well water of your own, perhaps some legal eagle may wish to test this out....) However until this question is settled I shall go on using siphons to move my own water around my garden whether they allow it or not. (See my article on using syphons to move the water for you in Kitchen Garden issue February 2000 and on wells in July 2001.)

However to get back to the main point of this article; assuming you have actually got a source of water how is this to be most effectively supplied to the plants. Usually almost all watering is done by one of two methods, spraying or sprinkling overhead or by hose or can to the soil at the base of the plants. Both methods are simple and straightforward but have serious drawbacks. When you spray or sprinkle water some of it is immediately lost as the smallest droplets evaporate while still in the air -that's assuming it is not actually raining when you water, though this would actually be the best time!!!

Much more water will evaporate from the wet soil and wet foliage engendered by the spraying and so be lost. This also cools down the foliage and the soil which is not generally desirable most of the time, though I suppose it could be advantageous in extremely hot conditions but then one would have to be careful not to scorch the foliage at the same time. I have seen leaves killed by being wetted in very hot sun, but truly not often in England. The wetting of the foliage also makes it prone to attack by many fungi who can only 'germinate' on wetted surfaces such as the mildews mentioned above. The wetting of the soil surface also encourages weed seeds to germinate, makes it easier for slugs and snails to move around and worst of all encourages the plants to bring their roots up to the wet surface when in fact they should be diving deeper.

This formation of a surface mat of roots is not in itself that much of a problem if you continuue to water regularly but if you stop, or only do so sporadically, then they are in the wrong place and the plant will suffer. This has long been acknowledged and the old advice of not splashing a little water on each plant's roots every day but doing a good soaking to each once a week is quite correct, -and can be done on a daily basis by thoroughly treating one seventh of your plants each day in turn over the week. But far better for most established plants of any size is applying the water more directly to the roots at a deeper level.

The simplest method is to bury a flowerpot beside or between the plants, even better is a plastic bottle made into a funnel by cutting the bottom off. These will slowly trickle water down to the roots but can be filled in very quick time thus speeding the whole job up. A simple hole can work but soon becomes a muddy puddle and needs re-excavating too often however planting several plants around the rim of a bowl shaped depression can work well though there is still rather too large a surface wetted for convenience. Running a watering trench alongside a row of plants can be a very successful method and is suited to such as tomatoes but may require more water than can be supplied by a can.

For many years I used trench irrigation for my raised beds. By simply stopping the ends with a pile of mud the trench between could be filled with water, this then seeped into the beds on either side. The surface of the beds remained dry and as the bottom of the trench was well panned from constant traffic little went straight down. (I was able to use a hosepipe from the mains as at that time I paid a nominal charge for unlimited use.) Although I did not apply any more water than a sprinkler running for the same time the water was much more effectively placed and my increases in yields were most satisfactory, however the current metering regulations prevent me using the mains water as it is too damned expensive. (And I am not happy about the potential pollution by chlorine etc.). Also and not a minor point is that mains water is rather cold and this in itself does not help the plants. In fact the trench method is better here as well as it enables the water to warm up somewhat before it reaches the plants. Even allow a can of water to warm up before use.

Seep, trickle and perforated hoses buried in the soil can be very effective but are most suited to permanent crops such as soft fruit as the extra aggravation of burying these under vegetable crops is a hassle. Laying them on the surface at least gets away from the losses due to evaporation in the air of sprinkled water but still gives rise to the others of some soil surface evaporative losses and cooling, surface rooting and pest encouragement. But of course they can be very low labour once installed, but are then banned along with hosepipes in a drought. They are most effective when buried but then there is the problem of not knowing what is actually happening when they have been buried some time. I once installed such a system of seep hose to be most disappointed as in fact one accidentally large perforation allowed most of the water to flood away in one spot unnoticed while all alongside the rest of the pipe it was as dry as a bone. Never again.