Irrigation

Watering the garden

When to water and how much.

Many garden failures are due to incorrect watering, either using too much, or too little water. Too much leads to insufficient air in the soil and rotting of plant roots, called 'wet feet'. Too little water reduces yields and, with certain plants, leads to premature running to seed, called 'bolting'. Some plants, notably certain trees, will not respond to water applied after they have reached the stage of water stress, until the following season. Basically, leaf vegetables require as constant a supply of water as possible. Plants grown for their seeds or fruits should be watered less frequently and more deeply. Under the same watering regime as leaf vegetables, they will produce foliage at the expense of fruit.

An example of the effects of watering is the humble potato. Watering prior to flowering has no noticeable effect. Watering at flowering results in an increase in the number of tubers. Watering after petal fall increases the size of the tubers. Another is peas. Watering prior to flowering can decrease yield, watering after petal fall increases yield.

Water should be applied at the appropriate time. Watering during the heat of the day stresses plants and can cause heat scorch due to concentration of the sun's rays. The leaf pores that allow the plant to transpire moisture are stimulated to open by irrigation. If it is hot and/or windy, it may result in more moisture being lost than is applied! Plant moisture use is greatest during the night, so watering in the evening may be best. If humidity and fungal disease is a problem, then watering in the morning is better.

Frequency of water application and depth of watering depend on the type of soil you have and the humus content. Well composted soil holds more water than the same soil without compost. Sandy soil holds less water than clay, so it needs more frequent and lighter applications. Because of the quicker drainage of sandy soil, overwatering is less critical than with clay soil.

Watering too frequently leads to excessive root development near the soil surface. Under these conditions, plants become very sensitive to water shortage. Deeper, less frequent application promotes deeper rooting into soil that remains moist for longer. Sandy soil should be watered about twice a week, loamy soil about once a week and clay soil about once every ten days. The amount of water applied depends on prevailing weather conditions. Windy weather produces lower soil moisture levels, hot weather can too. Cool, still conditions reduces watering needs. The crops' level of development must also be considered. At the seedling stage, plant roots are very shallow and prone to stress from lack of adequate moisture. Somewhat later, there is little leaf area to transpire water and the plants needs are not great. When the crop has developed fully, the large leaf area transpires a lot of moisture leading to high water loss from the soil.

The ABC Country Hour broadcasts the evapo-transpiration results for many localities once a week. This is the water loss from open water in millimetres. It can be used to calculate the amount of water needed by your crops. Since rainfall can be quite localised, you will need a rain gauge to calculate the difference in rainfall between your location and that of the recording station, so you can adjust the evapo-transpiration estimate.

To measure the rate of water application of your sprinklers, place three flatbottomed, straight sided jars or cans, next to a sprinkler, half way to its limit and near the limit of its throw. Turn it on for ten minutes and measure the depth of water in each of the three vessels. Add the three figures together and divide by three to get the average and divide by ten. This is the amount of water you are applying in millimetres per minute.

Methods of Applying Water

There is a wide variety of methods of irrigating. The simplest is a bucket, the most complex (and expensive) is trickle irrigation. One consideration to take into account when deciding the most appropriate for you, is the nature of the soil. Sprinklers lead to considerable water wastage by evaporation and can lead to a concentration of salts from the water at the soil surface. Overhead watering can compact sensitive soils, creating a crust that seedlings find difficult to break and which inhibits gas exchange between the soil and atmosphere. Flood irrigation, a sound alternative where crusting is a problem, is not possible if there is no clay sub soil. Trickle irrigation only suits widely spaced plants (you wouldn't use it for carrots, for instance), but uses the least water. Hand held hoses allow tailoring the application to individual crop needs, but requires a lot of labour.

In my own rather large garden, a variety of irrigation methods are used. Overall watering is accomplished by butterfly sprinklers. The orifice in these is quite wide, so the water needs no filtration, an important consideration with gravity fed water from a not very high dam. Filters dramatically decrease the amount of water flow. Crops that need more than the base amount of water are watered by hand held hose. The rose has a lot of tiny holes that allows a good flow of very small droplets, minimising soil compaction. Transplants are watered in by watering can. The fruit trees' needs are met by trickle irrigation. Some potatoes are grown outside the irrigated garden area and are watered by flood irrigation between the ridges. This minimises the risk of blight caused by wet leaves.

While mulching reduces water loss from the soil, reducing irrigation needs, some mulches reduce the effectiveness of watering. Hay and sawdust mulches shed water, so it is worth considering flood irrigation where these materials are used. Alternatively, trickle tape, or soaker hose can be put down prior to mulching.

Some Individual Crops

Celery is most affected by water shortage, closely followed by lettuce. As well as potatoes, celery, tomatoes, cucumbers, pumpkins, squash and peas are prone to fungal disease caused by wet leaves. Leeks and garlic have a much higher water requirement than their cousin, onions. Irregular water supply cracks cherries and carrots and causes blossom end rot in tomatoes. Radishes need a constant supply of moisture or they become too strongly flavoured and woody. During a recent drought I ran out of water. Despite the lack of irrigation or rain from January until March, the garden survived. The pumpkins failed to achieve their usual size, but they never tasted so good.

  

Crop Total Water Requirement Interval
Peas (early) 35-50 mm 2-3 waterings between flowering and pod swell.
Peas (mid to late) 50-90 mm
Beans (early) 100 mm 3 applications
Beans (late) 150 mm 5 applications
Onions 150 mm 4-5 applications
Carrots 150-180 mm 5-6 applications
Brassicas (early) 140 mm 4 applications
Brassicas (late) 80 mm 2-3 applications
Potatoes 180-200 mm 4 applications

Trickle irrigation

Trickle, or drip irrigation uses low pressure, well filtered water and a permanent network of small diameter polythene pipe. There is a wide variety of outlets to choose from. The cheapest is micro tube, also called spaghetti tube. The length of the tube determines the flow rate, which is generally adjusted to between 2 and 4 litres per hour. Moulded drippers are also available. There are two main sorts, pressure compensating and non pressure compensating. The latter are only suitable on short runs, or where the irrigation line slopes downward toward the end of the run. Micro sprays are available with a variety of spray patterns and outlet rates (50-90 litres per hour). They water an area of about 1 metre diameter.

This method of irrigation has the advantage in orchards of not watering the weeds or sod between the trees. Labour is minimised and poorer quality (salt content) water can be used. A big disadvantage is the need to monitor the drippers closely, as they clog easily. Some drippers are more easily cleaned than others, so it is best to spend the few extra cents required to buy the better sort. In a drought drip irrigation may not meet the trees' water needs. The trees respond to drip irrigation by creating a cluster of roots where the water enters the soil. This may lead to uneven root development. Where the early part of the season is wet, followed by a prolonged period of no rain, the tree responds quite slowly to the change from overall soil moisture to the concentrated area around the dripper. Capital cost is the highest for this method of watering.

Mulching

The word mulch comes from the old English word "melsc", meaning rotten hay. In modern parlance, it means any material used to cover the soil as a preventive to moisture loss and weed growth. The organic grower also prefers to use materials that will decompose and so add to the level of humus in the soil.

Rocks, hay, aluminium foil, plastic, leaves, straw, sawdust, gravel, compost, paper and grass clippings are all in common use. Each has its own advantages and disadvantages.

Rocks

Rocks are frequently used around trees. In an arid climate, water rising through the soil condenses on the underside of the rocks as they cool during the night time and take quite a while to warm up during the day. They mediate the soil temperature, protecting it from being too high or too low. In the cool climate garden, they have been used around warm climate plants, such as tomatoes and pumpkins. The heat they store during the day is released slowly at night, reducing the effect of the cool nights.

The disadvantage to rocks is that weeds grow readily between the rocks. Since it is advantageous to use fairly big ones, moving them in order to weed could prove an arduous task.

Hay

Hay is a feeding mulch. That is it contains nutrients that the plants need and they are released gradually as the plants need them. It is a relatively short-lived mulch on this account. Unfortunately, most hay contains a multitude of weed seeds. When used a foot or so thick (fluffed up), this doesn't matter too much. If the layer is thin and/or compressed, then the weed seeds will germinate and take root in the mulch. Since many will be grass seeds, this could spell disaster.

Hay is successfully used to grow potatoes. The weeds where the potatoes are to grow are flattened, not cut. The seed potatoes are placed on the ground 30cm or so apart each way. The hay is then pulled apart and allowed to fall on the ground. When the hay is trampled, it should be about 20-30 cm thick. If the fertility of the soil is low, compost should be spread beforehand. The fluffing of the hay allows any seeds to fall through the hay to the ground where they will not be a nuisance until after the potatoes are harvested. The turf will by then have completely decomposed, needing much less effort to till. The potatoes all grow on the surface of the soil, protected by the hay from greening. Yields much higher than by the conventional method are frequently reported.

Aluminium Foil

Reflective aluminium foil might seem a strange material to use, since it is quite expensive. It does have two great advantages however. It lasts a long time and it repels aphids. Apparently, the insects really dislike their habitat being brightly lit by reflected sunlight. Irrigation must be below the foil, so trickle irrigation is mandatory.

Plastic

Plastic, generally black, is cheap, easy to apply and effective. Like aluminium foil, irrigation should be of the permanent trickle type. The disadvantage of plastic is that it does not decompose top create nutrients; it falls apart under the influence of sunlight into unsightly, unecological shreds. It is somewhat more permanent underneath an organic mulch of wood chips. Black plastic is also said to warm the soil, an advantage in the spring in a cool climate.

Leaves

Leaves are what nature uses to mulch trees and on that account have a lot to offer.

Straw

Straw is one of the best mulches. It is light and easy to handle and relatively weed seed free. It is not as water repellent as hay. Being lower in nitrogen than hay, it is also a longer lasting mulch.

Sawdust

Many writers are somewhat unkind to sawdust. If the soil is at all lacking in fertility, then it will rob the soil of nitrogen while it decomposes. Composting before use solves the problem. It does tend to be somewhat water repellent, so do check beneath it following irrigation. Eucalypt sawdust contains growth inhibitors, which are readily decomposed by sprinkling with wood ashes or lime.

Cardboard

Used under sawdust, this material suppresses many stout perennial weeds. We use this on our garden paths. It is also a useful mulch under young trees. We slope the cardboard inward and anchor it with rocks.

Paper

Paper, particularly newspaper, is available in large quantities for nothing. It blows away readily, and so needs covering with some other material that won't. Some people are concerned about the heavy metals used in coloured inks, so they do not use those sections of the newspaper that are printed in colour.

Grass Clippings

Lawns are prolific producers of high quality material that needs some care in use. Put on too thickly, the grass decomposes to a fly-breeding, smelly mess. The surface is a water repellent crust. Used thinly, grass clippings are a feeding mulch, being high in nitrogen.

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Jonathan Sturm 2003 - 2011

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