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Effective Watering in the Garden

Keith Townsend

Water is probably the most essential factor of all for successful plant growth and survival. Very few gardens can grow successfully without some supplementary watering at various periods throughout the year. There is often, however, some difficulty in achieving the best and most economical usage of available water to the best advantage of the garden.

A practical idea of the moisture level in clay and loam soils can be obtained by taking a sample from at least 20cm down. Squeeze the soil in your fist. If it has any tendency to stick together and maintain its moulded shape, it is moist enough to maintain most plant life. If it is impossible to mould it, then it needs watering. This test will not work in sandy soils, but an inspection of the soil at a similar depth will give an indication of the moisture content.

The best approach to watering is to wet at least the whole depth of the root zone at each watering. A concept of "little and often" is dangerous. It encourages surface rooting and the plants lose drought tolerance. Heavy and occasional watering is a better approach.

A general programme for trees and shrubs should be along the lines of a heavy watering each 2-3 weeks in summer and each 3-4 weeks in winter. This is of course, dependent on any rain which may fall.

Mulched, drought tolerant native plants may not need watering at all once they are established. There are a number of benefits in grouping together plants which grow in similar environments. Not the least of these is a saving of water and effort which can be made by planting dry-land plants together.

Over or Under Watering?

  1. An over-watered garden may lose 15 litres ofwater per square metre on a very hot, dry day. A minimally watered garden may only lose 5 litres in the same period.

  2. Overwatering produces soft plants with poor ability to cope with stress.

  3. Frequent light watering produces large losses by evaporation from the soil and shallow roots.

  4. Infrequent deep watering produces small losses by evaporation, deeper roots and greater drought tolerance.

  5. To soak the top 30cm of dry sand takes about 25mm of water.

  6. To soak the top 30cm of dry loam takes about 40 to 50mm of water.

  7. To soak the top 30cm of dry clay takes about 70mm of water.

  8. One millimetre of rain will add about 1 litre of water per square metre to a soil.

  9. A sprinkler delivering 10 litres per minute (600 litres per hour) will supply an area of 40 square metres with 600/40 =15 litres per sq. metre/hour = 15 mm per hour.

Common Watering Problems

  1. Not Enough Water - often caused by light dally waterings. Remember - water more thoroughly and not as frequently.

  2. Too Much Water - low spots in the garden, trenches around trees or garden beds, undrained garden borders. All these may cause overwatering and result in killing plants or giving them a set-back. Wet patches also promote the growth of weeds, and can be the source of fungal attack by water borne fungal organisms which revel in poorly drained areas.

Improving Water Management

Organic matter does wonders for the soil. It increases the amount of available water a soil will hold and also improves water penetration rate. Compost or animal manure dug in at the rate of at least one kilogram to the square metre will be very helpful.

Coarse sand dug into heavy clay will open it up and increase the rate of water penetration; within limits, the more coarse sand the better.

Mulching will prevent drying of the soil and also check weed growth which robs the soil of moisture. A mulch of any organic material can be spread over a garden bed or around a shrub. Keep the mulch away from the stems or trunks, otherwise stem rots may develop. If you eventually dig the mulch into the soil, add nitrogen fertiliser to help it decay speedily, saving the plants from nitrogen shortage.

Drainage is probably the most neglected aspect of watering. It is especially important in controlling salts. If you have a good deep soil, excess water will seep down below the root zone, taking dissolved salts with it. Drainage will be no problem.

If your topsoil or subsoil is a tight clay, you will need to take special precautions. Garden beds with improved soil may be prepared on top of the clay. Do not remove the original soil, but instead, build up the soil above the ground level and plant in the mound. This method is particularly successful for growing Australian native shrubs and trees, many of which have a pronounced dislike for poorly drained situations. Mounding is also helpful in areas where hard water is a problem - as discussed below.

Mounding diagram
Mounding allows you to apply water where it will soak deeply. Mulching will help to prevent evaporation.

"Hard" or Saline Water

Some gardeners are faced with particular problems because they need to use bore water with a high mineral content for their gardens.

The 'hardness' of water is measured by the amount of dissolved salts present in a measured quantity. The combination of salts may vary from area to area, but the critical figure is usually that of Total Dissolved Salts (TDS), which is expressed as parts per million.

Water with TDS of up to 500 parts per million usually has little effect on most trees and shrubs, however certain soft indoor plants, ferns and orchids may be adversely affected toward the top of this range.

TDS readings of 500 to 1500 parts per million indicate that the water is far from ideal, but may be used for most gardening purposes. There will however be a range of plants with low salt tolerance which may be affected. As the reading increases, the necessity for care in the manner in which the water is applied also increases.

Water with TDS readings above 1500 parts per million are not recommended for widespread garden use, but it is an unfortunate fact that there is a lot of groundwater in the tropics which falls within this range, and many gardeners are faced with little alternative. Using water in this range does require more care and attention, but there is no doubt that quite successful gardens can be grown.

Hard water may affect your soil (and therefore the health of your plants) in a number of ways:

  1. Salts may be concentrated in the soil until they reach a level toxic to the plant.

  2. Soil pH may be increased to a level which is unsuitable.

  3. Plant nutrients and minerals may be 'locked up' in chemical reactions with the water salts, and may become unavailable to your plants.

Methods of Use

As mentioned above, the higher the TDS of your water, the greater the care you need to take with its use. One of the readily apparent problems is loss of foliage if the water is sprayed on to plants. This is caused by deposits of minerals (usually Calcium Carbonate) on the leaves, which block the pores and cause loss of the leaf. Water which causes this problem should never be sprayed but needs to be flooded on the ground surface.

Drip irrigation can also be a problem if watering is limited to an amount which only wets the immediate root area of the plant. In such a case the salts deposited by the water are concentrated in a limited area and continual irrigation will concentrate the salts to a level far above that which the plant can tolerate.

High concentrations of calcium will often result in blockage of dripper heads, necessitating regular cleaning.

   Water distribution diagram
   Hard water will deposit salts at the edge of the water penetration area - sufficient should be applied to soak beyond the root area.

Gardening practices which may be helpful when using hard water are:

  1. Do not spray irrigate - or if watering trees, use a low trajectory spray which keeps the water below the leaf level.

  2. Use flood irrigation in channels or drains, or alternatively, trickle or adjustable dripper heads which are able to supply a fair rate of water. Water supplied by a trickle system should be sufficient to soak far beyond the limits of the plant root system - salts tend to be deposited at the edge of the water penetration area, and this will ensure they are taken away from the plant root area.

  3. Water only when necessary, and then in sufficient quantity to last the plant at least 2-3 weeks. It is far better to water heavily each 2-4 weeks than to lightly water every few days.

  4. Mulch heavily where possible - mulching reduces evaporation from the surface and therefore reduces the deposits of salts left behind by evaporation. It also of course reduces the amount of water which needs to be applied.

  5. Mounding of beds provides the opportunity to apply hard water in the channels between the mounds, allowing it to soak deeply and moisten the subsoil. This is a particularly useful technique to establish young plants; if rainwater, or an alternative good quality water, can be applied to the plants, while the hard water is used to cool and moisten the lower levels of the soil. A heavy mulch assists to prevent the hard water being drawn to the surface by evaporation.

The salvation of those gardens condemned to the use of hard water is a good wet season - this will flush away most of the accumulated salts and give plants a new lease of life. In the meantime, however, while waiting for that good wet, we can make conditions much more acceptable by adopting the methods outlined above.

This article is extracted from the publication "Across the Top - Gardening with Australian Plants in the Tropics", compiled by Keith Townsend and published by the Society for Growing Australian Plants, Townsville Branch Inc. The book is available from Society for Growing Australian Plants, PO Box 363, Aitkenvale, Quensland, 4814 - $24 plus postage. Alternatively, phone orders can be placed with Keith at 07 4778 4661.

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