Q: G'Day Bob. I am looking at installing a drip irrigation system to feed off a rainwater tank.
To determine what size tank would be adequate I need to know what quantity of water and what frequency is adequate to sustain shrubs and small trees which are mostly native. Can't find any website which assists. Thanks John - Albany Creek
A: I am afraid, John, you will not find precise answers to the particular aspects of your question anywhere. Each plant has to be treated on its own merits and circumstances. The same species of plant will require different quantities of water depending on where it is growing.
There are just too many variables, and I will try and explain some of them in point form below, to try and give you and others an idea of what people need to think about.
My advice to you is at the end of it all, but the gist of what I say here is not just for you, but for others as well who read this answer and are pondering what to do in these difficult ‘water restricted’ horticultural times in Brisbane.
- Initially, of course, is what are the sizes of your shrubs and trees, how old are they, and what species of plants are they? This is then not as simple as it seems.
- Depending on the species of plant…in some cases one could say the smaller the plant the less water and the larger the plant the more water that will be required. BUT…are we then referring to a shrub or a tree? In the case of a shrub, what I have just said could more or less ‘possibly’ be true, but again … this depends entirely upon the species of plant. If it is a tree, then maybe the tree needs no watering at all. It may be mature enough to withstand the drought. Once again…it depends on the species of tree and other circumstances as set out below.
- The terms ‘small’ and ‘large’ are very relative…for it goes back to the species of plant and their natural mature height. The age or maturity of the plant then comes into play. Generally speaking, the mature plants will require less water, as their root systems would have grown deeper and spread out wider into the soil structure.
- ‘Native’, in relation to plants, can mean anything. There are native plants which are drought resistant but also those that require moist soils. One should NOT exclude ‘exotic’ plants (those from overseas) in one’s thinking. There are so many exotics that are drought resistant as well.
- An important consideration to be made is the ‘aspect’ in which the plant is growing in. Less water is required in a shady position, as against that in full sun … even if it be the same species of plant.
- The most important consideration is the type of soil your plants are growing in.
- Sandy soils require more water, and more frequent watering, than a soil with a loamy character.
- Depth of soil needs to be considered. Deeper soils require less watering than shallow soils with an underlying clay or shale subsurface.
- Soils with good organic content require less watering.
- Are you spreading mulch around your plants? This affects the amount of water you need to apply.
- “Deeper watering’ will result in an overall saving on water used as against ‘shallow watering’. What I am getting at here is that a good soaking means that the water will have time to really soak in and penetrate the soil to a greater depth. There will be less evaporation of this water and it will last longer in the soil. This also encourages plants to send their roots deeper into the soil. As against this, just watering the surface layer of soil means that only the top layer of soil will become moist. This moisture will readily evaporate, and a higher frequency of watering is then required to give the plant its needs. Shallow watering also means that root systems grow only into the top layer of soil, seeking surface moisture. They will miss out on whatever moisture is available deeper down.
My advice to you, John, is to install as large a rainwater tank as you can afford. The larger the tank the greater the storage capacity and the longer you will be able to depend on it.
An aside: Even for a ‘drip feed’ irrigation system, you will still need a suitable pump, unless you have sufficient elevation of the tank to provide a good gravity flow and water pressure.
Some plants will do well with one dripper at their base, whilst the larger plants may require three or four drippers around them.
It is all a matter of judgement at each particular site, depending on numbers of plants, their species, soil and the environment. I do hope the above gives you some idea of what you should do.
