Jeff's Tips


Soils and Plant selection


Soils ain’t soils and plant selection

I have been gardening on heavy northern Sydney soil for many years and have had my fair share of plant losses. My main problem was due to the fact that I planted plants that naturally grow in light soils and hence have a week root system that is unable to penetrate my heavier soils. When there is adequate rainfall and soil moisture this is not too much of a problem. However, when the soil dries out, these plants are the first to die as they do not have a root system extensive enough to get enough moisture.

In the past, when selecting plants, I often did not worry too much about what soil type they grew in as I was more concerned with height, attractiveness of the flowers, light requirements and leave texture etc. I now take into account the plants natural habitat and soil type that it grows in after reading a few years ago, two excellent Australian Plant Society books both written by Ken Newbey. The first was West Australian Plants for Horticulture -1 (published in 1968) and the second was West Australian Plants for Horticulture -2 (published in1972). In Ken’s second book on page 135 under the heading of conclusions all become clearer as to why I was having plant losses. While the book is written around Western Australia, the comments he makes about soil types, climate and selection of species applies to all, well most I suspect of Australia. Some, but not all of the points he made are (and I quote):

  • Species which occur naturally in shallow soils should do well in deep soils of the same type but the reverse is not the case. Species occurring in shallow soil have very strong root systems necessary to penetrate the hard clay subsoils. Species growing in deep soils do not have this strong root system and have extreme difficulty in penetrating the hard clay subsoil. Species growing in deep soils have a much more even moisture content throughout the year with only very temporary waterlogging whereas the shallow soil species have a wider range of moisture content in the top soil and more chance or waterlogging.
  • Using the five topsoil types as a standard – sand, sandy loam, loam, clay loam and clay - plant species are usually adaptable enough to do well in one soil type either way. There is a possibility that the species may be suitable two types either way e.g. a species which occurs on loam should do well on sandy loam and clay loam and may be successful on sand and clay. A large number of our species are selective in their soil type and this guide should be followed carefully.
  • Species which occur naturally in waterlogged areas usually do well in drier situations but the reverse is not the case. This means that species which occur naturally in dry situations should be planted where they will not get excess water.
  • Rocky soils are often essential for growing species in exposed places so that they form a stable root system and not suffer from root movement which is either retarding or fatal to the plant. Rocks also assist drainage.
  • Within reason, the rainfall is not of a great importance. What moisture is retained in the soil is the main factor. For instance, rocky ridges in the heavy rainfall areas may have lower annual moisture content than areas surrounding massive rocks in semi arid areas where there is excessive run off.

The more I garden the more I realise that there is more to selecting plants than just using the criteria that they are ‘local’ or look nice. I suspect that we have a lot to learn about reduced rainfall and the effect of climate change on how we garden. We may need to be lot more clever in plant selection than we have been in the past to help minimise plant losses.