Lesson 2 culture of native plants

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Determine cultural practices to maintain healthy native plants. 



Australian Natives are generally easily cultivated under a wide variety of conditions within the garden.  

Species natural to any given area with usually 

perform better then those introduced from other 



The climate, soil, aspect and the characteristics of 

the plant should all be given consideration before 

choosing appropriate species. Once you have 

learnt to develop a good plan and also understand 

the growing conditions required, a native garden 

will provide you with years of beauty and pleasure 


Galls on Acacias are often caused by wasps 

Treatment –remove and destroy damaged tissues 




There are three main things which affect the way a plant grows. They are environmental factors such 

as temperature, light or moisture; nutrition (ie. the supply of food to the plant and the influence of pest 

and diseases on the plant's health. You should strive to gain a broad appreciation of these three factors. 

With such an understanding comes the ability to make your own decisions about how to grow a particular 

plant in a particular place. 


Environmental factors 

Consider where the plant grows naturally. 

This may give you some idea of its requirements (eg. Banskias tend to occur in well drained soils, 

indicating that they need good drainage; plants which grow above the snowline will probably tolerate very 

cold conditions, etc.). A plant which is grown outside of its natural environment can often still be grown 

successfully, but you may find that it will grow differently (eg. tropical plants which are grown in the 

southern states tend to be smaller in size; in other words the plants may need more protection than they 

do in the north). 

  Consider light and temperature conditions. 

Characteristics such as foliage colour, flowering, fruiting, rate of growth, etc. are largely controlled by 

temperature and light conditions. It is helpful to think of plants as having "optimum", "tolerable" and 

"intolerable" ranges of environmental conditions. For instance, for a particular Grevillea, optimum growth 

may be achieved if temperatures stay between 20 and 30 degrees centigrade. The same plant may 

tolerate temperatures as low as minus 5 degrees C. and perhaps as high as 50 degrees C., but above or 

below these extremes the plant will die. Many plants will lose the brilliant colour in their leaves if they do 

not get ample light. Flowering and subsequent fruit development will also be affected by low light levels 

for many plants. 

  Similarly rainfall, wind, hail, frost, etc. will all affect plant growth. 



  Both northern and southern rainforest species tend to require reasonably fertile soil conditions. 

  A large number of Sclerophyll plants have evolved in relatively infertile soil conditions.  Many of these 

plants will grow better in soil which is not overly fertile. 


Pests and diseases 

There are hundreds of pest and disease problems which commonly affect native plants. Many insect 

attacks occur in the natural environment as well as in cultivation. For example, many Eucalypt species 

often have anywhere from 15 to 50 percent of their foliage eaten by insects annually.  

If the plant is healthy, most pests and diseases will not be of any great concern and will probably not 

need treatment. For example, wasps lay their eggs in the flowers and leaves of wattles causing abnormal 

swellings and distortions (ie. galls) as the grub develops.  Although the galls are unsightly most plants will 

normally recover from the injury. For some plants, a severe gall infestation can however, eventually lead 

to death. 

  The best way to fight pests and disease is to grow hardy and resistant species.  When you propagate 

plants, discard the weaker growing ones.  When you buy plants, buy the healthiest looking ones 

(even if they are smaller or more expensive). 

  There are some diseases which are very damaging and almost impossible to combat. For example, 

Phytophthora cinnamomi, or Cinnamon fungus, is a soil- borne disease which attacks the roots of a 

wide variety of plants, causing a thinning of foliage and eventual die-back of the plant.  The disease 

moves in the soil water and is more likely to occur if the ground is wet.  It is virtually impossible to 

control: if you suspect that you have it, take a soil sample to your Department of Agriculture (or a 

University Botany Department) for analysis.  If your fears are confirmed, all you can do is improve the 

drainage in your soil AND try to stick to plant varieties which are resistant to the fungus.   

  Chemical sprays can be used in your native garden to control pests and diseases but you should be 

aware that: 

      - Some chemicals will kill off the beneficial insects (ie. the insects which naturally eat your pest;  

      worms which carry rotting material into the lower levels of the soil; bees which pollinate your  

      flowers etc.) 

      - Chemicals may harm wildlife (either through direct feeding of sprayed plants or insects or by   

      contamination of water and soil). 

      - Some chemicals are very dangerous to humans; others are much less of a problem! Know your  

      chemical thoroughly before use it!   


Sawfly larvae attacking a Eucalypt 

Inkspot Disease 

A common fungal problem on Anigozanthus leaves 

The best control is to choose species or hybrid varieties 

which are more disease resistant  




A plant is only as good as the soil it grows in. Soil provides plants with their nutrients, it holds the plant 

firm in the ground, and it provides air and water. The soil's physical structure and biological and chemical 

nature are critical to good plant growth.  



Soil physical structure 

Soil is composed of  

  Solids and  

  Non-solids (pore-spaces containing air, water, etc.). 


The solid component of soil is made up of four different types of particles in varying percentages. 

  SAND and GRAVELS: particles between 0.02 and 2 mm diameter 

  SILTS: particles between 0.02 and 0.002 mm diameter 

  CLAYS: particles less than 0.002 mm diameter 

  ORGANIC MATTER: in varying stages of decomposition. 


The nature of a soil is mainly determined by how much of each of these 4 parts goes to make up the soil. 

This will help determine how much pore space is present in the soil. Too much or too little of any one of 

these components will cause the soil to have undesirable characteristics eg. soil with a large proportion of 

small particles will have poor drainage, whereas soil with a large proportion of large particles will have 

poor water retention. The physical structure of the soil can also be altered by human intervention for 

example heavy foot or machine traffic, or over cultivation can result in compaction of the soil causing a 

reduction in pore space between the soil particles. 


Chemical nature of soil 

  Plants obtain their food in the form of nutrients from the soil. There are around 50 different nutrients 

used by plants. Some are needed in large quantities (eg. Nitrogen, Potassium and Phosphorus), 

whilst others are only needed in small amounts. 

  Nutrients must be in the right balance in the soil (ie. too much of one nutrient can stop another 

nutrient being used, even if it is there in an adequate quantity). 

  The acidity of the soil can affect the plant's ability to use nutrients; thus the acidity (or pH) must be at 

the right level for the plant to perform at its best. 

  Each type of plant has a different set of chemical conditions which are ideal for it (ie. what is best for 

one type of plant may not be best for another). 


Soil profile 

Topsoil generally has a higher percentage of organic material and more nutrients than the lower layers. 

Topsoil generally drains better than deeper soil. The change from the topsoil to deeper layers is normally 

gradual, allowing roots to adapt to changes as they grow deeper. Plants generally do not like sharp 

changes through the soil layers; compost, sand or topsoil which is added to the planting hole should be 

thoroughly mixed into the surrounding soil to avoid sudden changes in the soil layers. 


Biological nature 

Harmful or beneficial organisms in the soil can directly affect plant growth and health. For example 

organisms such as nematodes can directly attack plant roots, mycorrhiza may increase the availability of 

nutrients to plant roots, nitrogen fixing agents such as Rhizobium bacteria may help convert atmospheric 

nitrogen into a form suitable for uptake by plant roots, and earthworms will help improve soil structure and 

fertility by the digestion and movement of organic matter through the soil and by increasing pore space by 

the presence of their burrows.  


What type of soil do you have? 

Soil can be named by a simple test: 

  Take a pinch or two of soil in the palm of your hand and add just enough water to make it stick 

together (not too wet or dry). If it doesn't stain the fingers and feels coarse, it is SANDY SOIL. 

  If it can be rolled into a ball which holds together, while still feeling gritty, it is LOAMY SAND. 

   If it can be rolled into a cylinder while still feeling gritty, but where the cylinder barely holds together it 


  If the cylinder is more solid and doesn't crack or feel gritty, but doesn't bend without cracking, it is 


   If it is like a loam except it's also sticky, it is CLAY LOAM. 

   If it is very sticky and when bent, the cylinder doesn't crack at all, it is CLAY. 




Soils which have poor structure can be improved in the following ways: 


Clay soils  

  Add lime, gypsum and commercial preparations such as Multicrop Clay Breaker and Agrosol have 

the effect of causing soil particles to aggregate to form soil 'crumbs' (or 'peds')  This makes the soil 

easier to dig, helps water to be absorbed, and assists earthworms and other small organisms to carry 

organic matter deeper into the soil. 

  Organic matter can be incorporated into the soil. 

  Sand can also be incorporated into the soil to help improve drainage, but generally large amounts of 

sand would be required, so it is rarely done. 


Sandy soils:  

  Adding clay and organic matter will aid water retention 


In both cases the addition of organic matter has extra benefits, including raising the soil's fertility, 

encouraging small animals (eg. worms) and beneficial soil micro-organisms (eg. bacteria, fungi), and 

improving the soil's ability to resist ("buffering effect") sudden or extreme changes in soil temperature or 



Watch the soil acidity 

Acidity affects how plants take up nutrients.  Extremely acid or alkaline soils can prevent plants absorbing 

some types of nutrients. The plant will suffer from a nutrient deficiency, even though the required nutrient 

is in the soil.  For example, in very acid soils all the major nutrients will be in short supply; in alkaline soils, 

phosphorus and some of the trace elements will be unavailable for plant growth. 


Most plants prefer slightly acid to neutral soil, so check your soil with a pH kit (available from nurseries 

and garden centres). Apply lime if you have acidic soil, and use sulphur to lower the pH of alkaline soils.   


Soil pH 

pH is a measurement of the hydrogen ion concentration in a particular medium, such as soil. More simply 

it refers to the acidity or alkalinity of that medium. 


The pH is measured on a logarithmic scale ranging from 0 to 14 with 7 being considered neutral, above 7 

being considered alkaline, and below 7 as acid. 

The pH of a growing media or a soil is important to plant growth. Each particular plant has a preferred pH 

range in which it grows. If a plant is subject to a pH outside its preferred range, its growth will at least be 

retarded, or it may even die. Very low pH (less than pH = 4.5) and very high pH conditions (above pH = 

9) can directly damage plant roots. 

Very high and low pH values can also affect plants as follows: 


1. As the pH of a soil changes so does the availability of nutrients. The majority of nutrients are best 

available at a pH range of 6 to 7.5. Somewhere in this range is generally considered to be the ideal for 

growing the majority of plants, although there are plants that prefer higher or lower pH conditions. In 

some circumstances, particularly at very low or high pH conditions, some nutrients may become 'locked 

up' and unavailable to plants. Although the nutrients are present, the plants can’t use them. At very low 

pH conditions, toxic levels of some nutrients, such as manganese and aluminium, may be released. 


2. As the pH of some soil is raised, more negative charges are produced on some colloid (particle) 

surfaces, making them capable of holding more cations. This allows some soils to hold larger quantities 

of nutrients. 

Soils and growing media that contain clays, or some of those derived from volcanic materials are most 





Cations are atoms which have lost electrons.  As such they are particles which have a positive charge. 

Many important plant nutrients occur in a soil or nutrient solution as cations (i.e. potassium, calcium 

and magnesium).  These particles will be attracted to particles which have a negative charge. Thus 

they will remain in the soil, or the growing medium, and become available to the plant roots for a 

longer period of time. 


Organic matter such as peat moss, and fine particles such as clay, have a lot more negative charges 

on their surface, hence a greater ability to hold cations (higher cation exchange capacity) than larger 

sand or gravel particles.  Soil or growing media with a very low cation exchange capacity will require 

more frequent application of nutrients than ones with a higher cation exchange capacity.  When a 

nutrient is applied to a soil (or growing medium) with a low cation exchange capacity, but high water 

holding capacity, the medium might remain moist, but many nutrients will be lost with drainage of 

excess irrigation water - so becoming leached more rapidly.  A higher cation exchange capacity will 

reduce this tendency. 



3. Like plants, micro-organisms have a preferred pH range in which they thrive. Altering the pH may 

severely affect the populations of both beneficial and detrimental micro-organisms. For example, the 

bacteria that convert ammonium to nitrogen prefer a pH above 6. Most mycorrhizal fungi prefer a pH 

range between 4 and 8. 


Adjusting pH 

On a new growing site, soil pH should always be tested. Commercial growing media should normally 

have a pH between 6 and 6.5 (though for some types of plants, the ideal pH is higher or lower than this).  

If a soil’s pH is either too high or too low, it can be changed to a certain extent. Slow-release lime will 

raise pH, and sulphur chips will reduce it. But this will always be a temporary amendment. It is far better 

to grow the plants that suit your soil, rather than have a constant, time and money consuming, fight with 



pH and Nutrient availability  

 The ease with which plants can take up nutrients is greatly affected by pH. Extremely acid or alkaline 

soils can often prevent plants using available nutrients. Nutrient deficiency can be caused by incorrect 

pH. The ideal pH is often slightly different for each nutrient. Iron and calcium differ more than most. The 

only answer is to compromise - go for a pH in the middle. 


Optimum pH for nutrients:  


6 to 8 


7 to 8.5 


6 to 7.5 


6 to 10 


7 to 8.5 


6 to 10 


4 to 6 


5 to 6.5 


5 to 7 

Copper & Zinc 

5 to 7 






The soils ability to withstand rapid pH fluctuations is known as ‘Buffering Capacity’. 

The greater ability a soil has to withstand fluctuations the greater the amount of acid which must 

be incorporated with a material to alter the pH. 

  Sandy soils that have little clay or organic matter have low buffering capacity 

  Soils that have lots of mineral clay and organic matter have a high buffering capacity 

  Soils with low buffering capacity need less lime to raise the pH then soils with a high buffering 







Perform the self assessment test titled ‘test 2.1.’ 

If you answer incorrectly, review the notes and try the test again. 




Water and Air 

The plant roots need water and air just as much as nutrients. 



Nutrients (except carbon and oxygen) must be dissolved in water for plants to absorb them.  Water itself 

is also needed by the plant for metabolism, where it is important in the processes of respiration and 




Soil air is essential! It is richer in carbon dioxide, and poorer in oxygen, than atmospheric air. Only a few 

plants (specifically water plants) can survive with very little air about the roots. The amount of soil air 

depends on the size of pore spaces between soil particles. Thus, water-logging will damage plants by 

depriving the roots of oxygen, not because there is too much water. 


Soil temperature 

The rate of absorption of water and nutrients is affected by the temperature of the soil. Too much heat or 

cold will slow the whole metabolism down. Soil temperature is not always the same as atmospheric 

temperature. Mulching a plant, or adding organic matter to the soil, will even out the fluctuations in soil 

temperature. As with most organisms, plant roots will grow within a particular range that varies between 




Humus is organic matter that has decomposed into a stable, ‘colloidal’ form. A colloid is a solid that is 

held in suspension (like jelly). Because of its colloidal form, humus can retain nutrients and prevent them 

from leaching out of the soil. Plant roots can extract nutrients from humus. 


Organic matter 

The average soil contains around 2 to 4 percent organic matter.  Organic content tends to drop in the 

course of normal cultivation therefore the addition organic matter to any soil-type will improve its quality. 

Only soils already exceptionally high in organic matter, such as peat soils, will not benefit from additional 

organic matter.  


Organic matter benefits the soil in many ways, including: 

  Adding valuable slow-release nutrients to the soil. 

  Acting as a buffer against sudden chemical or temperature changes which can damage plant roots or 

adversely affect soil micro-organisms. 

  Helping to improve soil structure. 


Organic matter can be added in the following ways: 

  Incorporate home-made garden compost during the growing season. 

  Use organic mulches on top of the soil. Mulches such as leaf-mould; shredded pruning material; 

composted woody waste, and similar materials, are all ideal for the purpose. As material breaks 

down, particles will be worked into the soil. Always mulch onto warm moist soil. 

  Leave crop plant roots in the soil, if possible, where they will decompose and increase the level of 

organic material. 

  Incorporate well-rotted manure during the growing season. 

  Grow green manures whenever crops are not growing, particularly during winter months. The green 

manures will protect the soil surface, as well as providing plenty of organic material to dig in. 


Soils with good organic matter content are generally easily worked – that is, they have a ‘good tilth’.  If 

you squeeze a handful of soil into a ball in your hand and it remains in a hard lump, then it has a poor tilth 

- hard clods will probably result when it is cultivated. If it crumbles, then it is well granulated with a good 

tilth.  Soils with good tilth are less subject to wind and water erosion. 




Why is water important to plants? 

Plants absorb more water then any other material, most of it entering the plants through its root system 

from the soil.  

  Seeds require water to initiate enzyme activity required to activate germination 

  Water is used in photosynthesis and all other metabolic processes associated with plant growth 

and development 

  Lowers the temperature of the plant’s leaves when water is dissipated through transpiration 

  Water dissolves nutrients before they are up-taken by plants. 

  Water functions as a transport system within the plant moving nutrients to sites where they are 

converted into products of photosynthesis, and then transports the synthesised materials to sites of 

storage or use within the plant 

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