Caring for Established Native Plants in the Garden

Rodger Elliot

Much has been said and written regarding soil preparation and the care of new garden plants during the current period of extended dry conditions, but it could be that the established plants we already have in our gardens tend to be overlooked. We assume they will simply survive.

In areas of natural bushland we are seeing many examples of plants, which are not surviving. One highly publicised example is the poor conditions of the River Red Gums in the Murray-Darling Basin, many of which are now dead or dying.

How can we ensure the survival of our gardens when they are not receiving the rainfall they once enjoyed and when supplementary watering must be done according to restrictions or we may even be relying on tank water - from sometimes empty tanks!

There are two major factors for the success of cultivated plants. The first relates to species selection and choosing the right plants for each situation in the garden and the second is to do as much as we can to maintain good growing conditions in our soils.

The selection of suitable plants for the conditions available in our gardens

While realising that here we are looking primarily at caring for garden plants, which are already established, it should still be borne in mind that the initial selection of suitable species is of prime importance. Some established plants may not tolerate the challenges of extended dry periods without supplementary watering, simply because they were not the most appropriate plants for the region in the first place.

We gardeners are often responsible for setting some unlikely targets for success when we choose plants that are from differing prevailing conditions to those where plants are to be grown. It should however be said that it is from these adventurous gardeners that we often find out that some plants are much more adaptable to cultivation than the so-called 'gurus' have stipulated in books or on radio or TV.

The basic framework trees and large shrubs in a garden should always be plants of known reliability in the region. Such plants are usually able to survive times of climatic stress, even if they just 'hang in there' rather than thriving.

It is still realistic to acknowledge that during times of extreme stress these plants will appreciate some assistance, perhaps in the form of supplementary watering. From another point of view, garden plants which require virtually no assistance whatsoever from us, even in times of climatic extremes, are likely to thrive to the point where they may become weedy during more favourable times.

Encyclopaedia of Australian Plants Rodger Elliot and David Jones are the authors of the definitive publication on Australian Native Plants suitable for cultivation - the Encyclopaedia of Australian Plants. This monumental work, which commenced with Volume 1 in 1980, is now up to Volume 8 and the project will finally see completion later this year or early in 2009 when Volume 9 is published. It's a task that will have taken almost 30 years to complete! The Encyclopaedia of Australian Plants is available though major book sellers nationally although you may have to hunt around for some of the early volumes. Most volumes are also available through the Australian Plants Society (Victoria).

Established trees and large shrubs can be of immense value in a garden and this is often under-estimated. They can provide very important shade or hedging, screening and windbreak functions. It has also been proved that trees and shrubs can modify the temperature of surrounds and of buildings by several degrees, through shelter from hot winds in summer and cold air in winter. Of course there can be some drawbacks by creating shady dry sites because of trees but generally their benefits outweigh their disadvantages.

Large plants, which are allowed to die through lack of care, cannot be replaced overnight. It may take 5 - 10 years to replace a tree, which may have been able to be saved by noting its signs of stress and providing the right assistance. So undoubtedly it is important to try and maintain conditions that are beneficial for existing established plants.

Soil conditioning and improvement

It is a widely known fact, but still often under-estimated, that the addition of organic matter to soils is of immense value if we wish to grow healthy garden plants.

In heavy clay soils the organic matter separates the fine soil particles, allowing better aeration of the soil and improved drainage.

An important point is not to over-do the use of organic matter, as it may initiate soft whippy or tender growth which will be susceptible to extended dry periods or frost damage.

Clay soils may also be improved by the addition of gypsum. It can be applied in a powder or liquid form and will also assist in improving aeration and drainage.

Not all clay soils respond to gypsum. Many readers may already know the way to test if clay soils can be improved by applying gypsum, however I have taken the liberty of providing the information for those who may not know of this little science experiment.

1. Take a 500 - 750 ml jar with a tight-fitting lid.
2. Fill one-third to half of the jar with the soil in question.
3. Fill the jar with water and make tight with lid.
4. Shake vigorously until no lumps of soil are evident.
5. Leave for 12 - 24 hours.
6. If water is still cloudy from fine particles of clay then you can apply gypsum at the rate of 1 - 1.5 kg per square metre. It is best to incorporate gypsum into the soil using a fork.
7. If water is clear, gypsum is unlikely to be of assistance in improving drainage. There are quite a number of other soil-conditioning products that could be beneficial for clay soils that do not respond to gypsum.

In sandy soils organic matter helps in the retention of moisture and nutrients, rather than everything leaching out extremely quickly, as is often the case. Fine sandy soils are renowned for shedding water off the surface (the descriptive term is hydrophobic). Even with added organic matter, sandy soils can still be difficult to 'wet'. It could be that the best mode of attack is to apply soil-wetting agents, which usually help to get water soaking to a greater depth. Wetting agents are not a long-term fix. Most wetting agents need to be applied regularly as per the manufacturer's instructions.

The use of low spreading groundcovers with moderately dense foliage is also beneficial in all soils. Such plants can dissipate heavy rainfall or hand watering as they allow for the water to fall gently to the ground and hopefully soak in, rather than just pounding the soil or simply running off over the top (see Mulching).

The beauty of a fork!

A fork with three or four prongs is a wonderful tool for making it easier for water to penetrate soils. Simply push a strong fork to its depth into the soil at about 30 cm intervals. Then move it gently backwards and forwards a few times to loosen the soil a little and this action should enable penetration of water to a greater depth. To have an uneven soil surface will also be beneficial in preventing the quick flow of water over the surface. This is a method of attack that is especially successful for clay-loam and clay soils.

For established trees the most important area for water penetration is the ground just inside to just outside the foliage drip line, as this is where most of the feeder roots are located.

Following on from the fork process leads to mentioning our ability to harvest rainwater on our properties by disallowing it to simply flow onto the road or into drains.

There is value in creating swales or shallow water-gathering depressions on the higher slopes above established trees and large plants. Being able to trap rainwater, which can then soak slowly into the soils is a very positive way to go. The main aspect to keep in mind is to have these depressions running at right angles to the prevailing slope. These depressions and swales can also be wonderful habitat for frogs, skinks and other beneficial animals, especially if they have a build-up of leaf litter that can provide shelter for the animals.

For people who are initiating a garden it is the location of permeable paths (not made from bricks, tiles, concrete or asphalt) that can also be utilised in a similar manner to swales and depressions. Strategically located paths can improve the water harvesting capacity and thus benefit your plants.

In all soils the presence of organic matter can also encourage the presence of earthworms, which may further improve the growing conditions in the soil and are a real bonus with their unpaid labour in the garden.

Mulches

Organic mulching involves the use of plant material such as leaves and twigs as a layer on top of the soil. This shields the soil from direct sunshine, keeping the root area cooler and reducing evaporation. Mulch can also reduce soil damage from very heavy rain and erosion from run-off.

Aspects that should be considered in regard to mulching include the following:

• The mulch should not pack down too tightly - a mixture of particle size and shape is best.
• If mulch is spread too thickly any light rain or overhead watering, may simply be absorbed by the mulch layer and not reach the root zone below. The recommended depth for organic mulch is 3 - 5 cm. It is best to replenish mulch at regular intervals rather than having it too thick. Organic mulches break down and sometimes it is beneficial to lightly fork into the topsoil the broken down matter before replenishing with new mulch.
• Ideally, mulch should be applied when the soil is moist, it is then that the mulch will assist in the moisture being retained within the soil.
• Organic mulch can be undesirable in areas of bushfire risk.
• Organic mulch can be undesirable in areas subject to heavy frosts. Moisture within the mulch can freeze, making the temperature at ground level lower than when no mulch is used. In frosty areas it is best to rake back any mulch from around frost-tender plants to expose bare, flattened earth, which thus reduces frost damage.
• Organic mulches can deplete nitrogen levels in soils as they break down and applications of nitrogen-rich fertilisers are usually beneficial in replacing that which is lost.

Other materials highly suitable for mulching include coarse sand, gravel, pebbles and materials such as re-cycled plastic chips. These materials can sometimes be fairly expensive and it should also be borne in mind, a sand mulch can provide an ideal environment for the germination of weed seeds.

Woven mulch mats or weed mats can be extremely useful, and can be covered with a layer of organic mulch or pebbles to achieve a more aesthetic result. Black plastic is not recommended as a mulch, as the soil below can become sour if wet for extended periods or alternatively may become very dry because rainfall or supplementary water may not be able to penetrate the plastic.

The value of groundcovers as mulch is very high. They may of course also deplete the soil moisture as the roots gather water to keep the plants alive but the hot sunshine and strong winds are not as likely to penetrate the cover and reach the soil surface. Most plants shed water rather than soaking it up as mulches can do and they also help prevent compacting of the soil from heavy rain.

Pruning

Judicial pruning of established plants can really help to maintain their good health.

Many Australian plants have a summer dormancy period (differing from winter-deciduous plants) and removal of foliage, once plants are no longer in active growth is worth doing. If pruning is done while plants are producing young growth this will only stimulate soft growth which may suffer during periods of extended dryness.

For trees, the removal of some branches could be really beneficial as it means that the roots will expend less effort in gathering that vital life-giving moisture. Stress levels can be relieved!

Many shrubs will also benefit from similar pruning and this could lead to the rejuvenation of mature plants when the autumn or winter rains arrive.

Our established plants will appreciate any measures we can adopt to help them survive lack of soil moisture. Perhaps there are readers who would like to share some of the successful methods they have put into practice in recent times!

From "Growing Australian", newsletter of the Australian Native Plants Society (Victoria), March 2008.

Australian Plants online - 2008
Association of Societies for Growing Australian Plants