Horticulturalist John Gabriele recommends careful thought before shaping native plants.
Australian native plants have come a long way over the past few decades and now hold a prominent place in Australian gardens and landscapes.
Natives have a reputation for being low-maintenance, but like any plant in a designed garden or landscape, with a little attention they can become so much more.
Pruning
Pruning plays an important role in the successful cultivation of native plants if they are to display at their best.
The pruning of any tree or shrub should be undertaken with a goal in mind. For many native shrubs, the aim is to increase the number and quality of flowers, as well as foliage density and improved plant vigour. Apart from improving overall plant appearance, correct pruning will eliminate several potential problems that can result in susceptibility to pest and disease infection, compromising habit and aesthetic appeal, or structural weakness in the case of advanced trees.
Managing trees in industrial and urban landscapes requires attention to detail and in conformity with the Australian Standard AS4373 Pruning of Amenity Trees. The recommendations given in the Standard are intended to apply specifically to urban and amenity trees. The objective of AS4373 is to provide contractors with a guide defining uniform tree pruning procedures and practices to minimise the adverse or negative impact of pruning on trees. Too often poor examples – particularly of tree pruning – are seen in landscapes where contract-maintenance crews have had little or no training in correct pruning techniques.
Understand the plant
An understanding of the plant is paramount in making decisions about when and how to prune. Features such as flowering time, age of flowering wood, seasonal growth, vigour, and plant habit need to be considered before making that first cut.
We can look to nature and find examples of ‘natural pruning’ in storm damage, wind pruning on exposed sites, and defoliation caused by the feeding habits of herbivores and insects. But to purposefully remove branches from plants in cultivation is an unnatural act. For this reason, care needs to be exercised in pruning to ensure plant health is not adversely affected. Improper cuts can lead to unsightly shapes, and imbalances result in asymmetrical growth, overly dense or sparse areas, and an overall unattractive appearance.
Additionally, irregular or poorly planned cuts can lead to the development of stubs or uneven branching, which detracts from the plant’s natural beauty and symmetry.
The type of pruning undertaken will ultimately be determined by the type of plant being cultivated. Many of the native shrubs grown in utility and urban landscapes flower on current season’s growth, and plants of this nature are best pruned immediately after their main flower flush. These include Callistemons, Phylotheca, Prostanthera and Hybrid Grevilleas. Pruning up to one third of the season’s growth at this stage encourages rejuvenation of dense, compact growth. Tip pruning is also recommended as part of a regular maintenance routine.
Remedial pruning
In the case of remedial pruning of larger trees, which involves the removal of large limbs that may be diseased or structurally unsound, target pruning should be employed.
Target pruning involves removing only those branches identified as existing or potential problems. Pruning cuts should be made using a systematic approach to minimise damage to the tree. It takes time to
prune with skill and is not a task to be rushed. Either you’ll make cuts that take years to undo, or you won’t make the cuts needed to fix a systemic issue in time.
The step cut and undercut method of pruning will ensure minimum damage occurs to the branch framework and prevent bark splitting (refer to AS4373 for detail). When removing branches, flush cutting to the point of origin between the branch and trunk should be avoided at all costs as this can lead to rapid internal decay of branches. When branches are pruned to the collar, a callus of repairing cells forms around the injury, sealing it from further infection. These cells are lost by flush cutting, leaving the tree open to infection from pests and diseases. Stubs are also unacceptable. They not only look unsightly but may also result in epicormic shoots. Branches should always be pruned to the branch collar, which will vary in size and shape depending on species.
Epicormic shoots often appear as clusters of small branches emerging directly from the trunk or large branches. Epicormic shoots can be beneficial; however, they can create structural weakness and require proper management. That’s why pruning should be undertaken carefully to avoid creating wounds that could trigger epicormic growth.
Managing strap-leaved plants
Pruning isn’t restricted to trees and shrubs.
In a recent social-media forum with landscape-maintenance managers, criticism was levelled at the pruning techniques used on strap-leaved plants, particularly Lomandra spp. and cultivar varieties in high-visibility landscapes. We’ve all seen these or other strap-leaved, grass-like plants being pruned into a formal spherical shape, or the pruning back of foliage of Doryanthes spp. (Gymea Lily) to display a more upright appearance. Not only are such practices detrimental to overall plant health, but the aesthetic value of these plants is also compromised. The questions then need to be asked: why is this type of pruning becoming increasingly common in the landscape-maintenance industry, and what drives it?
A common response is to manage plant height to provide line of sight. This is a particularly widespread practice for car-park settings. That then opens further discussion on correct plant selection in the first instance. If the selected species is unsuitable in terms of mature height, make a better selection. There are plenty of low-growing native and exotic species with similar habits to Lomandra. Examples include Poa,
Festuca, Liriope, and some Dianellas that will grow under 50cm at maturity. Many utility landscape designs rely on tried and tested tough plants that work well under harsh conditions.
Many of the management problems relating to the ongoing maintenance of selected plant species arise because the species aren’t given appropriate consideration at the design phase.
In most cases, the maintenance required for strap-leaved plants involves hand removal of dead material from within the centre of the plant. Pruning of Lomandra and other strap-leaved plants can, however, be justified, particularly when plants become congested or vigour declines. At that stage a hard prune will rejuvenate clumps.
Hard pruning is best undertaken in late winter or very early spring prior to the main growth flush.
Dome pruning
In the case of ‘dome pruning’ Lomandra, there are several factors that would justify a different plant selection.
This type of pruning reduces the photosynthetic area of the foliage, blocks new growth, and impedes airflow that will ultimately lead to a decline in plant vigour. The regrowth often has jagged edges from pruning cuts with mechanical shears, and if pruned after the main growth flush in spring, plants will be compromised moving into the heat of the late spring and summer months ahead.
Dome pruning of strap-leaved plants in general leads to an unrealistic shape that can often be at odds with the overall design aesthetic. If a dome shape is required in a landscape, plants that are more responsive to pruning techniques to maintain such a shape should be installed in the first instance. Some examples include Buxus, Westringia, Pittosporum, and other hedging/topiary plants. These make better selections where specialist amenity pruning is required. It makes no sense to prune strap-leaved plants into domes if the design intent was to have formal geometric symmetry in the first instance. Strap-leaved plants provide a flowing design aesthetic. Their natural habit should be celebrated, not restricted to a controlled shape at odds with their natural beauty.
Important skill
The bottom line is that whatever type of pruning is being conducted by landscape-maintenance crews, operators should have a thorough understanding of plant physiology, when and how to prune depending on specific species requirements, and be able to make recommendations on better species selection when necessary.
If your crews are not trained in specialist amenity pruning, it might be time to consider upskilling. We need to ensure pruning practices maintain the delicate balance between function and form of plants within a landscape to avoid compromising the aesthetic integrity of the design and the overall health of installed plants.