Principles of Pruning

Audrey Gerber

Many Australian natives plants and Proteaceae grow into large, straggly, extremely woody plants if left untended. This growth habit is suitable in some landscaping situations where the plant is used as a screen, eg. on roadsides and central highway strips. However, in a commercial plantation where we require plants to be productive and compact we need to develop strategies to manage plant growth.

Why do we prune?

To control growth and develop a suitable shape

Managing the shape of the plant ensures that each plant has sufficient space to grow. and enables easy access of machinery and personnel between rows. A good pruning programme also re- stricts plant height, thus making picking easier.

To encourage flower production

In plants grown for commercial flower production, flowers are most commonly produced on current season's growth. Annual pruning stimulates production of new, flower-bearing growth. Exceptions are some proteas, hanksias and waratahs where the flower-bearing stem grows for 2 seasons before flowering. Pruning strategies for these must be adjusted to avoid removing seemingly unproductive stems which would produce flowers in the following year.

To promote plant health

An open canopy allows free air movement and light penetration which discourages pests and diseases. If chemical sprays must be applied they are more evenly distributed throughout an open canopy. New growth from the centre of the plant following picking or pruning is stimulated by free movement of light and air, and vigorous, healthy growth is encouraged.

To remove diseased or damaged branches

Shoots that have been broken by machinery or damaged by wind, frost, or hail are removed to encourage new growth to replace the damaged portions. Branches with pests or disease damage are also removed to reduce the risk of the infection spreading.

To rejuvenate old, unproductive plants

Plants vary in their response to severe pruning, which can be used to stimulate new productive growth. The best response comes from those plants with a thickened stem base, known as a lignotuber. Old, woody stems of these plants can be cut off at the base, resulting in young vigorous regrowth from the lignotuber.

How do plants grow?

• on the tip of the shoot (terminal buds), and their growth results in elongation of an existing stem.
• on the sides of the shoot, generally in the fork where the leaf meets the stem (lateral buds, or axillary buds). Growth from lateral buds produces new shoots at an angle to the main stem.
• on other parts of the plant (adventitious buds), generally mature stems and roots, often as a response to injury, such as occurs in pruning. Not all plants are capable of producing adventitious buds.

What techniques do we use?

Techniques used in structuring and controlling plant growth:

Pinching

The soft growing tip is pinched out between thumb and forefinger. This stops the shoot from growing longer and encourages side shoots to grow from the lateral buds of the uppermost mature leaves. Pinching is used widely to encourage branching of young plants for increased complexity.

Heading cut (heading, heading back, cutting back)

Pruning to shorten branches is done using the heading cut. The top portion of the shoot is removed, and side shoots grow from the lateral buds on the shoot portion remaining, thus increasing plant complexity. In commercial flower production the picking cut is a heading cut, and the side shoots form the next season's flowering stems. A heading cut which removes less than a third of the shoot generally results in many short side shoots. A heading cut which removes more than 2/3 of the stem generally results in production of few, very vigorous side shoots.

Thinning cut (thinning, thinning out)

The thinning cut removes the entire stem at its point of origin. Thinning is done to reduce the canopy and does not result in a growth response. Thinning is done to remove old, unproductive stems and reduce competition within the plant. Light and air penetration within the plant are improved following thinning.

When do we prune?

As mentioned before, the picking cut is a pruning cut, and is, therefore, applied at the time of flowering. Regrowth following picking forms the next season's harvest. If necessary, a clean-up prune is applied after harvesting is finished. The shape of the plant is assessed and altered if deemed necessary. Branches blocking light and air movement through the plant, or trailing on the ground are removed with thinning cuts. The number of bearers is assessed and adjusted to balance vegetative and reproductive growth.

If pruning is done to correct or rejuvenate plant growth, it is best done in late winter. Pruning before spring growth starts takes advantage of the naturally vigorous growth occurring at this time of year. Pruning in mid-late summer can retard plant growth.

What's the ideal shape for commercial production?

• Complex for maximum productivity.
• Clear base for weed control
• Picking at waist height
• Open canopy for light and air

With the vast range of products grown in the wildflower industry of Australia it is impossible to design a single pruning strategy which will apply to all plants. Developing pruning strategies requires a simple knowledge of the plant's growth. based on observations. You must have clear in your mind what you are trying to achieve by pruning, and some knowledge of what the plant is capable of doing.

From the newsletter of the Grevillea Study Group, July 2002.

Audrey Gerber - New and Innovative Industry Development Officer, Department of Natural Resources and Environment, Victoria

Australian Plants online - 2006
Association of Societies for Growing Australian Plants