April 25, 2020
When it’s spring in Marin, it’s fall in the Southern Hemisphere, but do the Australian native shrubs in our Marin gardens think it is spring or fall? It turns out; they are not confused at all. Our spring pruning season is their pruning season, too, and Australian shrubs require very little pruning once you set the shape of the young plant.
The number one rule of care for young Australian shrubs and sub-shrubs is tip pruning. This will encourage compactness, lateral growth, and bushiness. Tip pruning should be a constant practice for the first few years of an Australian shrub’s life. It is as simple as it gets in the garden and doesn't require tools, just pinch the terminal buds off using your thumb and forefinger.
Removing the terminal bud eliminates the source of the hormone auxin. The terminal bud produces auxin to inhibit the elongation of lateral buds below it. This is called apical dominance. By removing the source of the inhibitory hormone, we simulate lots of lateral growth. I don’t know about you, but in my garden, I like my bushes to look bushy.
Apical dominance is an important tool for the survival of uncultivated plants. Energy is devoted to upward growth for more sunlight, which allows more photosynthesis. Sunlight is essential in turning carbon dioxide and water into glucose, which a plant uses for energy and to make starch and cellulose. Apical dominance allows a plant to grow tall and outcompete its neighbors.
Pinch off terminal buds in the spring and during the warmer months of the year when the plant is actively growing and when the plant is young. If you leave shaping until the plant is almost fully developed, it is tough to correct. Think of your Australian shrubs as puppies. If you teach them how to behave from the very beginning, you get a lifetime of ease and enjoyment. If you let the puppies do as they please, they will grow up to be very difficult to train.
Other types of pruning may be necessary with Australian natives. When branches are broken or infested by insects, particularly borers, you can prune back to healthy wood close to a leaf node or limb. If you are pruning to regulate unwanted growth, use the same best practices you do on your other garden shrubs, clean cuts that slope away from buds. Remember, it is best not to prune in winter because cold temperatures can damage the resulting new growth.
Did you let your Australian native shrubs do their own thing like me? The result may be a monstrosity that is no longer an asset in your garden. If they have no redeeming qualities, most Australian native shrubs can be cut to the ground level. Do this during the spring, and they should rapidly produce new growth, especially if given supplemental water. Once the new growth begins, start your tip pruning to ensure the shape and size you want moving forward. Cutting to the ground works particularly well with Callistemons, Melaleucas, and Leptospermum, but older Grevillea may not come back. My Grevilleas have become giants that hummingbirds love. I decided to prune the lower branches flush to the trunk, which means there will be no new leaf growth, but they look more tree-like.
Be sure to use sharp tools for pruning and always disinfect them between plants to be sure no plant pathogens are hitching a ride. The cutting parts should be dipped, wiped, soaked or sprayed with a disinfectant. Use alcohol straight or bleach that is mixed one part bleach to nine parts water.
For more tips on managing your garden be sure to visit http://marinmg.ucanr.edu/