If you like Australian native plants, and particularly if you like attracting native birds to your backyard, then she oaks are plants well worth considering for your garden. She oaks come in a variety of sizes – from ground cover, through to shrub, small and large trees, and many species have adapted to tolerate wet conditions, dry climates, swamp areas and coastal regions. She oaks were often used in the 1970s for street and school trees, but are making a return to popularity as a windbreak and as a screening plant. Don looked at the three ‘best’ she oaks, and what they have to offer Australian backyards.
The word ‘casuarina’ derives from the word ‘cassowary’ – she oaks are characterised by their fine, textured evergreen foliage, that looks like the feathers on the bird. She oak foliage means that it is often mistaken for a pine tree, and like pine trees, she oaks have little cones full of seeds. For these reasons, she oaks attract many native birds. Black cockatoos like the she oak cones, while finches and rainbow lorikeets are more interested in the seed. Willie Wagtails, Pee Wees and Butcher birds all favour she oaks for nesting trees. Don suggests that large groups of she oaks set them off to best advantage, and foliage of a bank of she oaks creates a wonderful whistling sound when the wind blows through it.
She oak foliage is not made up of leaves, but rather very fine ridged branches or ‘branchlets’. Don showed that when a branchlet was pulled apart and looked at under a lens, you can see the tiny teeth-like points, which are the true leaves, at the end of each branchlet. The number of teeth will tell you which species the tree belongs to. These branchlets and reduced leaves give casuarinas a great advantage in the harsh Australian climate. The gaseous exchange that plants require to live occurs at stomates, which on other plants are located on the surface of leaves. In she oaks, the stomates are found on the inside furrows of the ridges of each branchlet, which means that little water is lost by the plant.
Water loss is further minimised by the plant as the branchlets fall to form a thick blanket underneath the tree. Branchlets do not poison the soil, as it was once thought, but rather act as a wonderfully rich mulch which suppresses weeds and helps to stabilise soil from erosion. The roots of the she oak actually produce a kind of free fertiliser by taking nitrogen from the air and putting it into the soil.
Don looked at three species of she oak:
Casuarina cunninghamiana (or River Oak) will grow to a height between 15 and 35 metres. It is often seen on river banks and in urban plantings and is often used as a windbreak.
Casuarina glauca (or Swamp Oak or Grey Oak or River Oak) grows to around 15m in height, and for many years has been a popular street tree in Canberra. A fantastically useful plant for difficult and wet areas, it will tolerate near-swamp conditions, and even some salinity. The River Oak and Swamp Oak are best planted 15m (50′) away from water and sewer pipes so are more suited to large gardens or parks.
Allocasuarina torulosa (or Forest Oak or Rose Oak) will tolerate most soils, and quite a degree of salinity. A popular Sydney street planting, this tree reaches about 15m in height. It has been a favourite in Australian gardens for its rich burgundy foliage, which stands out against the green-grey older growth and its deeply furrowed bark.
Most nurseries will stock perhaps one or two species of Casuarina and Allocasuarina. However, Don suggests that to see a really good variety of these plants, look up your local Forestry Commission Nursery.
At the Cumberland Nursery of the State Forests of NSW, plants start from $2.50 a tube, but if you buy in bulk, it can work out to be cheaper.