Native blackthorn

Native blackthorn

Plants are amazing organisms. They produce the oxygen needed by all of the animals on this planet. They are able to convert the simple molecule, carbon dioxide, into the complex glucose molecule using the energy of sunlight and then to produce all of the molecules needed by living things from this. Thus, they are the base level of the majority of food chains. Unlike animals, they cannot move to a more favourable environment if conditions become adverse or hide from predators. Many plants, however, have evolved features which reduce the amount of predation by herbivores and so increase their chances of survival. These include toxins in leaves and fruit, stinging hairs, thorns and spines.

Bursaria spinosa, Blackthorn
Bursaria spinosa. Photograph by Neil Murphy.

The native blackthorn (Bursaria spinosa), also known as sweet bursaria, uses spines to deter animals eating it. This important shrub of eucalypt woodlands is found along the east coast of Australia from Tasmania to Cape York and extending from Victoria to South Australia. It has short, spiney shoots at its lower levels where browsing animals reach. The same spines, however, make this shrub a safe haven for small birds, such as wrens and finches, many of which build their nests within its branches. The size of this plant varies according to its habitat but in cultivation it remains a short to medium shrub. It is an extremely hardy, low maintenance and fast-growing species that thrives in well-drained soil in either full or partial sun. An annual pruning ensures a full, bushy shape.

Over summer, this shrub produces clusters of perfumed, white flowers, that attract nectar feeding birds and insects of all shapes and sizes. The flowers are followed by heart-shaped seed capsules which attract small seed-eating birds. From a wild-life attracting perspective, this is an adequate reason to propagate this plant. But there are further reasons. One of the pollinating insects attracted to the flowers is the brightly coloured parasitic ichneumon wasp. It lays it eggs in the caterpillars that feed on surrounding gum trees. The hatched larvae devour the caterpillars, pupate within the caterpillar skin and emerge to seek a new sweet bursaria bush.

Bursaria spinosa. Photograph by Heather Knowles.

An even more interesting relationship exists between this plant in Victoria, an ant and a butterfly. The larvae of the rare, colourful Eltham Copper Butterfly (Paralucia pyrodiscus lucida) live under native blackthorn bushes in the nest of the tiny Notoncus ectatommoides ant. Each lava has four allocated ants that guide it into the shrub at night to feed on the small obovate leaves and return it to the nest in the morning. The larva secretes a honey-like exudate from its abdomen on which the ants feed. Months later, in summer, the larva transforms into a beautiful copper-coloured butterfly that feeds on the plant’s nectar, pollinates the flowers in the process and begins a new symbiotic cycle when the female lays her eggs at the base of the shrub.

Whilst making a distinctive feature (the flowers and seed pods are lovely in cut flower arrangements), border and barrier plant (to deter unwanted visitors), native blackthorn is recommended as a lantana replacement plant and in erosion control. It is an important honey producer. The leaves contain the chemical aesculin which is used in homeopathic medicine as a blood thinner and in the treatment of varicose veins and haemorrhoids. This chemical is also used in microbiology to aid in the identification of bacteria.

Bursaria spinosa. Photograph by Heather Knowles.

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