Grass trees epitomise the Australian bush: they’re beautiful, ancient, hardy, thrive in nutrient-poor soils and respond to wildfire by flowering profusely. They're iconic plants, recognisable even to budding botanists.
All 66 species of grass tree are endemic to (only found in) Australia. They're all perennial, flowering plants. The smallest species grows to about 1m, others reach 6m tall. Most species are extremely slow growing. Studies of some of the taller species found that trunk height increases at about 0.8cm to 6cm per year, but this varies with local environmental conditions1. In any event, grass trees are often very long-lived; some are estimated to be 350 to 450 years old!2
Xanthorrhoea are monocots (meaning they only have one cotyledon, which is the leaf attached to the embryo within the seed). Some form a ‘trunk’ from old leaf bases stacked on top of each other and stuck together by a naturally occurring resin. Some species have a branched trunk, others, like X. gracilis, don't form an above-ground trunk at all.
Grass tree leaves are narrow, linear and stiff. Where present, old leaves often hang down and form a ‘skirt’ around the base of the trunk. The length of the skirt is a good indication of the time since the last fire – the longer the skirt, the longer the duration without fire.
Grass trees are even more interesting below the soil surface! They have a root system, where microbes called mycorrhiza surround the roots in a symbiotic relationship, which helps the plant take up nutrients.
Many species have an amazing ability to survive fire. A fire may burn their leaves and blacken their trunks, but the trees usually survive: the living growth-point is buried underground, protected by tightly packed leaf bases. In fact, some grass trees are stimulated by fire – in the spring after a summer bushfire, large numbers of plants can flower.
However, not all species are fire-tolerant: the threatened Grey Grass Tree (X. glauca angustifolia), which is found on JC Griffin Reserve in Victoria, is killed by hot fires and needs protection in the fire-prone landscapes in which it occurs.
Grass trees may take several years to flower. Flowers form on a spear-like spike, which can be up to 4m long! Flowers are arranged in a spiral up the spike, and produce a great amount of nectar, attracting a wide variety of insects, birds and mammals.
Once pollinated, the flowers form a tough, pointed fruit capsule that’s typically matt-black. One flowering stem of the X. johnsonii grass tree can produce up to 10,000 seeds!