The common grass tree (Xanthorrhoea preissii) is endemic to Western Australia and grows naturally in sand, loam or gravelly soils to a height of four metres. They have a lifespan of up to 600 years with a growth rate of only 1–2cm per year. Under The Environment Protection and Biodiversity Conservation Act 1999, it is an offence to take native vegetation including grass trees from public lands or state forests. Permits are required for commercial salvaging where the site has been approved for urban or industrial development. Land owners wishing to harvest grass trees from their property, must comply with state and commonwealth legislation.
Structure
The trunk is composed of a mass of old leaf bases held together by natural resin which can take ten years before it begins to form. The centre of the trunk is filled with a fibrous material. The needle-like foliage reduces moisture loss during periods of hot weather which makes the tree drought resistant. The root system is shallow with the main purpose being for anchorage. Surrounding the roots are microbes called mycorrhiza fungi which are essential for nutrient uptake in deficient soils and also protect roots against some pathogen fungi.
Common pests
Scale
Scale insects are sap-suckers which have either a waxy or armoured covering. Juvenile scales (crawlers) disperse to favourable sites on the leaf and start feeding. The crawlers eventually become immobile, and start building their protective covers, but are still sucking the needles. This activity, if left unchecked, may eventually kill the grass tree. Control scale with an application of horticultural oil, but not during hot weather.
Mealybugs
Mealybugs often have a number of overlapping generations per year. Their development is dependent on temperatures above 25°C with high humidity. After hatching, the juveniles (crawlers) search for suitable feeding sites in sheltered areas. Control is best achieved in late October at crawler stage with a systemic insecticide such as imidacloprid or acetamiprid. Populations reach peaks during spring and autumn.
Bardi grubs
The term ‘bardi grub’ is used to describe the larva of the Longicorn beetle (Bardistus cibarius), but may also apply to other native boring larvae. A decaying trunk or a thick ‘skirt’ of dead foliage provides the perfect environment for attack by these native borers and wood boring moths. The female moth lays her eggs into this foliage and the emerging grubs bore their way into the fibrous centre of the tree. This damage disrupts the tissue that carries water and nutrients to the crown. In severe infestations the tree will rot and fall over. There is not a registered chemical control, but burning of excess foliage (thatch) every 3–4 years will make this an unsuitable haven for pests. Burning off does little damage to the green crown, as it actually promotes growth as it would in a natural fire.
Diseases
Root disease — root rot
Root rot is a fungal pathogen Phytophthora cinnamonni and related pathogens, cause the roots to rot, preventing the grass tree from taking up water and nutrients. It is usually present in damp or poorly drained soils but will also survive drought. As a result the central leaves wilt and turn brown, causing the crown to collapse and the trunk to rot. Phosphorous acid sprays can be applied in the early stages of decline. However, as the disease may be present without showing symptoms, sometimes control is not successful.
Phytophthora may be in existing soil as a dormant spore or it can be introduced in to the home garden via contaminated soil. There have been examples of Phytophthora moving through the soil water from a higher elevated garden, to another garden at a lower level some distance away, affecting a variety of susceptible plants such as roses and conifers.
The Department of Agriculture and Food's Agwest Plant Laboratory can test soil samples for for a nominated fee.
Foliage disease
There are few foliar pathogens recorded for Xanthorrhoeas. The plants have adapted to dry conditions by having thick waxy needles to prevent water loss, as well as acting as a barrier to fungal attack. Foliar leaf spot pathogens are more common on the coastal plains than further inland.
Anthracnose
Symptoms of anthracnose (Colletotrichum xanthorrhoeae) are first visible as black flecks in the leaf tissue. These flecks develop to encompass the entire leaf and will spread to the rest of the crown. Apply mancozeb to protect leaves from further infection. However, if the disease has progressed too far, control may be ineffective.
Other factors contributing to decline
- Run-off from lawn phosphate fertilisers into garden beds.
- Transplant shock. This may take months to appear and is more common during summer.
- Insufficient drainage in clay soils causing waterlogging.
- Mulching too close to the trunk.
- Drought stress.
- Saline bore water.
A good indication to determine if the tree is to survive, is to pull gently at the centre needles. If they come away, this indicates the crown is rotting and unfortunately there is no remedy. In some cases burning the ‘skirt’ will promote growth as mentioned previously.
Unfamiliar pests
The Department of Agriculture and Food, Western Australia (DAFWA) is on the lookout for animal and plant pests, diseases and weeds that could pose a threat to agriculture and the environment.
If you discover something unfamiliar, please send a photo to the Pest and Disease Information Service (PaDIS) by email: info@agric.wa.gov.au or phone them on Freecall: 1800 084 881.
Please read the sending specimens for identification web article before sending, or bringing in, samples to the Pest and Disease Information Service, 3 Baron-Hay Court, South Perth, 6151, WA.