Citrus australis (RUTACEAE) Native Lime, Round Lime

Large shrub or small tree to 12 m in dry rainforest north from the Beenleigh area; endemic to (solely found in) Queensland. Branchlets often zigzag, with stiff thorns in leaf axils. Leaves simple, alternate, to 55 mm by 30 mm, elliptic to obovate or almost rhomboid, entire or somewhat scalloped towards the tip; juvenile foliage linear. Blade dark green, with numerous conspicuous oil dots (and hence aromatic). Stalk to 6 mm in length. Flowers white or pinkish, about 1 cm across, fragrant; solitary in leaf axils. Appear August to November. The dense, prickly habit makes this species a useful nesting site for small rainforest birds and as a screen plant. Propagate from fresh seed or cuttings, which are slow to develop roots. Could also be budded onto exotic citrus rootstock. Hardy but slow growing, needing some protection when young. Must have excellent drainage and prefers a sheltered position. Appreciates mulching and extra watering during dry periods, and organically rich soil. Moderately frost tolerant.

Fruit a green to greenish-yellow, globular berry to 65 mm in diameter; rough-skinned (similar to a miniature wrinkled lime). Ripe November to December. Edible acidic pulp; ripe fruits are eaten raw or can be made into marmalade and added to cold drinks for extra flavour. Citrus is Latin for lemon (cirtron) australis means southern (hemisphere). 

Six small citrus species known as limes are native to Australia. Five are found only in the rainforests of the north. The sixth, the Desert Lime Citrus glauca, grows in semi-arid areas of south-east Australia. Two of the five rainforest limes occur naturally in South-East Queensland – the Finger Lime Citrus australasica and the Round Lime Citrus australis sometimes called called Dooja How.

These thorns provide fairly good protection for browsing mammals (apart from the most dextrous ones, such as humans) but would not prove too problematic for birds, which this plant probably relies on more to spread itself. It is likely that frugivores and scavengers would be more likely to use fallen fruit, which would have greater chance of propagation when scattered, either in dung or just as a result of being torn apart and eaten.

The leaves can be quite diamond-shaped, and in many cases scalloped towards the tip, sometimes pronouncedly so, but not as much as its close relative the Finger Lime.