Crotalaria cunninghamii Seeds | Green Birdflower

Green Birdflower (Crotalaria cunninghamii) seed — Green Birdflower (*Crotalaria cunninghamii*): Fabaceae, genus Crotalaria , the rattlepods, native to Inland Australia — the arid and semi-arid interior of Western Australia, the Northern Territory, South Australia and western Queensland, on red sand plains and dunes.

LATIN NAME: Crotalaria cunninghamii

COMMON NAME: Green Birdflower

Plant Profile

Nineteenth-century botanists receiving a drawing of this flower are said to have suspected a hoax, and it is easy to see why. The green corolla is shaped so that the standard petal forms a rounded body, the wings and keel project as a head and beak, and the darker veins of the standard run where a bird’s wing feathers would. Nothing else in the pea family assembles its petals into an animal silhouette quite so literally, and the effect is strongest when the flowers are seen in profile along a stem, hanging in a row like birds on a fence wire.

The plant is a spreading, many-branched shrub of the dry inland, growing on red sand and sandy loam where summer rain is unreliable and frost is rare. Stems and leaves carry a dense coat of hairs that gives the whole plant a silvery, felted surface, and those hairs cut transpiration while reflecting some of the glare. Leaves are simple and rounded rather than the compound blades of most peas — a desert economy in which one blade does the work of many leaflets. Flowers are produced in loose clusters along the upper stems, opening a soft apple-green with a translucent quality, and the plant can carry bloom for much of the warm season if water allows.

The keel encloses the stamens and the style, and it must be depressed before pollen is released onto a visitor; large bees are the pollinators best able to work a flower of this weight and depth. Field observations of the species are sparse compared with its fame in cultivation, and the bird-like shape should be read as a pollinator-manipulating structure rather than as evidence of bird pollination — a good illustration of how easily a resemblance is mistaken for a relationship.

Fertilised flowers make a swollen, balloon-like pod that dries to a papery brown. Inside, the small seeds work loose and knock against the pod wall, which is precisely how the genus earned the name rattlepod. The pods stay on the bush well into the dry season, so seed is normally collected from standing plants rather than from the ground.

Science & Biotechnology

Rattlepods manufacture pyrrolizidine alkaloids — monocrotaline, retrorsine, senecionine and trichodesmine among them — as a chemical defence against herbivores. These compounds are hepatotoxic, and heavy grazing of Crotalaria in northern Australia has produced the liver disease long known there as Kimberley horse disease, or crotalosis. The lesson travels with the plant: the ornament is safe to handle, but stock should not be allowed to make a meal of it, and dried material in hay is as dangerous as the fresh plant.

Paradoxically, the same chemistry made the genus useful in the laboratory. Monocrotaline, obtained from a related species, is given to rats to produce pulmonary arterial hypertension, a standard experimental model for studying the vessel remodelling that underlies the human disease. That work is the reason the alkaloid profile of the rattlepods is so well documented, and it is a reminder that a plant’s most famous compound may also be its most dangerous one.

Growers in the genus have another, gentler reputation: several species suppress root-knot nematodes when used as a cover crop, partly by trapping the nematodes in their roots and partly through the alkaloids released into the soil. Crotalaria cunninghamii is a dry-country shrub rather than a green-manure crop, but it shares the root biology and the nitrogen-fixing nodules that make its relatives valuable in poor sandy ground.

Uses & Benefits

In cultivation the green birdflower is grown as a drought-tolerant garden shrub for warm, dry districts — a specimen for a gravel bed, a hot bank or a large container, where its grey felted foliage and green flowers look unlike anything else in the pea family. Its roots help stabilise sand, and in its native range it colonises disturbed ground after fire. It is frost-tender, resenting wet winters far more than cold nights, and it flowers best on lean soil in full sun; rich feeding produces leaf at the expense of bloom.

Growing from Seed

This is a hard-seeded legume, and the coat must be broken before sowing. Rub each seed over fine abrasive paper, or nick the coat with a sharp knife on the side away from the hilum, then soak in tepid water until the seeds swell and sink. Sow about twice the seed thickness deep in a sandy mix — two parts coarse sand, one part loam, one part compost — and keep the pan warm and only lightly moist. Pot on when the seedlings are a hand high, and plant out into a sharply drained position once the roots have filled the pot.

Tip-prune young plants to make a bushy frame, and shorten spent flowering stems to encourage another flush. Water established plants deeply but infrequently. The felted leaves are naturally resistant to chewing insects, and the commonest trouble is root rot in a wet, heavy soil — best prevented by planting on a mound and mulching with gravel rather than compost.

Our Take

Grow it for the flower, and give it the driest, sunniest spot you have. Among its relatives, Crotalaria pallida is the softer, paler rattlepod for a warm border, Sesbania cannabina is the tall quick screen for damp tropical ground, and native indigo brings indigo colour into the same sandy bed. Tephrosia candida is the other silver-leaved legume I keep beside it for contrast. Nick the seed, keep the soil lean, and it will flower itself into a curiosity that visitors refuse to believe is real.

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