Cotton-Bush

The genus Maireana comprises approximately 57 species, almost all endemic to Australia, where they are widespread across the arid and semi-arid interior.

• Named after the 19th-century French naturalist Joseph François Maire
• The center of diversity lies in the dry inland regions of Western Australia, South Australia, and New South Wales
• Maireana sedifolia (Bluebush) is one of the most widespread and ecologically significant species, forming extensive shrublands across the Australian outback
• Kochia scoparia (syn. Bassia scoparia), sometimes also called cotton-bush, is native to Eurasia and has become naturalized and invasive in parts of North America
• Asclepias species referred to as cotton-bush (e.g., Asclepias eriocarpa, Woollypod Milkweed) are native to western North America, particularly California and the Great Basin

These plants evolved under the selective pressures of increasingly arid climates:
• Australia's interior has been drying progressively over the past 15 million years since the Miocene
• Adaptations to aridity include reduced leaf surface area, dense trichome (hair) cover, and C4 or CAM photosynthetic pathways in some species
• Fossil and pollen records indicate that chenopod shrublands (including Maireana) became dominant in Australia during the late Miocene to Pliocene as tropical forests retreated

Maireana sedifolia (Bluebush) — the most representative 'Cotton-Bush' of Australian deserts:

Growth Form:
• Perennial shrub, typically 0.5–1.5 m tall, occasionally reaching 2 m
• Dense, rounded to spreading habit with multiple woody stems arising from the base
• Root system is moderately deep and extensive, enabling access to subsurface moisture

Leaves:
• Small, succulent, cylindrical to narrowly obovate (~5–15 mm long)
• Densely covered with fine, white to greyish trichomes (hairs), giving the plant its characteristic silvery-blue appearance
• Leaves are semi-deciduous — shed during extreme drought as a water-conservation strategy
• Cross-section reveals a specialized water-storage tissue (hydrenchyme) beneath the epidermis

Flowers:
• Small, inconspicuous, wind-pollinated (anemophilous)
• Solitary or in small clusters in leaf axils
• Lack showy petals; perianth segments are membranous and reduced (~2–3 mm)

Fruit & Seed:
• The fruiting perianth develops into a distinctive, papery, fan-shaped or disc-shaped structure (a 'fruiting wing' or 'samara') ~10–20 mm in diameter
• This winged structure is often densely covered with silky hairs — the 'cottony' feature that gives the plant its common name
• Wind-dispersed (anemochorous); the papery wing allows seeds to tumble across open ground
• Seeds are small (~2–3 mm), lens-shaped, and can remain viable in the soil seed bank for years

Kochia scoparia (syn. Bassia scoparia):
• Annual herb, 0.5–1.5 m tall, with erect, much-branched stems
• Leaves linear to lanceolate, 2–6 cm long, softly hairy
• In autumn, the entire plant turns brilliant red-purple and develops a cottony texture from dense seed clusters
• Seeds enclosed in hairy, brownish perianth segments

Cotton-bush species are keystone components of arid shrubland ecosystems:

Habitat:
• Maireana sedifolia dominates chenopod shrublands across the Australian interior, often co-occurring with Atriplex (saltbush) species
• Found on calcareous plains, sandy rises, and stony desert pavements
• Tolerates highly saline and alkaline soils (pH up to 9.0)
• Kochia scoparia thrives in disturbed, saline, or alkaline soils across steppes, roadsides, and abandoned farmland

Drought Adaptations:
• Dense trichome layer reflects solar radiation, reducing leaf temperature by up to 5°C
• Trichomes also trap a boundary layer of still air, reducing transpirational water loss
• Succulent leaves store water for use during prolonged dry periods
• Can survive on as little as 150–250 mm annual rainfall
• Deep taproot system accesses groundwater at depths of 2–5 m

Ecological Role:
• Provides critical shelter and forage for native fauna, including the endangered Plains Wanderer (Pedionomus torquatus) in Australia
• Seeds are an important food source for granivorous birds and ants
• Host plant for specialist insect herbivores, including certain moth and beetle species
• Plays a role in stabilizing soil and preventing wind erosion in degraded rangelands

Fire Ecology:
• Maireana species are generally fire-sensitive and slow to regenerate after burning
• In contrast, Kochia scoparia is a fire-adapted annual that proliferates after disturbance

Reproduction:
• Primarily by seed; germination triggered by rainfall events exceeding ~20 mm
• Seeds exhibit dormancy mechanisms that prevent germination during brief, unreliable rain events
• Some Maireana species can also regenerate from root suckers after damage

Cotton-bush species are occasionally cultivated in xeriscaping, native gardens, and ecological restoration projects in arid regions.

Light:
• Full sun essential — requires a minimum of 6–8 hours of direct sunlight daily
• Will not tolerate shade; becomes leggy and sparse under low-light conditions

Soil:
• Extremely well-drained soil is critical; waterlogged conditions cause rapid root rot
• Tolerates poor, sandy, rocky, or clay soils
• Highly tolerant of saline and alkaline conditions (up to EC 8–12 dS/m)
• Ideal pH range: 6.5–9.0

Watering:
• Once established, Maireana sedifolia requires little to no supplemental irrigation
• Water sparingly during the first growing season to establish roots
• Overwatering is the most common cause of failure in cultivation
• Kochia scoparia requires moderate moisture during its growing season but tolerates drought once mature

Temperature:
• Maireana sedifolia tolerates extreme heat (up to 45°C) and mild frost (down to approximately -5°C)
• Kochia scoparia is frost-tender; grown as an annual in temperate climates

Propagation:
• Maireana: best propagated from semi-hardwood cuttings taken in late summer; seed germination can be erratic and may require scarification or soaking
• Kochia: easily grown from seed sown directly in spring after the last frost; germination rate is typically high (>80%)

Common Problems:
• Root rot from overwatering or poorly drained soil
• Leggy growth in insufficient light
• Kochia scoparia can become invasive outside its native range — check local regulations before planting