Parthenium argentatum aka Guayule

Parthenium argentatum, known as guayule, belongs to the daisy family. This evergreen shrub grows wild from Texas into Mexico. Its native range spans several states in northern and central Mexico. It lives in deserts and dry scrub. It also grows on limestone ridges and lime-rich soils. These habitats help explain its need for dry, well-drained ground.

Guayule forms a dense shrub with a rounded crown. Sources give a height of about 3–3.3 feet [0.9–1 metre]. Its bark is pale gray-brown, with shallow cracks and darker ridges. Thin, gray-green twigs bear many fine, silver hairs. The leaves grow in an alternate pattern along the stems. Each leaf is simple, rather than split into leaflets. Leaves range from narrow and pointed to egg-shaped. Their edges may be smooth, toothed, or shallowly lobed. Silver hairs cover both sides of the gray-green leaves. The plant keeps its leaves through the year.

Small flowers range from pale yellow to white. They form flat clusters at the ends of branching stems. Even a little rain can prompt flowering through much of the year. The flowers have both male and female parts. Insects and wind can carry their pollen. Plants can also set seed without sexual fusion. Guayule is self-fertile. Its small, dry fruits are achenes. These gather in tight clusters at the stem tips.

Grow guayule in full sun, with very good drainage. It cannot grow in shade. Sandy soil suits it well, though cultivation reports also support other non-clay soils. The source lists clay as suitable elsewhere, so that point remains unclear. It accepts mildly acid, neutral, and alkaline soil, including very alkaline ground. Established plants withstand drought and prefer dry soil. PFAF lists USDA zones 7–10 but warns about British winters. Plants dislike temperatures below 14°F [-10°C] and suffer damage below 5°F [-15°C]. Wet winters and changing winter temperatures also cause trouble.

A sunny garden edge or a south- or west-facing wall suits its stated needs. Its silver foliage and rounded form provide its main visual traits. Guayule also supplies rubber from crushed roots or stems. Wetter growing sites can reduce latex yield, even when plants grow well. PFAF reports no known food or medicinal uses. It also lists no known hazards or weed potential. The supplied sources name no specific pests or diseases.