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Oncosiphon pilulifer

stinknet

Attracts Pollinators Deer Resistant Drought Tolerant

Overview

Oncosiphon pilulifer is an aromatic annual in the Asteraceae, reaching 6-24 inches (15-60 cm) tall and 6-16 inches (15-40 cm) wide in a single growing season. Stems branch from the base and carry finely dissected, fern-like leaves 0.8-2 inches (2-5 cm) long that release a strong resinous odor when crushed. From late winter into spring the plant produces numerous spherical flower heads 0.2-0.4 inch (5-10 mm) across, composed entirely of tiny yellow disc florets with no ray petals, giving each head a button-like outline. A single plant can set thousands of seeds, which germinate after autumn and winter rains. In its introduced range across the Sonoran Desert of Arizona and parts of southern California, dense stands form on disturbed soils, roadsides, and desert flats, where the dried plants increase the dry fuel load and raise spring fire risk. The pollen and crushed foliage can trigger allergic skin and respiratory reactions in sensitive people. Oncosiphon pilulifer is listed as a noxious or invasive weed in Arizona and is targeted for removal in many Sonoran Desert preserves. It tolerates poor, compacted soils and full sun, completing its life cycle before summer heat. Growth is limited to a single season, and control depends on removing plants before seed set.

Native Range

Native to the winter-rainfall region of the Western Cape in South Africa. It has naturalized in Arizona and southern California, where it spreads on disturbed desert soils, roadsides, and vacant ground.

Suggested Uses

Oncosiphon pilulifer has no horticultural use and is treated as an invasive weed in its introduced range. It is encountered on roadsides, vacant lots, and disturbed desert ground rather than in planted gardens.

How to Identify

Look for finely dissected, bright green leaves and a strong resinous odor released when the foliage is crushed. The yellow flower heads are spherical, 0.2-0.4 inch (5-10 mm) wide, and lack ray petals, clustered at the branch tips. Plants form low, bushy clumps that dry to tan by early summer.

Appearance

Size & Dimensions

Height6" - 2'

Width/Spread6" - 1'4"

Colors

Flower Colors

yellow

Foliage Colors

green

Bloom Information

Bloom Period

~10 weeks

Winter Spring

Flowering occurs from late winter through spring, typically February to May in the Sonoran Desert. Each plant carries many rounded yellow heads over several weeks. Bloom timing follows the amount of autumn and winter rainfall.

Detailed Descriptions

Foliage Description

bright green

Growing Conditions

Sun Requirements

SunFull Sun

Requires 6-12 hours of direct sunlight daily

• Full Sun: 6+ hours of direct sunlight

• Partial Shade: 3-6 hours of direct sunlight

• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range6.0 - 8.0(Neutral)

357912

Soil Types

sand loam rocky

Drainagewell drained

Water & Climate

Water Needs

Low

Drought Tolerance

Drought tolerant when established

Care & Maintenance

Care Guide

Oncosiphon pilulifer grows in full sun on poor, compacted, or sandy soils of low fertility. It germinates with autumn and winter moisture and needs little supplemental water once growing. The plant completes its life cycle in one season and dies after setting seed in late spring. It is not grown ornamentally and is managed as an invasive weed across its introduced range. Removal before flowering prevents the large seed bank that sustains infestations. Hand-pulling is feasible for small stands because the root system is shallow.