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Erigeron bonariensis

flax-leaved horseweed

Overview

Erigeron bonariensis, formerly placed in Conyza spp., is an erect annual or short-lived herb growing 8-40 in (20-100 cm) tall, with greyish, roughly hairy stems that branch toward the top. The narrow, greyish-green leaves are 1-4 in (2.5-10 cm) long, linear to lance-shaped, and hairy, the lower ones sometimes toothed. From late spring through autumn the plant carries many small flowerheads about 0.2-0.3 in (5-8 mm) across, each with tiny whitish to pale-yellow florets and no spreading ray petals, grouped in a cluster whose side branches often overtop the central stem. Each head ripens into a tuft of small seeds tipped with pale bristles that are carried on the wind. A single plant can produce tens of thousands of seeds, and the species establishes quickly on bare and disturbed ground. It is a weed of crops, vineyards, roadsides, and pavements across temperate and warm regions, and many populations have evolved resistance to glyphosate and other herbicides. The plant completes its cycle in one season and relies on prolific seeding rather than perennial regrowth.

Native Range

Native to South America, particularly Argentina and neighbouring countries. It has spread as a weed to North America, Europe, Africa, Asia, and Australia, naturalising on disturbed and cultivated ground.

Suggested Uses

This species is not cultivated as an ornamental and is treated as an agricultural and environmental weed. It has limited use beyond minor traditional medicinal applications in parts of its native range.

How to Identify

Recognised by erect, greyish-hairy stems and narrow, hairy, greyish-green leaves, with side branches that often grow taller than the main flower cluster. The numerous small flowerheads lack spreading ray petals and open only slightly. The seed heads form fluffy pale tufts, and the overall grey, hairy look separates it from the greener, less hairy Erigeron canadensis.

Appearance

Size & Dimensions

Height8" - 3'4"

Width/Spread8" - 1'4"

Colors

Flower Colors

white

yellow

Foliage Colors

green

Bloom Information

Bloom Period

~20 weeks

Spring Summer Fall

Flowering extends from late spring through autumn, broadly May to October in the northern hemisphere and across much of the warm season in milder climates. The small heads open in succession over a long period, so flowering and seeding overlap on the same plant. Warm, dry conditions extend the flowering season.

Detailed Descriptions

Foliage Description

grey-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 Range5.5 - 8.0(Neutral)

357912

Soil Types

sand loam clay rocky

Drainagewell drained

Water & Climate

Water Needs

Low

Drought Tolerance

Drought tolerant when established

Care & Maintenance

Care Guide

Erigeron bonariensis grows in full sun on poor, dry, compacted, and disturbed soils, and tolerates heat and drought once established. It germinates in flushes after rain in autumn and spring and develops a shallow fibrous root system. The species needs no cultivation and is generally managed as a weed rather than grown. Seedlings emerge readily on bare ground and grow quickly to flowering size within a few months. Dense stands compete with crops and ornamentals for water and nutrients, and herbicide resistance complicates chemical control in many regions.

Pruning

No pruning is required for this annual. Plants are cut, hoed, or pulled before the flowerheads mature to limit the large output of wind-blown seed. Removing plants while young limits seeding more than tackling mature, seeding stands.