Woollyleaf bur ragweed (Ambrosia grayi)

Synonyms: Franseria grayi, Franseria tomentosa, Gaertneria grayi, Gaertneria tomentosa

Common Names: Bur ragweed, woollyleaf bursage, lagoonweed

Description:

Woollyleaf bur ragweed (Ambrosia grayi) is a native perennial in the ragweed genus, found across the central and southern Great Plains from Nebraska and Colorado south through Kansas, Oklahoma, and Texas [1][2]. Despite being native, it is a documented, aggressive weed of pastures, cropland, and roadsides, spreading by tough underground roots into dense colonies [1][3]. MISIN tracks it because it crowds out desirable forage and native plants and can be difficult to eliminate once established [1][3].

Habit:

Woollyleaf bur ragweed is a bushy perennial, usually about 1 to 2 feet tall, with a silvery-gray look from the dense woolly hairs covering most of the plant [1]. Its alternate leaves are broadly oval with irregular lobes and a soft, felted texture [1][2]. It is easiest to spot from August through October, when it produces small, greenish-yellow flower heads typical of the ragweed genus, followed by spiny burs [2]. Look for the gray, woolly, colony-forming growth in disturbed sandy soils ? pastures, fields, and roadsides ? especially where it forms dense patches rather than scattered single plants [1].

At-a-glance cues:

1. Bushy perennial, about 1 to 2 feet tall, with a silvery-gray, woolly-haired appearance [1]
2. Broadly oval, irregularly lobed alternate leaves with a soft, felted texture [1][2]
3. Small greenish-yellow flowers, August through October, typical of the ragweed genus [2]
4. Grows in dense colonies rather than scattered single plants, in disturbed sandy or saline soils [1]

Leaves:

1. Arrangement: alternate along the stem [1]
2. Shape: broadly ovate (egg-shaped), irregularly 3 to 5-lobed or occasionally untoothed, on long stalks [2]
3. Margin: irregularly lobed rather than deeply dissected [2]
4. Surface: densely woolly-hairy, giving a dusty white to gray-green look [1][2]
5. Size: about 0.4 to 1.8 inches (1 to 4.5 cm) long [2]

Stems:

1. Habit: an erect, bushy perennial herb, about 1 to 2 feet (30 to 60 cm) tall, branching from the base [1]
2. Texture: covered in fine, woolly hairs [1]
3. Spread: creeps outward from tough underground roots (rhizomes), forming dense colonies [1][3]

Flowers:

1. Color: small, greenish-yellow, inconspicuous [1]
2. Arrangement: separate male and female flower heads on the same plant, typical of the ragweed genus ? small male heads clustered above, female heads below [1][2]
3. Bloom time: August through October [1][2]
4. Wind-pollinated, like other ragweeds, rather than insect-pollinated

Fruit and seeds:

1. Fruit type: a small, spiny bur enclosing a single seed, typical of the ragweed genus
2. Ripening: fall, following the August?October bloom [1][2]
3. Dispersal: burs catch on fur, feathers, clothing, and equipment; the plant also spreads vegetatively ? a piece of root containing a node can grow into a new plant, so tillage and cultivation can spread it through a field [1][3]

Habitat:

Woollyleaf bur ragweed is native to the central and southern Great Plains of the United States, occurring from Nebraska and Colorado south through Kansas, Oklahoma, New Mexico, and Texas [1][2]. It grows in moist to disturbed ground ? pastures, rangeland, cropland, and roadsides ? and often favors sandy or saline (salty) soils [1]. It is not native to Michigan or the Great Lakes region.

Reproduction:

Woollyleaf bur ragweed spreads two ways, and its underground roots are the more troublesome route [1][3]. It creeps outward from tough, branching roots (rhizomes), and any piece of root that includes a growing point can sprout into a brand-new plant ? which means tilling or cultivating a field can spread it further rather than control it [3]. It also produces spiny burs that catch on fur, feathers, clothing, and equipment, carrying seed to new ground [1].

It flowers from August through October, and its burs ripen soon after [1][2]. Because established colonies regrow from root fragments, controlling this species means going after the root system, not just cutting the visible plant.

Impact and Damage:

Woollyleaf bur ragweed is not a typical hazard for people in a yard, but it does carry a livestock risk: eaten in large amounts, it can cause nitrate poisoning in cattle, though grazing animals usually avoid it because it isn't palatable [1]. Where it does spread into pastures, hayfields, and cropland, it can be a real management headache ? a peer-reviewed weed-science study describes it as "a pernicious perennial weed" of the southern Great Plains that spreads by root fragments and has proven difficult to control with standard tillage and herbicide programs [3].

In natural and semi-natural areas, its dense, rhizome-fed colonies crowd out desirable native plants and forage species, effectively taking over the ground they occupy [1]. Because the plant regrows from any root fragment left behind, an established colony is slow and persistent to clear [1][3].

Similar species:

Several other ragweeds and bursages are confused with woollyleaf bur ragweed.

Woolly fruit bur ragweed (Ambrosia eriocentra) is a genus relative, but it is a woody desert shrub of the Southwest with sharply toothed, holly-like leaves, quite different from woollyleaf bur ragweed's herbaceous growth and softly lobed, woolly-gray leaves.

Common ragweed (Ambrosia artemisiifolia) is the most familiar ragweed to most homeowners, but its leaves are deeply, fern-like dissected into many narrow lobes, unlike woollyleaf bur ragweed's broader, only irregularly lobed leaves; common ragweed is also an annual, not a colony-forming perennial.

Western ragweed (Ambrosia psilostachya) is another rhizomatous perennial ragweed with a similar spreading habit, but its stems are slender and not noticeably woolly, unlike woollyleaf bur ragweed's stouter, densely hairy, gray-felted growth.

Monitoring and rapid response:

Signs to watch for:

1. Silvery-gray, woolly-leaved colonies spreading in pastures, hayfields, cropland edges, or roadsides on sandy or saline soil, rather than scattered single plants [1]
2. Small greenish-yellow flowers August through October, followed by spiny burs [1][2]

What you can do:

1. Dig out young, isolated plants by hand, removing as much of the root as possible ? any root piece left behind with a growing point can resprout [1][3]
2. Cut or mow colonies before the flowers set seed in late summer to limit new burs, understanding that mowing alone won't stop root-based spread
3. Avoid tilling or cultivating through an established patch, since that can chop roots into pieces and spread the colony rather than kill it [3]
4. Bag and remove cut plant material and burs rather than leaving them on site
5. Natural-area and rangeland managers: because this species regrows from root fragments, plan for repeated, multi-year monitoring and removal rather than a single treatment, and prioritize small outlier colonies before they merge into a larger patch [3]

When to call a professional:

1. Established colonies in pastures, hayfields, or cropland, where root-based regrowth has made standard tillage or one-time treatment ineffective [3]
2. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. Nebraska Invasive Species Council. Bur Ragweed (Ambrosia grayi). neinvasives.com
2. Integrated Taxonomic Information System (ITIS). Ambrosia grayi (A. Nelson) Shinners. itis.gov
3. Sheikh T, Wheeler TA, Dotray PA, Zak JC (2001). "Biological Control of Woollyleaf Bursage (Ambrosia grayi) with Pseudomonas syringae pv. tagetis." Weed Technology 15(2):375?381. semanticscholar.org

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