Lacy Ragweed

Ambrosia tenuifolia Sprengel

Synonyms - Franseria strigulosa, Franseria tenuifolia.

Family: Asteraceae.

Names:

Lacy Ragweed - Lacy refers to the lace-like appearance of the finely divided leaves. Ragweed refers to the ragged appearance of the plant. Ambrosia is from the Greek meaning 'food for the gods' and used for species of aromatic plants. Tenuifolia is Latin for narrow leaf.

Summary:

Erect, rhizomatous and stoloniferous herb forming dense colonies, 300-600 mm tall with finely divided, lacy leaves. It has annual tops with a perennial root system.

Description:

Cotyledons:

Two.

Leaves:

Grey green. Alternate near top, opposite near bottom.
Stipules -
Petiole -
Blade - Broadly lance shaped 60-80 mm long, 40-50 mm wide. Slightly hairy. Deeply divided twice into slender thin lobes about 1.5 mm wide.

Stems:

Woody at the base, 300 -600 mm tall. Simple or occasionally branched. Bristly hairs.

Flower head:

About 12 male flowers are grouped into a cupped structure about 2.5 mm round and these are in spike like clusters at the ends of stems and branches.
Female flowers, few and single in the axils of upper leaves.

Flowers:

Male and female flowers on the one plant.
Males are green white and peep out of a set of fused bracts.
Bracts - Fused
Ovary -
Perianth -
Stamens - 5
Anthers -

Fruit:

Seeds:

3-4 mm long. Black. Top shaped. Pointed beak surrounded by 4 to 6 small lumps.

Roots:

Strong, short rootstock with long creeping rhizomes running parallel to the soil surface giving rise to new shoots.

Key Characters:

Biology:

Life cycle:

Perennial. Flowers from spring to summer. Seeds germinate in autumn and form an extensive root system over winter. Flower stems emerge in spring and flowering starts in late spring and continues to late summer if conditions are favourable. Top growth dies in autumn. New shoots emerge from the rootstock and rhizomes in autumn.

Physiology:

Reproduction:

Flowering times:

Spring to summer.

Seed Biology and Germination:

Probably has dormancy. Germinates in autumn.

Vegetative Propagules:

Rhizomatous roots and stoloniferous shoots.

Hybrids:

Allelopathy:

Population Dynamics and Dispersal:

Reproduces by seed and creeping roots that form dense colonies.
It hasn't spread quickly from known infestations. Movement in soil associated with road and railway works is probably the major form of long distance dispersal. Movement in water by floating seeds may occur at a more local level. The seed probably has a degree of dormancy.

Origin and History:

North America.

Distribution:

NSW, SA, VIC.

Courtesy Australia's Virtual Herbarium.
Not in WA according to Florabase.

Habitats:

Open areas.

Climate:

Sub-humid temperate regions.

Soil:

Light soils.

Plant Associations:

Significance:

Beneficial:

Detrimental:

Noxious weed of NSW and SA. Weed of roadsides, sand dunes, disturbed areas, fallows, stubbles and poor pasture.

Toxicity:

Not recorded as toxic.

Legislation:

Noxious weed of NSW and SA.

Management and Control:

Improve pasture density and growth.

Thresholds:

Eradication strategies:

Mowing, slashing and cultivation are ineffective. Treatment with herbicides over a number of years will probably be required because of dormant seed.

Herbicide resistance:

Related species have developed resistance to triazine herbicides.

Biological Control:

Related species have been subject to biological control programs.

Related plants:

Annual Ragweed (A. artemisiifolia)
Burr Ragweed (A. confertiflora)
Perennial Ragweed (A. psilostachya).

Plants of similar appearance:

References:

Cunningham, G.M., Mulham, W.E., Milthorpe, P.L. and Leigh, J.H. (1992). Plants of Western New South Wales. (Inkata Press, Melbourne). P728. Photo.

Lamp, C. and Collet, F. (1990). A Field Guide to Weeds in Australia. (Inkata Press, Melbourne).

Lazarides, M. and Hince, B. (1993). CSIRO handbook of economic plants of Australia. (CSIRO, Melbourne). #70.4.

Parsons, W.T. and Cuthbertson, E.G. (1992). Noxious weeds of Australia. (Inkata Press, Melbourne). P253-254. Photos.

Acknowledgments:

Collated by HerbiGuide. Phone 08 98444064 or www.herbiguide.com.au for more information.