Pale Knotweed

Polygonum lapathifolium L.

Synonyms - Polygonum lanigerum, Persicaria lapathifolia

Family: Polygonaceae.

Names:

Polygonum is from the Greek polys meaning many and gony meaning knee and refers to the many nodes on the stems.
Pale Knotweed.

Other names:

Pink Knotweed because it has pink flowers.

Summary:

A rather erect, many branched annual plant with oval leaves that are dotted with oil glands.

Description:

Cotyledons:

Two.

Leaves:

Alternate.
Stipules - (Ochrea) Membranous, flat topped, hairless, ribbed sheath where petiole joins the stem. Hairless.
Petiole - Short.
Blade - Lance shaped to broadly oval with a pointed tip, 70-200 mm long x 15-60 mm wide, dotted with oil glands, conspicuous veins underneath, usually has central brown blotch, rough due to low lying hairs on the edges and midrib. Base tapered.

Stems:

Erect or bending upwards, branching, stout, up to 1200 mm tall, hairless.

Flower head:

Dense, thick spike, 20-80 mm long. On a stalks (peduncles) with oil glands that is shorter than the spike. 2 or more spikes form a panicle on the ends of stems.

Flowers:

Pink. Less than 4 mm long. On stalks with oil glands.
Bracts - Small flat topped bracts underneath.
Ovary - Style is 2 branched.
Perianth - Less than 4 mm long. Lobes small usually with no or few oil glands (dots).
'Petals' - Small, pink.
Stamens -
Anthers -

Fruit:

Lens shaped nut, edges obtuse, 1.75-2.5 mm long, smooth.

Seeds:

Enclosed in fruit.

Roots:

Taproot.

Key Characters:

Flowers in paniculate, dense, stout spikes.
Leaves lanceolate.
Stipules glabrous.
Stems erect, stout, glabrous.
Nut biconvex.

Biology:

Life cycle:

Annual. Flowers in summer.

Physiology:

Reproduction:

By seed.

Flowering times:

Summer in western NSW.
Flowers in summer in SA.

Seed Biology and Germination:

Vegetative Propagules:

None.

Hybrids:

Allelopathy:

Population Dynamics and Dispersal:

Spread by seed.

Origin and History:

Africa. New Zealand.

Distribution:

ACT, NSW, NT, QLD, SA, TAS, VIC.

Courtesy Australia's Virtual Herbarium.

Habitats:

Wet areas, depressions, edges of swamps.

Climate:

Temperate.

Soil:

Often on clay soils.

Plant Associations:

Black box and Lignum.

Significance:

Beneficial:

Rarely grazed by sheep, lightly grazed by cattle.

Detrimental:

Weed of watercourses, roadsides, wet areas and disturbed areas.

Toxicity:

Suspected of causing dermatitis and death in cattle.

Legislation:

None.

Management and Control:

Thresholds:

Eradication strategies:

Cultivate before flowering.
Glyphosate, dicamba and sulfonyl urea herbicides provide good control.

Herbicide resistance:

Biological Control:

Related plants:

Black Bindweed (Polygonum convolvulus)
Creeping Knotweed (Polygonum prostratum)
Princes Feather (Polygonum orientale)
Sand Wireweed (Polygonum arenastrum)
Slender Knotweed (Polygonum salicifolium, Polygonum decipiens)
Small Knotweed (Polygonum plebeium)
Spotted Knotweed (Polygonum strigosum)
Tree Hogweed (Polygonum patulum)
Vietnamese mint (Polygonum odoratum)
Water Pepper (Polygonum hydropiper)
Wireweed (Polygonum aviculare)
Polygonum capitatum
Polygonum glabrum

Plants of similar appearance:

Redshank (Persicaria maculosa)

References:

Auld, B.A. and Medd R.W. (1992). Weeds. An illustrated botanical guide to the weeds of Australia. (Inkata Press, Melbourne). P205. Photo.

Black, J.M. (1965). Flora of South Australia. (Government Printer, Adelaide, South Australia). P283.

Burbidge, N.T. and Gray, M. (1970). Flora of the Australian Capital Territory. (Australian National University Press, Canberra). P154-155. Diagram.

Cunningham, G.M., Mulham, W.E., Milthorpe, P.L. and Leigh, J.H. (1992). Plants of Western New South Wales. (Inkata Press, Melbourne). P232. 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). #1002.9.

Acknowledgments:

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