Pigweed, Knotgrass or wireweed

Wireweed (Polygonum aviculare)

Wireweed is also known as prostrate Knotweed, Pigweed and Knotgrass, and is a common weed of amenity turf. It is a problem on heavily worn and compact soils such as council sports grounds.

On golf courses and sports grounds, Wireweed thrives in irrigated areas, is very resistant to heavy use and is often found in goal mouths.

Wireweed favours a soil pH greater than five. Soil compaction and heavy nitrogen use also favour it, but pigweed also tolerates drought and low fertility soils.

Wireweed is a good indicator weed for soil compaction.



Why is Wireweed such a Problem?

  • Wireweed often germinates and emerges during turf establishment.
  • Knotweed competes for moisture and nutrients. This is a big problem when you are trying to establish turf.
  • It is phytotoxic.


Wireweed Identification.

Wireweed is an aggressive low-growing warm-season annual or biennial dicot. It has that has small oval leaves with a purple sheath around the base of each leaf. The branch leaves are about half the size of the stem leaves and stems can root at the nodes.

Pigweed grows only from seeds and produces less than 50 seeds per plant. These can reach a density or 200-5000 seeds/m2 and you find these mainly in the top 5cm of the soil profile. Viable seeds have been found after 60 years. Soil moisture is the trigger for seed germination.

When you disturb a large seed bank and then irrigate, these seeds germinate and pigweed becomes a problem. This is often the case during sports ground or fairway resurfacing. When it germinates in early spring, Knotweed resembles a grass with long, dark green leaves, and in warm weather it can grow rapidly.

Wireweed flowers mainly in the Autumn and Spring with large numbers of flowers appearing along the base of the leaves.

Flowers develop in small clusters and are pink to white with 5 petals. The stems are prostrate, have multiple branches and have a wiry appearance.

Flower: Pink to white and around 2 mm in diameter and 2-3 mm long.
Leaf length: The leaves of pigweed areblue-green and 5-10 mm long.
Leaf width: Leaves are 4-12 mm wide, dull green and oblong to oval in shape.
Comments: Leaves are hairless and it has a strong, deep, fibrous tap root. It tends to germinate in the spring, and first appears when soil temperatures are only around 4C.
Habitat: Knotgrass is a weed of disturbed and trampled soils. It thrives in heavy soils in full sun, and flourishes in poor soils with little plant competition.

Impact of Wireweed.

Pigweed may cause dermatitis in sheep and humans. Horses and stock that consume large quantities of seed may develop enteritis.

Pigweed may also contain toxic nitrate levels, and horses have died from nitrate poisoning after eating it in hot dry weather.

Knotgrass also inhibits seedling growth and germination of couch grass due to its high content of water-soluble allelochemicals.




How to control Wireweed.

Both cultural and chemical control can manage Knotgrass in your lawn and turfgrass.

Cultural control.

Wireweed flourishes in compacted soils, and so relieving soil compaction creates less favorable soil conditions for this weed. Prostrate Knotweed has a thin taproot so you can remove this by hand, but you are best to do this on young plants growing in moist soil.





Chemical control.

You get the best results in the spring when the weed is actively growing from the seedling to flower stage. If you cannot stop it producing seeds then control is impossible.
Pre-emergent herbicides can work really well if you apply these at the right time. Ideally this should be in the late Autumn to early Winter, when the plants are as small as possible. You may need several applications on more mature plants. United Kingdom research estimates you need four to seven years to exhaust any seedbank.

Products containing Dicamba usually provide better results.



Herbicide research on Pigweed and Knotgrass

  • Among a group of eighteen herbicide treatments, only six control knotgrass over 80%.
  • These include products containing 2,4-D, Dicamba, Metsulfuron, or Chlorsulfuron.
  • Treatments relying on Bromoxynil, Triclopyr, Clopyralid, Quinclorac, Metribuzin, Rimsulfuron, Foramsulfuron, and Trifloxysulfuron sodium, do not provide adequate control of Pigweed in couch grass.
  • Diflufenican, the active in Warhead Trio provides long-lasting pre-emergence control.