Perennial sow thistle (Sonchus arvensis) is an aggressive plant that spreads through both wind-borne seed and creeping roots. It is native to western Asia and Europe and was introduced to North America as a contaminant in seed.
Stems are upright, leafy at the base and may grow up to 2m tall. They exude a milky juice when cut or broken. This sap is not irritating to skin.
Leaves are borne alternately and fringed with weak prickles or spines. They are long and thin and can grow up to 20cm long.
Flowers and Fruit
Flowers are yellow and similar to dandelions. A plant may have up to 20 flower heads, but only a few are in flower at any given time. Seeds are dark brown and tufted with white fluff to aid in wind dispersal. Seeds are relatively short-lived in the soil seed bank.
Perennial sow thistle is well-adapted to thrive in agricultural or urban development conditions - it spreads rapidly in loose, moist, fertile soil under full sunlight. It may become a serious problem in riparian areas where control options are limited.
Once an infestation is established, its extensive root system makes it very difficult to eradicate entirely. The milky, bitter sap may be unpalatable to livestock and sow thistle seed is difficult to clean from forage seeds such as alfalfa and clover.
New infestations must be controlled before the plant's extensive root system develops.
Seedlings may be hand-pulled but mature plants will break off at the soil surface and grow again from roots. Successful control will require multiple mowing or pulling operations to weaken the root system and allow competing vegetation to take over.
Herbicide resistance has been confirmed in some populations of perennial sow thistle, which limits chemical options for both homeowners and farmers. Consult a local landscape company or garden centre for herbicide recommendations and always use chemicals in conjunction with other control methods to delay resistance.