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Pest Status
Chapter 43
Sonchus arvensis L., Perennial Sowthistle,
S. oleraceus L., Annual Sowthistle
and S. asper (L.) Hill, Spiny Annual
Sow-thistle (Compositae)
D.P. PESCH KEN
Four species in the genus Sonchlls have been introduced into North America from
Europe and Asia. Three of them are weeds. Perennial sow-thistle. Sonchus arvensis L..
is a common weed in all provinces of Canada but it is particularly abundant in Ontario.
Manitoba. and the northern agricultural areas of Quebec. Ontario. and the Prairie Provinces
(Frankton & Mulligan 1970). It includes the varieties S. arvensis L. s. str. and S.
arvensis var. glabrescens Guenth .• Grab & Wimm. (Boulos 1961). It reproduces by
seed and an extensive underground root system. About 9750 achenes are produced per
flowering shoot (Stevens 1932) and 59500 per m 1 (Stevens 1924). The seeds are dispersed
by means of a pappus. Stevens (1924) noted that most seeds are not carried far
and that there is a difference in the persistence of the pappus in different lots. In our
experiments only one of 26 seeds which were released into the air at wind speeds
averaging 7 km per hour and gusting to 22 km per hour became detached from its pappus.
Eighteen out of 20 seeds flew out of sight at average windspeeds of 15 km per hour and
gusting to 38. The pappus of one seed became entangled on vegetation and fluttered in
the wind for 15 minutes. but did not separate from the seed (Peschken, unpublished).
Seedlings establish more readily where there is litter. and the moisture in low habitats
may help seedling establishment in pond and ditch margins (Stevens 1926). This is the
reason why S. arvensis is particularly serious in the irrigated area around Outlook,
Saskatchewan (W.J. King. 1980, personal communication). Perennial sow-thistle
thrives on low heavy soils and on lighter soils with a good moisture supply (Stevens
1924).
Fifty-nine shoots per m l reduced the yield of oats by 25% (Friesen & Shebeski 1960).
Shaskov el al. (1977) report reduction in wheat yield by 4.5% to 27% in Kazakh SSR. at
3-15 plants per mI. In Saskatchewan five shoots per ml reduced the yield of rapeseed by
12% and 10 shoots by 18'Yo (Peschken unpublished).
Control of perennial sow-thistle growing in grain is possible with herbicides, but
difficult in broad-leaved crops. Cultivation has to be repeated every three to four weeks
for at least one year to deplete root reserves (Derscheid & Parker 1972).
The annual sow-thistle. S. oleraceus L.. an annual or rarely biennial weed, reproduces
by seeds and has a strong tap root which may reach to a depth of 1 m and may
spread to 1.29 m (Kutschera 1960). It grows up to 2 m tall and it produces about 6000
seeds per plant (Salisbury 1961). It is a weed of cultivated fields, lawns, gardens. grainfields,
vineyards and occurs in all provinces of Canada. In Canada, it may harbor virus
diseases such as tobacco streak, alfalfa mosaic, cucumber mosaic. and the mycoplasma
disease. aster yellows (Gayed 1978. Holm el al. 1917. Smith 1972). The spiny annual
sow-thistle. S. asper (L.) Hill. is an erect annual or sometimes biennial weed up to 2 m
tall, with a strong taproot reaching down to 2 m (Kutschera 1960). Salisbury (1961)
estimates seed production to be about 18 000 per plant.
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