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Spiny Annual Sow Thistle

By: John Ippolito, PAg, Regional Crops Specialist, Kindersley

October 2017

Spiny annual sow thistle has increased greatly in frequency in the past 10 or more years. In the 2003 weed survey, the weed was ranked as number 34 and was found on 2.2 per cent of the fields that were surveyed. In the 2014-15 Saskatchewan weed survey spiny annual sow thistle was ranked as number six in frequency and was found on 24 per cent of fields surveyed. The frequency of it being detected in the field survey was similar in all crop types. 

There are a couple of reasons that may partially explain the rapid increase in frequency. It is not well controlled by group two herbicides and it is possible that some biotypes have resistance to this group of herbicides. The reduction in tillage has also favoured field scale infestations. In the absence of crop residue resulting from tillage the wind borne seeds would have accumulated on field edges and germinated in those locations. With reduced tillage we now find this weed dispersed throughout the entire field.

Spiny annual sow thistle is an annual plant that can also act as a winter annual. It grows from seed only with the seeds being dispersed by wind. The taproot helps to distinguish it from perennial sow thistle that will have extensive creeping roots. Mid and upper leaves of spiny annual sow thistle clasp the stem, have spiny toothed margins, are glossy and have rounded basal lobes close to the stem. Annual sow thistles are a similar species, but have deeply lobed leaves and are narrow at the base of the leaf.

Winter annual plants are likely to be controlled by pre-seeding herbicide applications containing glyphosate, because it is effective against this weed. The spring annuals may be controlled by early in-crop herbicide applications, but later flushes are possible. Control of these late flushes may be reliant on pre-seed or in-crop herbicides with some residual activity. In cereals where a group 4 herbicide such as dicamba can be used, it is recommended to provide the residual control of late flushes. In crops such as Liberty Link canola and glyphosate-tolerant crops, these secondary flushes may require a second application of herbicide.

Tillage is an effective control for this plant. Tillage also stimulates germination of the seeds and could be used as a practice to reduce the size of the seed bank for following crops.

Currently there are no recommended herbicides for this weed in peas and lentils where it may be more competitive. The best strategy at this time for these crops is to ensure good control prior to planting and maintain a healthy plant stand.

Actual yield losses from this weed are not well documented but spiny annual sow thistle is definitely a weed that producers may wish to watch for closely in the future.

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