Weed Wisdom

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by Dr. Bob Blackshaw

Cleavers is considered to be a weed of the Black Soil Zone of the Canadian Prairies but this may be changing as there are reports of it moving southward. Spotting early infestations in our region will be critical for its management.cleavers

There are two species of ‘cleavers’ on the prairies; cleavers (Galium aparine) and false cleavers (Galium spurium). They are very similar in appearance, life cycle, and response to herbicides so they tend to get lumped together. Both have spiny stems that allow them to climb up through crop canopies but there are differences in flower color and seed size. Cleavers usually have white flowers while false cleavers tends to have greenish or yellowish flowers. Cleavers also have larger seed (2.8-4.0 mm) than false cleavers (1.5-2.8 mm). Both species can have a winter annual or summer annual life cycle. The optimum germination temperature is 10-20 C so it is common to have plants emerge in September-October and again in May-June. Large, overwintered cleavers are more difficult to control with spring-applied herbicides. Thus, both fall and spring control practices may be needed depending on infestation levels.

A 2010 Alberta weed survey found cleavers to be the 3rd most abundant weed in agricultural fields; only wild buckwheat and wild oat were more abundant. Many canola growers consider it to be their #1 weed problem. Cleavers compete vigorously with crops and reduce yield but they can be even more troublesome in terms of interfering with swathing/combining operations and causing reductions in crop quality. The good news is there are several herbicides available for cleavers control but Group 2 herbicide resistance is widespread and Group 4 resistance has been identified in some populations. Thus, planning herbicide rotations and mixtures utilizing multiple herbicide groups will be required. Rapid identification and control will help keep cleavers from becoming a widespread weed in the southern prairies.