Dandelions aren’t a problem only for fussy urban lawn growers.
The prolific and hard-to-control yellow flowered weed can also be a significant pest for farmers, especially in pastures, alfalfa and reduced tillage fields.
They can severely interfere with the establishment of legumes and also cause losses in annual and winter annual crops.
It’s a growing problem. Reports of dandelions in field crops in the northern Great Plains have increased from around seven percent of fields in the 1980s to around 15 percent since 2000.
The 2002 Manitoba weed survey found dandelions to be the ninth most abundant weed in the province, up from 13th place in the previous survey five years earlier.
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Now a recently completed research study at the University of Manitoba has shed new light on the subject, concluding that post-harvest herbicide application provides the most effective chemical control.
“Timing is everything and it pays to spray dandelions in the fall,” say study authors Kristin Hacault and Rene Van Acker of the university’s plant science department.
The study, carried out in southern Manitoba fields in 2003 and 2004, had two facets: to determine the emergence timing of dandelions from seed and rootstock and to compare the efficacy of preseeding versus post-harvest applications of glyphosate, alone or in combination with other products.
The study found that dandelion emergence from rootstock occurred early in the growing season before spring tillage and seeding, while emergence from seed happened later between mid-June and mid-July and was much more dense.
The researchers learned that pre-seed and in-crop herbicide application had no effect on seedlings, which are the main source of population spread. Preseed application did control over-wintered plants, but regrowth was a problem.
The most effective control was found to be fall-applied treatments of at least three-quarters of a litre of glyphosate plus florasulam, which is sold under the name Pre-Pass, or tribenuron, which is sold as Express Toss *’Go. Glyphosate alone at a lower rate of half a litre per acre provided reasonable control.
“Fall herbicide applications are more effective than spring applications for control of perennial weed species such as dandelion, probably because herbicide movement toward the root is enhanced as plants prepare to overwinter,” said Hacault and Van Acker.
Fall application also controls newly emerged seedlings and established plants.
Nick Underwood of Fairview, Alta., an agronomist with Reduced Tillage Linkages, said while seedlings will not compete with the annual crop in which they are growing, they are preparing to be a major competitor the following season.
“They will do a significant amount of growing after the August or September harvest and like most perennial weeds they become a significant competitor with annual crops after their first year.”
Saskatchewan Agriculture’s website includes commentary about dandelion control through cultural practices. Clipping is ineffective because the plant does not produce an aerial stem other than the flower stalk.
Dandelion seeds require light on the soil surface to germinate, so maintaining a competitive stand of forage will prevent dandelions from becoming established.
