Weed of the week: dandelions – May 8, 2014

 

Dandelions are a serious pest for modern prairie farmers, but European pioneers considered them to be valuable plants.

They were brought to the new world as a medicinal herb and were considered a resilient, perennial forage plant.

It is known mostly as a problem weed on urban lawns, but it can also cause big headaches for farmers. For example, alfalfa producers cite it as a main reason for rotating that crop.

Tillage used to keep dandelions in ditches and pastures, but reduced tillage and continuous cropping have allowed dandelions to flourish. Applying phenoxy herbicides or a half rate of glyphosate in the spring merely reduce dandelion’s competition.

Light tillage and discing usually fail to stop its growth, but its reliance on a single taproot makes it vulnerable to deeper cultivation. Rod weeders are especially good at destroying the weed, but they are becoming hard to find.

Research has found that the weed has a limited effect on the overall yield for pasture grazing because it provides highly palatable feed for cattle and sheep. The large tap root allows it to rely on stored resources even in situations of over-grazing. It will often be the last plant standing in abused pastures.

Alfalfa seed crops are another matter, and the weed remains a serious issue for growers.

Fall seeded cereals can suffer significant damage if not controlled with a full rate of glyphosate or a combination of glyphosate and Group 2 or Group 4 herbicides before seeding.

Chemical control of dandelions began in the 1940s with the introduction of phenoxy herbicides.

The use of 2,4-D and other products produces limited results, but it can hamper the ability of older plants to produce seed and eliminate new seedlings when applied to grass pastures and hayland in the fall and combined with summer grazing.

A little epinastic bending of a main stem might signal pesticide success in many plants, but with dandelions it often only shows that there will be a delay in the pest’s seed production.

The best times to treat dandelions is early in the spring and later in the fall. Fall applications are the most successful.

Herbicides effectively control the plants only when they are small or actively growing

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New seedlings are generated by mid-season flowerings of mature plants that have escaped spring burn-off destruction, a pre-harvest glyphosate regime or a pulse crop desiccation. The plants are in a semi-dormant state when in the dry, shaded conditions of a mature prairie crop, so herbicides have a limited effect at that time.

Agriculture Canada and University of Saskatchewan research has found that dandelions are most vulnerable when they are preparing for winter after using stored energy to get through the shady July and August season.

The effect of winter stress combined with illness created by herbicides can be enough to kill them.

A full rate of glyphosate before heavy frost damage usually controls the weed and kills any post-harvest germinated pests. Some tank-mixed broadleaf products can enhance control, according to research by the Saskatchewan Soil Conservation Association.

Increasing the effectiveness of spring applied glyphosate with products such as DuPont’s Express, Dow’s Prepass and BASF’s Heat has been shown to be effective, but the strategy works best when the surviving plants have already suffered from a fall application.

Spring applications suppress growth and reduce yield damage. They also reduce the number of seeds produced by plants that survive the treatments.

Applications of 2,4-D in pastures and alfalfa are effective in the fall before the first frost or in the spring when the alfalfa is still short.

For more on dandelion control, see The Western Producer column by Thom Weir in the April 24 edition or read it online at producer.com. For more Weeds of the Week visit our archive.

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