Weed of the Week: dandelion

Like many plants we now call weeds, dandelion was brought to the new world as a useful plant. It has a history of being used as a medicinal herb and as food and feed for people and livestock.

It might be an annoyance in a lawn, but it causes significant yield reductions in grain and alfalfa fields.

The dandelion was relegated to ditches and pastures in the days of tillage, but reduced tillage and continuous cropping have created a nearly ideal environment for the pest. A few applications of peony herbicides or a half rate of glyphosate in the spring merely reduces the perennial weed’s competition.

The tap-rooted pest, which is formally known as taraxacum officinale, survives most shallow tillage, such as that of a vertical till unit or the trusty discer.

The single taproot is vulnerable to cultivation, provided it cuts below the growing point. In the days of cultivation, the weed was usually vanquished only after a sequential pass by the rod weeder, turning the weed’s root skyward.

Some research indicates that the weed has a limited effect on the yields of forage crops such as alfalfa and is highly palatable to livestock where alfalfa and grass pastures are grazed.

Dandelion can often injure alfalfa seed crops and is often the main reason for removal or rotation of those perennial stands.

It can be a significant problem in fall seeded cereals if not controlled with a full rate of glyphosate or a combination of glyphosate and a Group 2 or 4 product ahead of seeding.

Its significant stored-below-ground resources helps it tolerate over-grazing and will often be the last plant standing in abused pastures.

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

The use of 2,4-D and other products produced limited results, but could reduce stands of the weed when applied to grass pastures and hay land in the fall, along with summer grazing.

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

Early spring or late fall are the best times to spray dandelions, with fall applications being the most successful. The plants can be effectively controlled with herbicide only when they are small or actively growing.

Research at the University of Manitoba has shown that seed produced in the fall often doesn’t initiate many new plants in the spring. This is one of the keys to successful control of the plant.

New seedlings are generally from mid-season flowerings of mature plants that have escaped spring burnoff, 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 and as a result are less vulnerable to control.

In many cases, those applications might kill off the tops, leaving the root and a few leaves to start over with new growth in late August or September.

Research at Agriculture Canada and the University of Saskatchewan has shown that this is when the plant is most vulnerable: it has drawn on its resources and will be subject to the ravages of winter in only a month or two.

Applications of full rates of glyphosate to the plants’ leaves ahead of significant frost damage provide control of the weed and kill late germinated plants. Additions of other tank mixed broadleaf products can in some cases enhance control, according to research co-ordinated by the Saskatchewan Soil Conservation Association a decade ago.

Products that improve the spring effectiveness of glyphosate, such as DuPont’s Express, Dow’s Prepass and BASF’s Heat are useful in spring control strategies, but might control only the weeds’ tops, depending on the health of the plants.

Applications of 2,4-D are effective in pastures and alfalfa hay land before the first frost or when the legume is less than 10 centimetres tall in the spring.

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