What is the best timing to control perennial weeds?

Now is not the best time to control many perennial weeds in your fields. You are likely only going to get top growth control because all of the energy that the plant is producing is sent to the leaves and upper plant parts, not to the roots. Even after systemic herbicides are applied, the roots tend to survive and are still able to reproduce. An exception to this is milkweed at early bud and field bindweed at 10% bloom.

Late summer and early fall are definitely the best time to control perennial weeds. Typically, the application window runs from early September through October depending on what weeds are targeted. During this time the plant begins moving carbohydrates produced by photosynthesis to the roots for storage instead of to upper plant parts for continued growth. If herbicides are applied at this time, the chemicals are transported to the roots along with the carbohydrates, killing the entire plant instead of just the parts above the soil surface.

Many winter annual weeds, like field pennycress, shepherd's-purse, chickweed and henbit, should also be targeted for control in the fall. Winter annuals germinate in the fall, grow into a small rosette then overwinter and resume growth in early spring. With all winter annual, biennial and perennial species, adequate leaf tissue must be present and should be reasonably healthy to absorb the herbicide.

The most common herbicides used for broad-spectrum control of many weeds in the fall is glyphosate for grasses and broadleaves and 2,4-D or dicamba for broadleaves. A combination of these products may be the best solution for a mixture of different weeds. Favourable air temperatures should be a consideration immediately before, during, and after application. In general, the warmer the better, with daytime high temperatures at a minimum of 12ºC. Cold nights and cool, cloudy days will reduce and slow the effectiveness of the applications. The more active the weeds are growing, the better the herbicide performance.

Table 1. Effectiveness of herbicides on perennial weeds
Herbicide active ingredient Canada thistle Field bindweed Perennial sowthistle Quackgrass Yellow nutsedge
2,4-D
F/G
G
F
N
N
aminopyralid
G
F
G
N
N
clopyralid
G
N
F
N
N
clopyralid + 2,4-D
G
P
F
N
N
clopyralid + MCPA
G
P
F
N
N
glyphosate
G
G
G
G
F
picloram
G
G
G
N
P
s-metolachlor + benoxacor
N
N
N
N
G
MCPA
F
G
F
N
N

Note: G = good; F = fair; P = poor; N = No Control. Effectiveness ratings apply if herbicide is used according to label recommendations as to rate, time of application, etc., and favorable temperature and moisture conditions prevail. Degree of control is often the result of repeated applictions.

Note: This is not a complete list of all herbicide active ingredients that are registered to control the perennial weeds listed.

References


For more information:
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