How long do herbicides stay active against weeds in the zone of effectiveness, that is the top six inches of soil? Weed control by herbicide application or tillage/hilling must last until seven to eight weeks after emergence. At that time, the canopy should be closed sufficiently to exclude light from the soil surface thereby inhibiting weed establishment. It is critical that the crop canopy's health is maintained by proper fertilization, irrigation, and insect and disease control. Late season control of escaped weeds requires the use of post-emergence herbicides.

In Wisconsin trials (L. Binning), soil samples were taken weekly to determine the persistence of several common pre-emergent herbicides and their duration of weed control. At the time herbicides loose their weed control ability, the canopy should be at its densest or at about seven weeks after emergence.

See Table "Herbicide Dissipation and Loss of Weed Control."

The soil concentration of metribuzin applied at 6 oz/acre dropped to 0.1 ppm also after nine weeks with a loss of weed control. Potato rows treated with metribuzin at 4.5 oz/acre were fairly free of weed problems after 10 weeks. The metribuzin did a good job for seven weeks and after then, the potatoes themselves took over weed control. Metolachlor applied at 20 oz/acre, the recommended rate, had a soil concentration of 1 ppm just after application. After eight weeks, the soil concentration dropped to 0.2 ppm at which time weed control was lost. The same type of study was done with pendimethalin. At 5 oz/acre, it gave a soil concentration of 1 ppm at application which dissipated to 0.2 ppm after nine weeks at which time weed control was ineffective. Linuron took five weeks to loose weed control.

Table. Herbicide dissipation and loss of weed control.

Common name
Amount per acre
Soil concentration at which weed control is lost
Time from application to loss of weed control
20 oz
0.2 ppm
8 weeks
6 oz
4.5 oz
0.1 ppm
0.1 ppm
9 weeks
7 weeks
5 oz
0.2 ppm
9 weeks
8 oz
0.1 ppm
5 weeks


Weed Management Program for Fall Harvest:

Weed management program



Integrated Weed Management Model


A healthy potato canopy closes rows in six to eight weeks after emerging from the ground and excludes light from the soil surface.

Weed control is necessary during the seven to eight weeks after plant emergence. Repeated cultivation up to small young plants controls weeds up to three weeks after emergence. Application of pre-emergence herbicides is needed to control weeds to canopy closure. Early post-emergence herbicide applications may also be applied.

Weed control during the middle of the season may be necessary due to a poor canopy, weed escapes or plant senescence (dying) at the completion of the tuber bulking period (ten to thirteen weeks after emergence).


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