Weed Monitoring

Weed monitoring is the first step in any site-specific weed management program. Site-specific weed management (SSWM) is a strategy in which weed control is varied within a field to match variations in location, density, and composition of the weed population. This concept is based on the fact that weed populations are often irregularly distributed within a field.

Most herbicides are only effective against certain weed species. Regular monitoring is used to determine if treatments are working. Weeds often grow in patches, so it may not be necessary to spray postemergent herbicides or till the entire field to control them. Spot treatment can save time and money while achieving good weed control.

The most accurate method of estimating the population of weeds is to count the number of plants in an area of known size in several places. A quadrant, which may be square or circular, should be used to make weed plant counts. The number and location of counts needed to estimate the population will vary depending on the distribution pattern.

Figure 2.6.Sampling and estimating weed density (photo: Lemic, D.)

The size of the quadrant depends on the weed density. Small quadrants (0.1m2) are sufficient for weed populations of more than 200 plants per square metre. This would equate to more than 20 plants per quadrant. For lower weed densities, increase the quadrant size (up to 1 m2) to allow counts between five and 50 plants per quadrant.

At least five quadrant counts should be made in at least four sections within a field, giving 20 counts for the area. The more counts conducted, the more accurate the assessment.

Record plant count for each weed species found. The plant count is an appropriate time to record various aspects of the weeds and the stand. Note whether the plants appear small and crippled or are infested with insects or disease. It is also necessary to make notes about other weeds present. The records should be able to be queried and show changes in weed density and spectrum over time. These records can be an early warning of an emerging problem.