Scouting improves weed management

by Cheryl Wilen

While scouting for weeks in the nursery seems straightforward, there are actually a number of details that need to be considered. The goal is not only to reduce the impact of weeds on the crop but also to make your herbicide application or other methods of weed management more reliable. In order to do so, the following should be considered.

What to Look For

Preemergent herbicides are applied soon after plants are placed in the bed. Uniform coverage of the media is important to provide a “layer” of protection from weed seeds germinating and establishing. However, not all weeds may be controlled by your selected herbicide. About one week after canning, plants should be inspected for any weeds in the pots (fig. 1).

 Fig. 1. Looks for weeds that may come in with the liners. Photo: C. Wilen.  
If found, weeds should be removed by hand, trying not to disturb the media surface. You should also look over the bed as a whole and see if there are some sections or a pattern where there are more seedlings coming up.  This indicates that the herbicide was not applied uniformly and a reapplication may be warranted as soon as the pots are cleaned up. This is also a good time to look for weeds that are emerging in and around the bed. Weeds left uncontrolled will easily spread seeds into the bed and are also excellent places for snails to hide. Records should be kept regarding when the herbicide was applied and which weeds were not controlled.

How Long Do You Expect the Herbicide to Work?

Herbicide labels often have information about expected length of control. This should just be used as a general guide —  there is no replacement for verifying control in your situation. Irrigation amount and frequency, sunlight, organic matter, etc. will modify how long an herbicide can be expected to provide control. Additionally, some weeds may “break” or not be controlled before others (fig. 2). 

 Fig. 2. Weeds at various growth stages in pots. Photo: C. Wilen.   
At this stage, containers should be monitored at least every other week during cool periods of the year and weekly when warmer, as germination can be very fast as the soil temperature rises. Temperature and irrigation records are also helpful to better predict when the herbicide starts to lose efficacy. Your threshold for application of the next herbicide should be very low since almost all herbicides used in nurseries are preemergents and have limited effect on weeds once they emerge. To quantify weed emergence, it is better to use number of pots that have weeds (e.g., presence/absence) as opposed to weed cover or counts in the pots to base your decision on when to reapply. Any weeds that have emerged must be removed before the next herbicide application. 

Records should include when the herbicide was applied, the date of scouting, which weeds are emerging and any areas of higher weed pressure. The records you obtain during this period can be used to predict which weeds are not controlled with the selected herbicide and how long you can expect it to work in your nursery. Over time, you will use this information to decide what and when to apply even before you see breaks in activity. 

Time of Year

As noted above, scouting should be done more frequently when it’s warmer than in the cooler times of the year (fig.3).  

 Fig. 3. Scout for weeds at least weekly in warmer times of the year. Photo: C. Wilen.
However, because some weeds grow better under cool temperatures (think of bittercress and common groundsel) and some germinate and grow better in warm temperatures (such as spotted spurge and purslane), it’s important to be able to match your herbicide to the weeds you expect to be problems.  Monitoring areas surrounding the nursery or the bed can help provide you with information about when these weeds are germinating.  Modify your herbicide choice, if needed, to have better control of the weeds that will be growing in that period.

Surrounding Areas

Areas around the nursery and in and around the planting beds should be monitored for weed emergence as well. This not only helps you predict which weeds can be problematic, it also gives you a chance to control them with postemergence herbicides or mechanical methods before they flower and disseminate their seeds. It also helps to identify what the reasons are for the weeds growing in those areas. Are you applying too much water? Is drainage a problem? Are there holes in the nursery cloth? Are the weeds growing in spilled potting media? Correcting the problems that encourage weed growth will have an overall positive effect on weed management in the growing areas.

Cheryl Wilen is UC Cooperative Extension Area Integrated Pest Management Advisor, Los Angeles, Orange and San Diego Counties, and UC Statewide IPM Program; and Endemic and Invasive Pests and Diseases Initiative Leader.