Why scout for weeds?
While weeds are present in every orchard, there is variation in the weed species composition and density from orchard to orchard. Scouting for weeds is the basis for a good Integrated Weed Management (IWM) plan. Information gathered from weed scouting allows growers to:
- Evaluate the current year's weed control program
- Adjust control practices for the following year
- Discover weed stands and possible resistance before they spread throughout the orchard
- Select the best control option for species of concern, such as:
- Choosing appropriate management tactics for species present
- Identifying areas for possible spot treatments
- Selecting best cultivation method for weed stage
- Altering cultural practices to target weed life cycles
Post-harvest scouting offers an opportunity to evaluate the current year's orchard floor management plan, allowing you to see what weed species escaped management, where they are, and how severe the infestation may be. These are all valuable pieces of information when designing a management program to meet the specific needs of the orchard from year to year.
Keys to scouting
Most weed species are much more challenging to manage as they mature. Because of this, post-harvest scouting should start early and be repeated once more before the start of the season in order to catch weeds when they are young. Herbicide applications targeting mature weeds are often minimally effective, resulting in a less successful program and increased management costs. Three keys for successful scouting:
- Record weed infestations and use a map/GPS to show areas of escaped weeds. Below are links to UC IPM weed scouting templates for several common tree crops in California. Each template provides a chart with the most common weeds already listed and a designated area for mapping orchard weeds found while scouting.
- Accurately identifying weed species is crucial for effective management because herbicide recommendations, mechanical, and cultural control strategies vary depending on the species. While some species can look similar, they may have drastically different management requirements. For help identifying some of the weeds you may find visit: UC IPM- Weed Identification, the multi-state Weed ID tool, or other online or commercial weed ID guides
- Look out for different weeds in different management zones. A good place to start is by checking in the tree rows to evaluate the effectiveness of any previous herbicide applications. Check the ground cover in the row middles for any seedlings of annual and perennial weeds. Check orchard borders and at the ends of rows where new species are more likely to be initially introduced.
With the growing number of herbicide-resistant weeds in California orchards, control of escaped weeds can considerably reduce the long-term cost of an annual orchard floor management program. For example, spot treating two acres of glyphosate-resistant palmer amaranth with a tank mix of glufosinate and paraquat is much more affordable than trying to control it over an entire 50-acre block. There are currently thirty confirmed herbicide resistant species in California; for more information regarding the status of herbicide resistance in California visit the UC Weed Science post from January 2019, titled The Current Status of Herbicide Resistance in California. Remember to scout this winter so you can spot treat, rather than having an orchard full of herbicide-resistant weeds in the future.