Adapted from the article “Fall Weed Management Considerations in Prune Orchards” by Emily J. Symmes, UCCE Area IPM Advisor, Sacramento Valley and John Roncoroni, UCCE Perennial Crops Weed Science Advisor, Napa County in the October 2014 Sacramento Valley Prune News

Post-harvest weed surveys are essential for effective integrated weed management. Identifying the perennials, germinating winter annuals, and summer species that escaped the past season’s control tactics will inform the coming year’s control strategies. If you need to identify your weed population, online identification tools are here and here. Or, purchase the Weed Pest Identification and Monitoring Cards with descriptions and photographs of growth stages of 48 common California weeds which is handy for field scouting.

Surveys should be conducted after the first rain and germination of winter annuals. A fall weed survey form for prunes is available. Use this form to note the weeds presence, level of infestation, and locations (in or between tree rows). Map weed distribution in your orchard that catch emerging problems early (new species, possible resistance) on the form or with a GPS-based method. Early detection and intensive, localized control can prevent problems from becoming widespread. Weed monitoring, coupled with good record-keeping, can help you track the effectiveness of your management practices, detect early problems, avoid over- or undertreating, and ensure the right tactics, materials, and treatment timings are used.

Resistance management should be in the forefront when making weed control decisions. A good monitoring program can help identify early patterns of weed escapes and possible resistance. An integrated management approach needs to be adopted BEFORE detection of resistance in the population. Managing herbicide-resistance generally requires non-chemical methods (e.g., mechanical cultivation) coupled with appropriate and effective herbicides.

Incorporating multiple herbicide modes of action in the spray program is vital to managing resistance development. Including preemergence herbicides in the spray program reduces dependence on postemergence materials and increases the available modes of action. Tank mixes can increase the range of weeds controlled and the modes of action at work.

Even when tank mixing with multiple modes of action, chemistries must be rotated throughout the season to avoid applying the same selection pressures to weed populations generation after generation. Know your product’s active ingredient, its efficacy on the target weed(s), and its mode of action to ensure proper selection and rotation of chemicals. A searchable database of weed susceptibility to herbicides and this herbicide registration table will help with material selection.

Preemergence Herbicide Considerations:

• Must be applied prior to weed emergence or combined with a postemergence material.
• Efficacy is improved when applied to a clean soil surface. Remove leaves and debris prior to application.
• Require incorporation for activation within 21 days after application. Rainfall or irrigation (0.25-0.50 inches) works for most preemergence materials. Some materials require almost immediate mechanical incorporation for maximum efficacy. Refer to product label.
• Treatments should be applied in late winter (before mid-February) or split into two applications (first in fall and second in late winter). Many growers wait for the rainy season to apply because most herbicides are more effective when applied to moist soil. Weeds will have germinated and a postemergence herbicide will also be needed.
• Tank mixes of pre- and postemergence herbicides can be more effective than either one alone depending on the targeted weed spectrum.
• Preemergence use should be discontinued at least 1 year prior to orchard removal and replanting due to their long residual periods. Use cultivation or postemergence herbicides instead.

Postemergence Herbicide Considerations:

• Some growers apply multiple postemergence treatments rather than preemergence treatments or a combination of the two, particularly if orchard access is limited by conditions during preemergence treatment time.
• Postemergence materials are most effective in controlling small, actively growing weeds (less than 4 inches tall). Control of larger and/or moisture-stressed weeds is less effective with postemergence materials.
• Applying after irrigation can improve efficacy of many postemergence materials.
• Spray additives such as ammonium sulfate (AMS), spreaders and stickers, and citric acid can improve efficacy of many postemergence herbicides.
• Tank mix combinations of postemergence herbicides can control a broader spectrum of weeds than individual materials. Be aware of which weed species you are targeting and refer to herbicide susceptibility tables to choose the best material(s) for the job.
• Always be cautious of postemergence applications and the proximity to green crop tissue to avoid tree injury. Watch wind conditions and use hooded sprayers, drift-reducing nozzles, or low-volume applicators mounted to ATVs for good control and reduced risk of drift and herbicide damage.

Hairy fleabane populations with resistance to postemergence glyphosate and paraquat are present in parts of California, increasing the need for careful selection and use of herbicides with alternative modes of action. Even susceptible fleabane populations can be difficult to control with preemergence herbicides alone and postemergence herbicides are effective only when plants are small (fewer than 8 to 10 leaves).

Recent UC research has shown that various tank mixes combining one or more preemergence materials with postemergence herbicides were effective in controlling weed populations, including hairy fleabane. Refer to the table below for the pre- and postemergence products registered for use in prune and to compare modes of actions when selecting materials. Always check product labels of all materials included in tank mixes for specific mixing regulations and instructions.