Weed Management
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production WEED MANAGEMENT Kurt J. Hembree istachios, like other crops, are grown under annual and perennial weeds. Fields with a P a wide variety of soil types, management history of perennial weeds, like nutsedge, field methods, and irrigation systems. In order to bindweed, dallisgrass, johnsongrass, and maintain profitable production, growers should bermudagrass should be avoided, since they are make weed management an integral part of the difficult to eradicate once established. If one total program. A plan to control weeds should has to plant into fields already infested with be in place before planting trees and carried these or other troublesome weeds, one may be through the life of the orchard. Weed control is better served by dedicating efforts at especially critical during the first years of controlling these weeds before new trees are establishment, since heavy weed competition planted. Although this ties up land for a time, can reduce early tree vigor and productivity. it can help reduce weed control costs for many In addition to direct competition for water years after planting. Unfortunately, there are no and nutrients, certain weeds host several inexpensive or easy solutions available to important insects (like lygus, Green Plant Bug, control perennial weeds once established in and False Chinch Bug), diseases (like new tree plantings. So, starting off with Verticillium wilt), nematodes, and rodents. relatively clean fields can be a large advantage Where low-volume drip or micro-sprinkler for long-term effective weed management. irrigation is used, weeds such as yellow nutsedge, spotted spurge, sprangletop, and UNDERSTANDING THE WEEDS bermudagrass become prevalent, interfering PRESENT with water distribution and reducing irrigation Weed identification efficiency. Proper weed identification is essential for weed Since few herbicides are registered in management decisions to be effective. By pistachio orchards (particularly in bearing being familiar with the weeds present, you can fields), growers, PCAs, and field managers better understand how they grow and need to be familiar with all the tools available reproduce. Weeds have differing to select the most appropriate management characteristics, making them unique from each strategy and achieve effective control. These other. They have different life cycles (annual, tools include field selection, weed biennial, or perennial), have differing general identification, monitoring weeds, sanitation, plant structures (broadleaf, grass, or sedge), and mechanical and/or chemical control. grow flat or erect, reproduce by seed and/or other reproductive structures (rhizomes, tubers, GETTING OFF TO A CLEAN START stolons, etc.), and compete differently for Field selection valuable resources. Properly identifying weeds While many considerations go into deciding on is especially important when deciding on specific fields to plant pistachios, it is herbicide selection, rates to use, and important to realize that long-term weed application timing. There are numerous weed control strategies are directly influenced by site species commonly found infesting pistachio selection. In order to reduce the costs orchards. Some of the more important ones are associated with weed control, select fields that listed in Table 1. are known to be relatively free of troublesome 85 Table 1. Weeds commonly found in pistachio One such method of monitoring is as follows orchards and requires five simple steps: 1) Walk several locations of the field two or three times per Summer Winter Perennials Annuals Annuals year (winter, spring, and summer) and identify the weeds present, 2) Rate each of the weeds Barnyardgrass Annual Bermudagrass found on a 1-10 scale (1= very few present or bluegrass not a problem, 3= moderate infestation, and 5= Hairy fleabane Cheeseweed Dallisgrass Lambsquarters Chickweed Field bindweed very heavy infestation or dominates most other Large crabgrass Cudweed Johnsongrass weeds), 3) Make note of weeds escaping Marestail Filaree spp. Nutsedge, herbicide or mechanical treatment, new weeds, purple or a shift in the type of weeds, 4) Map or list Pigweed spp. Mustard spp. Nutsedge, particular areas that show new weeds, escapes, yellow Prickly lettuce Nettle or shifts in populations, and 5) Maintain these Sowthistle Shepherd’s purse records in a file or computer, where they can be Spotted spurge readily accessed and updated. An example of a Sprangletop weed monitoring record is shown in Table 2. Turkey mullein Table 2. Weed record for field monitoring Several publications are available that can help growers, PCAs, and managers to Date Field Crop accurately identify weeds. Some include: The Herbicide(s) Rate(s) Date(s) Growers Weed Identification Handbook, Weeds used used treated of The West, and Weeds of California. Various IPM manuals are also available at University of Weeds present Weed rating Notes California Cooperative Extension county 1. offices that include weed photos and 2. descriptions. Local farm advisors, specialists, 3. and consultants are excellent sources to help 4. identify weeds. If unsure of the weeds in your 5. field, collect samples and bring them to someone to be identified. Weed samples should It is desirable to become familiar with all be stored under cool conditions and transported the weeds in the field and to monitor each of in a sealed plastic bag to maintain sample them. If certain weeds are not controlled or not integrity. Computer programs are also available monitored, they can soon dominate a field. In on CD-ROM that have easy-to-follow some cases, these weeds may be resistant to instructions for the taxonomic identification of your current herbicide regime and require new weeds. Using a personal computer and a weed methods of control. To avoid this shift in program, many of the weeds found in weeds, it is necessary to monitor the field California can be identified in less than seven frequently and rotate with effective herbicides, or eight steps. Consult your local farm advisor mechanical, or other means. Accurate records for further information regarding the programs help to point out areas in the field where tough- available. to-control weeds (field bindweed, mullein, nutsedge, etc.) may pose a problem. By Maintaining accurate weed records routinely monitoring for weeds, you can alter Maintaining accurate, up-to-date weed records your herbicide program, based on changing is important to track the distribution of weeds weed patterns. A little time spent monitoring in the field. It also helps in choosing the most for weeds can help you select appropriate appropriate method(s) of control. The exact herbicides, rates, and other control options to method of collecting weed data is not set in reduce long-term costs. stone, and often varies from grower to grower. 86 Field sanitation weeds emerged next to the trees can be done Many of the troublesome annual weeds (like with spot applications of post-emergence hairy fleabane, groundsel, and marestail) are herbicides like glyphosate, paraquat, or disseminated by wind, often originating from sulfosate, or hand weeding during the season. outside the field. Typically they are growing Other disadvantages to disking include along field borders or nearby ditches and increased soil erosion on sloping land, soil canals. Controlling these weeds with compaction, dust, reduced water infiltration, herbicides, disking, mowing, and burning can and a general decline in productivity. help eliminate seed production. Many times More commonly, growers disk only the growers achieve excellent weed control within middles, leaving a four- to six-foot strip of soil the field, but fail to clean surrounding areas. undisturbed within the tree row. This facilitates Reducing this seed source by eliminating basin, flood, and low-volume irrigation. weeds before they set seed is a must to prevent Mechanical mulchers, equipped with tripping new invasions into the field. mechanisms (such as the Kimco®), can then be used to destroy weeds within the tree row as DEVELOPING A RELIABLE WEED long as trees are staked. Mulching at a shallow CONTROL PROGRAM depth (1 - 2 inches) will kill most seedling It would be ideal if one method of weed annual weeds. Weeds larger than three or four management could be used in all pistachio inches tall may already have well defined root orchards, but this is obviously not possible. systems and are not easily killed. Perennials There are many local conditions that influence will not be controlled and often require timely management decisions, including topography, post-emergence herbicide applications. To be soil structure and texture, permeability, type of effective on young annual weeds, several trips irrigation, availability of equipment and labor, through the field during the season are herbicide and cost, and accessibility to the required. Where low-volume irrigation is used, orchard during wet conditions. The specific tubing should be suspended high enough to method(s) of weed control will vary from site permit access by the equipment. As with to site, but usually include the combination of disking, mechanical damage to trees is a mechanical, cultural, and chemical control concern and should be avoided. options. Complete non-tillage is an option where weeds are maintained in the middles as a green Tillage vs. Non-tillage cover. This is an excellent way of reducing soil Tillage options are normally limited to the area erosion, especially on sloping soils. A flail between tree rows. In-row tillage can be mower can be used five to seven times per year practiced in new plantings when low-volume to help maintain the weeds in a low-growing irrigation tubing has not been laid out or has state. This also allows for better field access been suspended off the soil surface. In and reduced soil compaction. Mowing too established plantings, disking can be used to close to the soil surface will create dust and eliminate weeds growing between tree rows. should be avoided. A cover crop can also be The size of the weeds is usually not a concern, planted to compete with the weeds in the as long as the disk blades cut deep enough to middles.