Citrus integrated pest management (IPM) programs typically utilize a combination of control practices, like cultural, mechanical, and chemical, to minimize competitive effects of weeds on crop productivity. Weed management can be an expensive part of the total citrus production program, but resources invested here can provide significant economic returns.

Tree age and variety considerations

The smaller canopies of younger trees allow more light to reach orchard floor, promoting greater weed growth, which can lower yield by 23-33% compared to mature groves (Singh and Sharma 2008). Sensitivity of citrus trees to the various registered herbicides varies, depending partly on tree age. While mature trees tend to tolerate higher doses of herbicides, younger trees require greater attention to herbicide selection and potential damage from overspray or spray drift. Collars placed around the trunk of young trees are commonly used to reduce sunburn damage, but can also help protect young wood from damage caused by direct herbicide contact. Also, some herbicide products may only be labeled for non-bearing trees, meaning they can only be applied if a crop will not be harvested within 12 months following treatment. Consideration should also be given to product selection based upon citrus variety. For example, some products specify use only on oranges, thus prohibiting use on tangerines. Always refer to the pesticide label for these and other guidelines.

Influences of weeds

Weeds can impact cultural operations, tree growth, and yields by altering the spray pattern of low-volume irrigation systems, intercepting soil-applied chemicals (fertilizer and agricultural chemicals), reducing grove temperatures during freeze events, and interfering with pruning and harvest operations. The presence of weeds in a citrus grove can also affect insect populations. Weeds growing around tree trunks may also create a favorable environment for pathogens that infect the trunk and roots (Futch and Singh, 2010). Weed species compete with citrus trees in many ways and with varying intensities; management of more competitive weeds such as hairy fleabane, horseweed, johnsongrass, dallisgrass, and vetch should be prioritized. While some weeds (e.g., puncturevine, spiny cocklebur, stining nettle, bull thistle, and bristly oxtongue) may have low competitive effects on citrus trees, they can hinder labor operations and may also rank high for active management.

Cover crop benefits and complications

Vegetated orchard floors can accentuate frost hazard, often experiencing 3-5°F cooler ambient temperatures than do bare orchard floors, depending on vegetation height and atmospheric conditions (Steinmaus 2014). Alternatively, ground cover in the row middles can reduce soil erosion, reduce sand blasting during windy conditions and help retain nutrients. Ground covers can also be beneficial if they are less competitive than other weeds potentially present in the grove, and for erosion-prone situations such as on steep slopes or poorly structured soil. Cover crops may require additional management steps such as rotation to a different species or species mixture every few years to avoid pathogen buildup. Currently, there are no cover crops that will fit all situations and provide all possible benefits (Steinmaus 2014). Water requirements for vegetation regrowth after mowing can impact water availability within the grove, with grasses typically using more water than broadleaves post-mowing.

Weed ID is key

Proper weed identification is a critical in developing an effective management program. Weed species will vary with location, climate, season, soil type, previous site history, and current and past management programs. Weeds can be identified or grouped as: 1) broadleaf (including vines); 2) grass; or 3) sedge (WSSA 2015). Pay particular attention to key features like the shape and color of the leaves, stems, seed, seed head, plant size, root system, and especially flowers (if present). Weeds can be classified by their life cycle: annual, biennial, or perennial. Annual plants grow from seed, mature, and produce seed for the next generation all within a single year, presenting challenges for effective management (WSSA 2015). Annuals can be further divided into summer (sprout in spring, grow, mature, and produce seed and die before winter) or winter (sprout in the fall, grow, mature, produce seed and die before summer). Biennials have a two-year life cycle, growing from seed and developing a heavy root system the first year followed by seed production in the second year and then plant death. Perennials live more than two years with seed production occurring as early as the first year. For perennials species, vegetative structures such as rhizomes, tubers, and stolons pose management challenges, and are difficult to eradicate. If at all possible, manage perennial weeds before they begin producing these vegetative structures. A photo gallery of weeds, weed seedlings, and various weed anatomical features is available online through UC IPM at http://www.ipm.ucdavis.edu/PMG/weeds_intro.html. Additionally, an online identification tool is available through the UC Weed Research and Information Center at http://weedid.wisc.edu/ca/weedid.php.

Scouting

Scouting for weeds should be conducted in all areas in and near the grove , including tree rows, row middles, water furrows, ditch banks, fence rows, and adjacent perimeter locations. These sites may receive different cultural practices which can facilitate the persistence and spread of different weed species. Look for small isolated weed patches and manage them before they spread to other areas of the grove. Since weeds emerge all year long, schedule weed surveys throughout the year, especially after rains or soil disturbances. Scouting should occur even if weeds are not easily visible or appear to be dead. Re-growth from perennial plants is common; they may appear to be dead, but may be just temporally stunted. If weeds are properly identified in the seedling or vegetative stage, then proper control can be achieved through: 1) optimal treatment timing; 2) possible reduced herbicide application rate; and 3) reduced environmental impact from treatments. The weed species present will vary with season and location, because weeds are typically not distributed uniformly. For further information about characteristics used to identify, see the Weeds section of the UC IPM Citrus Pest Management Guidelines, http://www.ipm.ucdavis.edu/PMG. When scouting for weeds, records should be developed and recorded as to species abundance, location, and identity.

Weed management: cultural

Preventive programs are often overlooked, but are an important component of cultural practices and are cost-effective. Practices, such as sanitation, spot spraying, and/or hand removal of weed escapes before they produce new seed are examples of prevention. While preventive programs may not stop the spread of all weed species, these practices may slow the spread of undesirable species, thereby reducing long-term weed control costs.

Weed management: mechanical

Cultivation or tillage has been used in the past for many years in citrus production. Tillage is an effective method of controlling annual weeds effectively by severing weed stems and roots but is can be counterproductive for perennial grasses or sedges that can propagate vegetatively. Soil erosion concerns are cited as a reason why tillage use is decreasing as more groves are planted on raised berms. Also, citrus trees have a shallow fibrous root system and tillage increases risk of root and trunk damage. With the use of low-volume irrigation systems and closer in-row planting distances, tillage in both directions is no longer possible. Mechanical mowing is generally more expensive than tillage and can throw seed under the tree canopy, increasing weed pressure next to the tree trunk.

Weed management: chemical

Herbicides used in a citrus grove are generally divided into two groups: 1) soil-applied (preemergence) herbicides that should be applied to fairly clean soil surfaces prior to weed emergence, and 2) foliar-applied (postemergence) herbicides that are applied after weeds have emerged (Futch and M. Singh 2014). Preemergence herbicides are generally applied two to three times per year, so the maximum amount of herbicide is in the upper soil profile (0 to 2 inches) slightly before peak weed emergence. Herbicides applied too early, before weeds emerge, will not provide adequate weed control due to herbicide leaching or degradation on the soil surface or within the soil profile. Preemergence herbicides must be incorporated (mainly by rainfall or irrigation) and are usually broadcast on the entire orchard floor since growers do not know where weeds will emerge and to reduce risk of frost damage. Growers using drip irrigation or micro-sprinkler irrigation have a difficult time adequately incorporating preemergence herbicides, so usually try to treat prior to predicted rainfall (Rector et al. 1998). Soil type can influence herbicide selection and rate used. Many preemergence herbicides including Goal, Prowl, Surflan, Treflan, and Visor can be used on sandy soils without injuring citrus trees (McCloskey and Wright 1998). Tree age is also an important consideration when selecting which herbicide(s) to use.

Postemergence herbicides are used to control weeds that escape control by preemergence herbicides or mechanical cultivation. Postemergence herbicides can be systemic or contact in activity. Systemic herbicides are moved within the target plant, killing the foliage and root system of the treated plant. Contact herbicides are active only on those parts of the weeds the herbicide comes into contact with. Hence, adequate spray coverage of the weeds is more critical than with systemic materials. These herbicides are effective on small annual weeds and usually only suppress growth of perennials. It should be noted that the majority of organic herbicides are contact herbicides. Glyphosate is a widely used systemic postemergence herbicides used in citrus due to its efficacy on many weed species and relatively low cost (Sharma and Singh 2007), though continuous use over time will likely lead to the development of resistant populations in some weed species. To help reduce likelihood of herbicide resistance development, rotate and/or mix herbicides with different modes-of-action. For more information regarding herbicide usage in groves and for a list of herbicides registered for the use in California tree and vine crops, including citrus, can be found at the following websites:

http://ucanr.edu/sites/Citrus/KAC/Weed_Management/

http://wric.ucdavis.edu/PDFs/T&V_herbicide_registration_chart.pdf.

Herbicides may move through the soil to groundwater if used improperly. Factors influencing the rate of herbicide movement in the soil include, but are not limited to, irrigation practices, rainfall, herbicide solubility, soil type, and organic matter. Additional consideration should be given to products containing bromacil that are prohibited on deep, sandy, and ridge-type soils, and some product labels restrict the annual application of diuron within Highlands County. Please consult USDA-NRCS office or Web Soil Survey, http://websoilsurvey.sc.egov.usda.gov for information on soil type restrictions.

Always read the label of the product being used, and note that all registered pesticides are not necessarily listed on the UC IPM Online website (http://www. ipm.ucdavis.edu) or in this newsletter. Always check with the certifier to determine which products are organically acceptable.

To view the entire issue of Topics in Subtropics Newsletter, please visit: http://ceriverside.ucanr.edu/Custom_Program/Topics_in_Subtropics/

References

Futch, S.H., M. Singh, M. 2010. 2010 Florida citrus pest management guide, Florida citrus pest

management guide: Weeds, eds Rogers M.E., Dewdney M.M., Spann T.M. (Inst. Food Arg. Sci., Univ. of Florida, Gainesville), pp 125–137.

Futch S.H and M. Singh. 2014. 2014 Florida Citrus Pest Management Guide: Weeds.

Cooperative Extension Service, University of Florida, Institute of Food and Agricultural Sciences. Accessed: February 22, 2015.

McCloskey W. B. and G.C Wright. 1998. Applying Roundup to the Base of Lemon Tree

Canopies: Preliminary Effects on Leaves, Flowers, Fruitlets, and Yield. Citrus and Deciduous Fruit and Nut Research Report.

Rector, R. J., W. B McCloskey, G. C. Wright, and C. Sumner. 2003. Citrus Orchard Floor

Management 2001-2003: Comparison of a Disk,“Perfecta” Cultivator, and Weed Sensing Sprayer. Citrus Research Report.

Sharma S.D. and M. Singh. 2007. Effect of Timing and Rates of Application of Glyphosate and

Carfentrazone Herbicides and Their Mixtures on the Control of Some Broad- leaf Weeds. HortScience, Vol. 42, No. 5, 2007, pp. 1221-1226.

Singh M. and S.D. Sharma. 2008. Benefits of Triazine Herbicides and Other Weed Control

Technology in Citrus Management. The Triazine Herbicides—50 Years Revolutionizing Agriculture, Elsevier, San Diego, 199-209.

Steinmaus S.J. 2014. Production. Citrus Production Manual, Pp. 239.

Weed Science Society of America. 2015. Accessed: February 20, 2015. [http://wssa.net/].