Proper weed management is important for several reasons, but in general younger orchards are much more susceptible to the negative impacts of weed overgrowth. The full canopies of mature orchards limit the amount of sunlight reaching the orchard floor, which suppresses the growth of many weed species. Younger trees also have less extensive rooting systems, putting them into direct competition with weeds for water and nutrients. The presence of weeds provides habitat for insects, pathogens, and rodents, which can affect trees of all ages through direct damage to vascular tissue and foliage, and as disease vectors.
Rank growth of weeds also visually obscures irrigation infrastructure and signage, and in some cases even field roads. Weeds can also limit mobility of laborers during harvest, and snag on ladders, which is a safety hazard for workers. Frost damage is more likely in orchards with weed overgrowth because there is less transfer of heat from the sun to the soil surface and lowers the amount of heat that will radiate back into the air during the night. Further, weeds are a primary source of fuel for fires. Weed management can be broken down into four phases that correspond to different life stages of the citrus orchard: 1) best practices before the trees go in the ground, 2) best practices for young orchards, 3) best practices for established orchards, and 4) monitoring or scouting which is ongoing throughout the life of the orchard. Each of these phases or scenarios has a different set of challenges and somewhat distinct set of management opportunities and constraints.
Monitoring is critical in identifying how effective management practices are, is helpful in identifying what practices may be having unintended consequences (increasing a particularly problematic weed species for example), and provides clues for what can be done differently to achieve more satisfactory results. The first step in effective management is to know what weed species are present, their relative abundance, and where they're located (more on weed id later). This is important because different weed species have different tolerances and susceptibilities to herbicides and other control methods, and knowing where they appear in an orchard in a given year is a good predictor of where they may occur in the future. Keeping accurate records of weed species and locations and reviewing these records periodically will facilitate an adaptive approach when combined with knowledge of past management practices. Under irrigated conditions, weeds can and do germinate, grow, flower, and set seed all year long, but generally the biggest flushes of weed population growth are in the cooler months and warmer months. This is when they will be easiest to detect. However, it is worth noting that this is not necessarily the best time for management – all too often by the time they are noticed, they've reproduced, and in the case of annuals it's too late for control (annuals will die no matter if you kill them or not – what's important is to stop them from setting seed). The University of California Statewide Integrated Pest Management Program (UC IPM) offers some useful monitoring forms available online. These forms contain a section to sketch out the locations of weed species, infrastructure, and other landmarks within an orchard, and are already partially filled in with common weed species by season (summer vs. winter). Pay particular attention for the weeds that are most difficult to control: namely perennials (Johnsongrass, bermudagrass, nutsedge, bindweed, etc.) and species with known herbicide resistant populations in California (horseweed, fleabane, Palmer amaranth, junglerice, etc.). Also, limit monitoring to the orchard itself- weed propagule pressure can and will come from off-site, especially from roads and adjacent areas that aren't are not actively managed.
Weed management before planting is the best opportunity to reduce the overall weed seedbank, but more critically it is when the most effective management options are available to remove perennial weed species from the site. In the case of perennials this is also the most economically efficient opportunity for management as any perennial weeds left in place may incur management costs for the life of the orchard for ongoing suppression instead of a one-time cost for removal. This is one of the few times that cultivation won't have negative impacts on crop health and productivity. A useful strategy for perennials with underground storage structures is to repeatedly disc the field every few weeks to bring the roots to the surface and allow them to desiccate in the hot sun (this should take place during the summer months). For an integrated approach, supplement this strategy with a systemic postemergence herbicide, applied in the fall when the plants are preparing for dormancy. This is when weeds will have maximum translocation rates of photosynthate (carbohydrates) from the leaves to the roots. Because systemic postemergence herbicides follow this same path within the plants, this is when the weeds are most susceptible to complete kill from these herbicides. This can be repeated in the spring for regrowth and followed by discing 2-3 weeks later to again expose and dehydrate rhizomes. Once irrigation infrastructure is in place, another strategy is to irrigate to “flush” the weed seedbank. For annual species a burndown herbicide application can be used to kill emerging weeds – important to time this properly to catch the two main cohorts of annuals (cool- and warm-season germinators), so plan to repeat at least twice in one year. For perennials a systemic postemergence herbicide may work better, especially if they are not sprayed very soon after emergence when they are susceptible to contact herbicides. Repeat multiple times to greatly reduce weed seed load on site and minimize future management requirements. Additionally, properly timed preemergence applications can further deplete the weed seedbank by killing plants as they germinate in the spring or late summer/early fall. Any weeds that survive should be spot-treated with herbicide or hand rogued.
Once young trees are in the ground, it's important to minimize root disturbance, especially close to the trunks of the trees. This means more reliance on chemical control, but also keep in mind that the trees are also more susceptible to injury from herbicides at this stage from roots and foliage. The bark is also less lignified and herbicide is more easily taken up through this route than on older trees. Protect with wrappers and/or use a spray shield when applying herbicide to minimize potential damage from drift. Because roots are more sensitive to herbicide uptake it is important to select preemergence herbicide products that are labeled as safe for young trees. Postemergence herbicide options are 1) contact herbicides (includes all of the organic products), 2) systemic herbicides that are selective for grass species, and 3) glyphosate. Contact herbicides are only effective when applied to young plants below a certain height (this varies by product so refer to the label, but always better to catch them sooner than later), and are less effective on perennials than annuals. Grass herbicides, although systemic, are most effective on annual grasses, but will kill certain perennial grass seedlings below 4-6 inches in height.
Cultivation should be avoided whenever possible. Several studies demonstrate the negative impact of cultivation in citrus. These include but are not limited to damage to feeder roots, which impacts the trees ability to collect soil resources (nutrients, water, oxygen); increasing exposure to soil pathogens which can enter the vascular system of the tree through damaged tissue; and increased risk of soil erosion, especially in sloped orchards. Further, the additional passes with a tractor can contribute to compaction, especially in finer textured soils, which limits tree root growth and ability to uptake soil resources. Dust generated from cultivation practices can reduce the effectiveness of biocontrol of insects and mites, and also buries organic matter in the soil, providing a food source for certain insect pests. Cultivation also risks spreading root fragments of perennial weed species throughout the field and making weed problems worse.
A robust preemergence herbicide program can be helpful in managing orchard weeds, but preemergence herbicide efficacy and safety can be greatly modulated by several factors. Finer soil texture can bind up certain herbicides, making them less effective. Herbicides can quickly move beyond zone of seed germination in coarse soils, also making them less effective. Soil pH, especially in CA, can be high (alkaline) when calcium carbonate (aka caliche) is present in the soil, causing increased carryover or herbicide persistence and potential for injury. Knowledge of orchard soil characteristics and a careful review of the herbicide product label are critical in avoiding unintended crop injury. Many product labels will contain special precautions or rate modifications for specific soil characteristics or weather conditions. Herbicides are also broken down by sunlight on the soil surface if not immediately incorporated via rainfall or irrigation. Under prolonged moist soil conditions, certain herbicides are more quickly broken down by soil microbes, and are also moved out of the zone of seed germination. If permitted by the product label (limited by recommended application rate, max rate per year ,and pre harvest interval), sequential applications can be made to extend control.
Contact herbicides are not translocated within the weed's vascular system, they require more complete coverage as they only kill the parts they come in direct contact with. This is less of an issue for annuals (always attempt to target annuals before they set seed) but a drawback of contact herbicides is that they are much less effective at controlling perennial weeds unless applied to newly emerged seedlings. Suppression of perennials (not mortality) will likely require multiple applications of a contact herbicide. A positive aspect of contact herbicides is that their efficacy may be less affected by drought conditions than systemic herbicides.
There are few systemic postemergence herbicide products labeled for citrus. Systemic (translocating) postemergence herbicides don't require as complete coverage as contact herbicides, but they do require active growth and are highly impacted by drought. As always, young, succulent plants are more easily controlled than mature plants with waxy leaf cuticles that impede absorption. For many reasons, glyphosate usage may be restricted in some areas or situations. This is problematic because glyphosate is the only postemergence herbicide option for labeled for control of most perennial weed species in California citrus, especially broadleaves weeds (but many grass species as well). When glyphosate is not available, thorough weed management prior to planting and robust preemergence herbicide programs are even more critical to successful and effective weed management. Other postemergence weed management options include mowing (although this tends to select for more perennial weeds) and hand roguing which can be expensive when weed problems are extensive. Cover crops are another potential option, but more research is needed to help identify appropriate cover crop species, recommended cultural practices for their management in citrus orchards, and their potential positive and negative impacts on orchard productivity.
The first category of problematic weeds in citrus are those species that have known herbicide-resistant populations. This topic is covered more thoroughly in other UC publications, but for the purposes of citrus weed management the three main species of concern are horseweed (Erigeron canadensis), fleabane (Erigeron bonariensis), and Palmer amaranth (Amaranthus palmeri). These species are known to have populations in California with glyphosate resistance. To manage these species 1) use herbicides with multiple modes of action, 2) use full label rate of glyphosate (no “burndown” applications), 3) treat when small, and 4) avoid incomplete control of the targeted population and adjacent populations (ditchbanks, roadsides, etc.).
The second category of problematic weed species are those with tubers, rhizomes, and/or a long-lived seedbank such as nutsedge and Johnsongrass (Sorghum halepense). Yellow nutsedge (Cyperus esculentus) is found throughout California to 3300 ft in elevation. It produces round, smooth tubers about ½ inch in diameter that are usually found in the upper 6 inches of soil, but sometimes much deeper (particularly if buried during cultivation). Yellow nutsedge tubers are only found at the ends of the rhizomes. Purple nutsedge (Cyperus rotundus) is found mostly in the southern parts of California (Central Valley, South Coast, and Desert to 820 ft in elevation). Tubers are larger than those of yellow nutsedge (about 1 inch), oblong, rough, and scaly, and are chained together at the ends of the rhizomes. Nutsedge is best controlled by eliminating it from the site before planting the orchard (as described earlier), and past this time can be extremely difficult to manage. Young plants can be controlled with glyphosate, but once plants develop more than 5-6 leaves (often called the “FLS” or “five leaf stage”) translocation of carbohydrate from the leaves to the tubers is limited, meaning that only top-kill occurs. Further, yellow and purple nutsedge also do not typically reproduce from seed, meaning that preemergence herbicides are not very effective. Some sulfonyl urea preemergence herbicide products labeled for citrus are effective at controlling nutsedge when applied foliarly (postemergence), though repeated applications are likely necessary for more complete control. Nutsedge tends to dislike shade and is less of a problem once orchards are established and canopies are mature.
Johnsongrass is a rhizomatous perennial grass species with very long-lived seeds that can remain viable in the soil for more than 5 years. Like nutsedge, Johnsongrass is best to control before orchard is planted so that repeated tillage can be utilized that bring rhizomes to the soil surface where they will desiccate and die. This should be done in the summer when the soil is dry. Keep in mind that Johnsongrass can form plants from rhizomes as small as 1 inch long, so cultivation under moist conditions can exacerbate the problem by spreading rhizomes throughout the orchard. Otherwise glyphosate, specifically timed for immediately after plants finish flowering in August and September may be the best control option. This is when the plant will be moving photosynthate into the rhizomes and the herbicide will be more efficiently transported to this structure where it can kill the plant.
The UC Statewide IPM Program has several online resources available for integrated management of weeds in citrus. These include a photo gallery of common citrus weeds to aid in identification, a tutorial on how to identify weed species from common morphological traits (complete with illustrations), a chart of herbicide susceptibility by weed species, and a searchable gallery of herbicide injury photos.
Monitoring forms: http://ipm.ucanr.edu/PMG/C107/citrus-summerweeds.pdf
Weed ID and photo gallery: http://ipm.ucanr.edu/PMG/weeds_intro.html
Weed species herbicide susceptibility: http://ipm.ucanr.edu/PMG/r107700311.html
Herbicide symptoms: http://herbicidesymptoms.ipm.ucanr.edu
This article was first published in the Topics in Subtopics blog.