The diverse mix resulted in more consistent ground cover in this study, and ground cover led to greater weed suppression. However, cover crop incidence, not the specific cover crop treatment, was the primary driver for suppression and other effects on weeds in this study. These results are consistent with previous experimental results of cover crop and weed competition that highlight the importance of cover crop abundance, not diversity, in weed competition . It is important to note that these studies primarily measured cover crop and weed biomass, while the present study came to a similar conclusion by measuring incidence. Additionally, these studies are in annual cropping systems. Agricultural systems, whether annual or perennial, are designed to support ample plant growth, and this resource-rich environment favors the asymmetric competition that is associated with abundant, fast-growing, cultivated plants. Maintaining biodiversity is a major challenge for agroecosystems, but this study continues to challenge the importance of functional diversity for achieving agronomic management goals like weedy vegetation management. Agricultural plant communities are not diverse compared to plant communities in non-agricultural systems. The cover crop mixes in this study represented a significant increase in orchard plant diversity, essentially doubling species richness in the mature orchards . Among treatments, weed species richness was highest in the young orchard , where the orchard floor was relatively unshaded and still populated with many weed species carried over from the previous pasture system. Weed community assembly may be affected by cover crops during the early stages of orchard development, commercial hydroponic systems but more research is needed to understand the effects of cover crop competition on filtering weed communities over timescales relevant for orchard production.

Cover crop species in this study were primarily selected for their relevance to almond management goals other than weed suppression. When considering multifunctionality, there are significant tradeoffs between agroecosystem services associated with various cover crop mixes. Managing cover crops for maximum weed suppression, and therefore maximum abundance, may detract from other orchard or cover crop management goals. For example, rye was included in the multifunctional mix in this study and is known to be an important component species for weed suppression , but persistent residues from high-biomass species like rye could negatively affect on-ground almond harvest several months after cover crop termination. However, weed-suppressing cover crops are also likely to contribute to other ecosystem services. Large and abundant cover crops are more effective in exploitative competition due to asymmetric resource acquisition, such as root competition for soil nutrients . The same mechanism that facilitates competition in this example also facilitates improved soil structure and increased soil organic matter. In another example, cover crop functional diversity could enhance competition through niche differentiation, as well as enhance pollination services by increasing floral resource diversity. Abundant single-species cover crops may be the best for outcompeting weeds, but a multifunctional cover crop mix may be designed to enhance other orchard management goals and protect against environmental uncertainty. Weed suppression may be essentially aprerequisite towards achieving an abundant, competitive, and multifunctional cover crop, but cover crop species and management practices should be selected with consideration for other ecosystem services that may complement orchard production. Balancing multifunctionality against singular management goals like weed management presents opportunities for integration of cover crops into conventional cropping systems.

Conventional weed management is chiefly a tool for reducing biodiversity, but cover crops can reduce weed infestation while promoting functional biodiversity. Uncertainty of outcomes remains a challenge for the practical adoption of cover crops by orchard growers; specific cover crop management practices should be planned alongside specific management goals . Particularly important is uncertainty related to the timing of winter rainfall. In the almond study system, cover crop planting is timed to coincide with the beginning of winter rains, which would reduce demand for supplemental irrigation of the cover crop. Weedy plants also depend on this rainfall for germination, and timely planting of the cover crop can align cover crop emergence with weed emergence. Cover crop mix diversity could be one way to hedge against increasingly uncertain winter rains. In this study, the diverse mix had a level of diversity that led to more stability, even if that stability did not consistently lead to enhanced weed suppression. Adjustments in cover crop phenology could be an important line of future research. Perennial cropping systems have significant temporal flexibility compared to annual cropping systems, where relatively few options exist for growing a cover crop during the cash crop growing season. Optimization of when a cover crop is planted and terminated in the orchard could improve weed suppression or other ecosystem services. Furthermore, the timing of these management actions could differ across orchard cropping systems, depending on climate or various needs of the main crop. Examples that could facilitate different cover crop management timings compared to those used in the present study could include cropping systems such as citrus, which is harvested during winter months, pistachio, which utilizes off-ground harvest during the fall, or apples, which are frequently grown in climates with colder winters. While this study implemented cover crops on a time scale relevant for adoption in contemporary orchards, further understanding of the cumulative impacts of cover cropping on the decades-long scale of orchard lifespans could further improve temporal arrangements of cover crops.

Sustainable orchard cropping systems require vegetation management programs that produce accessible orchard floors while minimizing management intensity. Orchard systems in California require significant upfront investment that expose orchard growers to heightened risks related to climate change, water scarcity, and land use change compared to more flexible annual cropping systems. Sustainable management systems could reduce risk for orchard growers who manage over 1 million ha of almonds D.A. Webb, walnuts , stone fruit , and similar orchard crops . Weed management is an important area for orchard sustainability improvements, given that vegetation and vegetation management practices affect many environmental quality parameters across the orchard agroecosystem, including factors such as herbicide use intensity, soil health, water quality, and air contaminants . Rather than seeking vegetation-free orchard floors, growers could potentially cultivate orchard floor vegetation that contributes to ancillary management goals and provides additional ecosystem services . Cover crops offer flexible management options for creating a functional orchard floor . As one cultural management practice within a suite of integrated pest management practices, cover crops can provide a framework for understanding the seasonality and phenology of weed life cycles while also promoting grower acceptance for some level of orchard vegetation . Typically, commercial orchards in California will have a zone of high intensity weed management in a strip centered on the tree row, often 25-50% of the orchard floor, with less intensive weed management in the remainder of the alley between rows . The high intensity tree strip is maintained to keep weeds from interfering with irrigation infrastructure and minimize non-crop water use in the irrigated area. In crops that are harvested from the orchard floor, which includes many tree nut crops, the alley is generally managed to be weed free ahead of crop harvest in the late summer, where heavy plant residues could impede sweepers and other harvest equipment. Alley management in the winter can vary with grower preferences, but cover crops could easily be implemented in this zone so long as cover crop residues do not affect crop harvest operations. California has mild, rainy winters, which are conducive to cover crop growth. Furthermore, tree nut and stone fruit crops are deciduous, and dormant tree canopies allow ample light to reach the orchard floor throughout the winter, until trees leaf out in mid-February for almonds and later in the spring for other species. Therefore, cover crops in California orchards could have minimal negative impacts on the cash crop if they consist of winter annual plants, since these species have a predictable life cycle that could usually begin with winter rains without supplemental irrigation and ends during hot, dry summers common in the Mediterranean climate of central California. With this context, cannabis racking systems winter annual cover crop species could be used to displace winter weeds in orchard alleys. Literature focusing on cover crops and weed management often centers on annual cropping systems, with cover crops growing in the off season between annual cash crops resulting in temporal separation . Heavy cover crop residues drive weed control by limiting the emergence of weed seedlings before and during cash crop emergence . In contrast, cover crops in orchard systems have spatial separation between cover crops and cash crops, which increases the importance of interference with concurrently growing weeds .

Spatial separation also creates flexibility by reducing restrictions on the cover crop growing season imposed by annual cash crop planting and harvest, and information about the phenology of plant competition could help optimize the management of an abundant, competitive cover crop . Finally, California orchards undergo dormant-season management like pruning and orchard sanitation, which could create tradeoffs between these management practices and a winter cover crop. For these reasons, weed-suppressing cover crops require additional research that informs practical management guidelines relevant to orchard systems in this California. Specific cover crop management recommendations could help growers balance the many functions of cover crops and support various ecosystem services and management goals. Specific management recommendations also support adoption by reducing knowledge barriers of this complex cultural management practice. Would-be adopters of orchard cover crops need to develop a plan that addresses many aspects cover crop establishment and management and acknowledges potential tradeoffs. To address this need, we developed specific questions about cover crop planting date, the phenology of crop-weed competition, and intensified cover crop practices. Research on intensified cover crop management could help us understand how agronomic practices including planting rate, fertilizer or herbicide inputs, cover crop species mixtures, and cover crop termination practices interact with many aspects of agroecosystem function . Likewise, varied cover crop planting date information helps us understand how cover crop establishment affects cover crop development and when weed competition occur relative to cover crop establishment and the onset of winter rains. Our objectives were to assess how different aspects of orchard cover crop management affect winter weed management. We evaluated how cover crop management system and planting date impacted cover crop and weed biomass. We also evaluated how cover crop planting date affects cover crop and weed emergence rates. Finally, we evaluated how cover crop management systems differentially affect summer weed emergence through different levels of cover crop residue. Together, these research questions can provide information about to what extent covercrops contribute to overall orchard floor vegetation management and which cover crop management practices have the largest effect on weed suppression.We initiated two different experiments to separately examine the effects of intensified cover crop management systems and cover crop planting date in nut orchards. These were small plot experiments in research orchards with commercially relevant cultural practices, including tree spacing, tree strip management, and irrigation. The ‘intensification experiment’ involved a range of cereal rye cover crop management intensities, from minimal management to an intensively-managed forage intercrop, planted in a walnut orchard. The ‘planting date experiment’ involved two different multi-species cover crop mixes each planted at early and late planting dates in an almond orchard. These experiments focused on plant population and community characteristics of orchard floor vegetation in the orchard alleys only. We used the different orchards as a study system but did not intensively monitor orchard crop performance or yield. Namely, almonds maintain a leaf canopy for a greater portion of each year, but the almond orchard in this study was younger with a smaller tree canopy compared to our older walnut orchard. However, the orchard floor environment is generally similar in almond and walnut cropping systems, and each has similar cultural factors including irrigation, alley and strip management, and winter pruning and pest management operations. For cover crops to be a feasible management strategy, they should work in a variety of orchard systems, conditions, and life cycle stages. Therefore, understanding how cover crops influence vegetation management across different orchards is a key aspect of this study. The intensification and planting date experiments were managed independently of one another, but there is a shared treatment to facilitate comparisons between the experiments. Intensification experiment.