Perspective Chapter: Management of Weeds in Organic Farming System – Special Focus on Organic Vegetable Farms of the USA

Abstract

Weed management is an important challenge in all farming systems, but it is more complicated in organic farming due to lack of chemical options. Concern about potential increases in weed populations without the use of herbicides has limited the uptake of organic farming even though organic products are gaining popularity among consumers. Different weed management strategies differ in their ability to control weeds, and often have unique agroecological implications. Broadleaf weeds, grasses and sedges are the three major classes of weeds impacting quality and yield in both horticultural and row crop systems. In an organic production system, the major concern is how and when to adopt the management practices to make the system productive and profitable. Generally, an integrated management approach using organic herbicides, mechanical, cultural, and biological practices is considered best for weed control in organic system. However, a systematic literature on different methods for weed management in organic farming system is not well developed. This book chapter outlines major weed problems in organic farming and various methods of weed control. The chapter is expected to disseminate information on strategies for sustainable weed management in organic farming systems.

1. Introduction

Organic farming is an agricultural system that operates on the principle of utilizing internal resources within the farm system and resorting to external inputs only when necessary. By incorporating modern technology selectively and avoiding environmentally risky elements, the practice aims to maintain harmony between farming practices and the natural environment [1]. Through techniques such as increased organic matter content, crop rotations, and conservation systems, organic farming fosters soil health, minimizes compaction, and mitigates erosion without the environmental hazards associated with excessive herbicide use in conventional methods [2]. In the global market year 2021, the world organic foods market size was valued at 140.46 billion dollars [3]. In the USA, organic food market size was estimated to be worth over $52 billion in 2020 and is projected to have a 10% growth during the forecast period 2023–2028 with increasing demand from consumers [3]. Organic farming in the United States was initiated approximately three decades ago after the establishment of the 1990 Organic Foods Production Act that contributed to increase in certified organic agricultural production areas in the country [4]. The organic industry in the United States is one of the fastest-expanding agricultural areas [5]. Consumer demand for organic products is a primary driver of organic vegetable production. Many people choose organic vegetables because they believe these are healthier and contain less pesticides than conventionally grown vegetables [6]. As a result, farmers and producers respond to these demands by increasing organic vegetable cultivation.

Common organically grown vegetables in the United States includes fresh tomatoes, onions, and squash with largest number of organic operations in the USA totaling 1847, 1487, and 1347 farms, respectively in the year 2014 [7]. Table 1 lists top five organic vegetables grown in the USA and their major weeds and Table 2 lists most common and most troublesome weeds in the organic system within the United States [8].

Vegetable crop		Primary weeds
Common name	Scientific name	Common name	Scientific name (Respectively)
Tomatoes	Solanum lycopersicum L.	Dodder, field bindweed, nightshade, yellow nutsedge	Cuscuta campestris, Convolvulus arvensis L., Solanum nigrum, Cyperus esculentus
Onions	Allium cepa	Lady’s thumb, common purslane, red root pigweed	Persicaria maculosa, Portulaca oleracea, Amaranthus retroflexus
Squash	Cucurbita spp	Bindweed and perennial weeds	Convolvulus arvensis L
Spinach	Spinacia oleracea	Common purslane, Common nettle, goosefoot, Shepherd’s purse	Portulaca oleracea, Urtica dioica, Chenopodium album, Capsella bursa-pastoris
Sweetcorn	Zea mays	Nightshade, chickweed, common Lambsquarters	Solanum nigrum, Stellaria media, Chenopodium album
Potatoes	Solanum tuberosum	Sandburs and foxtails	Cenchrus spp. and Setaria spp.

Table 1.

List of common organically grown vegetables in the U.S. and their primary weeds.

Sources: Economic Research Service, USDA 2016 [6].

United States Weeds
Most common	Most troublesome
Pigweed spp. (Genus: Amaranthus spp)	Pigweed spp. (Genus: Amaranthus spp)
Common lambsquarters (Chenopodium album)	Nutsedge spp. (Genus: Cyperus)
Nutsedge spp. (Genus: Cyperus spp)	Common lambsquarters (Chenopodium album)
Large Crabgrass (Digitaria sanguinalis)	Morningglory (Genus: Ipomoea)

Table 2.

Major weeds of organic vegetable production system in the U.S [6].

Pigweed spp. Included Palmer amaranth, tall water hemp, prostrate and tumble pigweed.

Nutsedge spp. Included annual sedge, purple and yellow nutsedge.

Morningglory spp. Included tall, ivyleaf and pitted morningglory.

Crabgrass spp. Included large, smooth, and southern crabgrass.

For many organic growers weed management remains one of the most resource-intensive management activities from the perspective of time, effort, input costs, potential impact on crop yield and quality, capital investment, and energy consumption. There are few times of the year when a grower is not actively working on some aspect of weed management [9]. Numerous grower surveys of organic agriculture demonstrate that weed control remains a major and enduring challenge [10, 11, 12, 13]. Because of that, the growers need to combine the crop, soil type, and weeds to be controlled and which method will be most efficient for them.

Typically, weeds are the major limitations to yield in organic farming system. Conversion from conventional farming practices to organic production results in lower yield with an estimate of about 20% lesser yield during the first years but improves over time through better soil fertility and weed management strategies adoption [14]. However, yield loss in organic production is also correlated with pests and diseases contributing to organic-conventional yield gap.

There are only a few approved crop protection products for organic production system that limit the control of weeds, pests, and diseases organically [15]. Moreover, studies are showing that adaptation of organic agriculture performs best under integrated pest management conditions and might close or reduce the yield gap between organic and conventional systems [16, 17]. For instance, in certain cases, organically managed areas gave higher yields compared to conventional systems under conditions like prolonged periods of low rainfall due to better water-holding capacity of organically farmed soils. Over the years, improvement of management strategies and crop varieties may close the yield gap of organic-conventional farming [18].

In organic farming, a variety of strategies are employed for effective weed management. Common tillage practices, such as hand weeding, hoeing, and the use of mechanized equipment like cultivators and rototillers, are widely utilized [19]. However, the success of tillage depends on the timing and frequency of operations, and prolonged, continuous tillage may have adverse effects on soil quality [20].

Another weed management approach involves the use of dead organic mulches, derived from natural materials and waste by-products. These mulches, compose of materials like straw or wood chips, gradually break down, enriching the soil with organic matter and fostering beneficial microorganisms. Acting as a protective layer, they mitigate erosion, control weeds, regulate soil temperature, and conserve moisture, promoting sustainable and ecologically sound soil management [21].

Cover crops and living mulches, comprising grasses, legumes, or other broadleaf plants, play a pivotal role in organic farming by enhancing soil organic matter, suppressing diseases and weeds, and providing habitat for beneficial insects [22]. However, careful selection is paramount to prevent competition with the main crop and the potential proliferation of pests. Strategic choices, tailored to the specific needs of the farming system optimize the positive impacts of these practices, contributing to improved soil health and overall sustainability.

Crop rotation, involving the systematic planting of different crops on the same land each season, is crucial in organic farming. It helps prevent soil diseases, controls insect pests and weeds, and fosters soil health [23]. Diversifying crops applies varied selection pressure on weeds, preventing the dominance of any species.

An innovative method for controlling weeds in organic farms involves allelopathy. In this approach, crops release allelochemicals to hinder the growth of nearby plants/weeds [24]. Suppression of neighboring plants occur through root exudation, leaching from plant tissues, and volatilization from aboveground parts. Factors such as soil hyphae and arbuscular mycorrhizal fungi enhance the movement of allelochemicals, making this method effective in weed control without harming the environment or increasing management costs [25].

For comprehensive and sustainable weed control in organic vegetable production growers can adopt a combination of these practices described above, falling under the concept of integrated weed management. However, the management decision adopted by an organic grower depends a lot on economic goal of the grower. In many cases, significant price premiums can substantially enhance gross returns in organic farms even when the crop yield is lower than conventional farms. The price difference in return per unit product between the organic and conventional farms influence the economic threshold or break-even points for weed control in organic farms. Consequently, organic growers are economically motivated to tolerate higher weed populations in their fields [26]. This chapter aims to review different organic weed control methods for a better understanding of weed control strategies and developing a good decision-making practice for improved organic vegetable production.

2. Methodology

In the current study, data were collected from secondary sources through bibliographic search and based on the keywords of interest to the authors in google scholar to perform an integrative review. Bibliographical survey is one of the best ways to initiate a study and search for similarities and differences among the articles of interest. The electronic compilation of information is a major advance for researchers as it democratizes access and provides frequent update [27]. The authors finalized different sections of the chapter by topics and worked on their assigned sections before the chapter compilation. During the search some keywords used were weed control, weed management, organic farming systems, non-chemical weed management, and integrated weed management.

3. Ecological approach to weed control in organic system

Organic agriculture is believed to have several health and environmental values such as higher biodiversity, improved soil and water quality per unit area, enhanced profitability, and higher food nutritional value [28]. However, this production system requires intensive management in terms of nutrients, especially nitrogen, and weed management [29, 30]. Nitrogen in natural soil is limited and the supply is dependent on cover crops, organic fertilizers, composts, and soil organic matter [29]. The supply of nitrogen and the control of perennial weeds are classified as the two most important yield-limiting factors in organic production [31]. With sufficient available nitrogen, plants can have competitive advantage as they would have faster growth greatly suppressing weed development [32]. Thus, apart from weed management efficient and effective supply of plant nutrients, especially nitrogen, should also be one of the major priorities of organic growers.

Weed management is a great challenge in organic production system because it eliminates the option of synthetic pesticides. Thus, high manual weeding is one of the widely used option for organic vegetable farming such as sweet corn, spinach, beets, onion, and carrots [33]. Weed control is considered one of the significant problems in organic farming and their effective management is imperative to avoid yield losses and ensure food security.

In the United States, most of the organic production is concentrated in the Western U.S. especially in California [34]. In California, the growers have adopted large-scale conventional farming and used a “zero seed rain” approach for weed control [33, 34]. The zero seed rain system ensures that weeds are controlled before flowering so reproduction and dispersal of weed seed from the parent plant is mitigated [35]. In one of the organic weed management studies, for the “zero seed rain” system, beds and paths were scouted, and appropriate control measures were followed every 2 weeks throughout the growing period with the goal of avoiding weed seed inputs to the seedbank [36]. The hierarchy of weed management tools starts with managing weed seed rain and the seedbank, Merfield in 2023, explained that weed seed rain” is when weeds set viable seed that replenishes the weed seedbank [37]. The importance of minimizing the weed seed rain, and therefore minimizing the size of the weed seedbank. The researchers claimed that using the zero seed rain approach reduced their weed seedbanks and ultimately minimized the weed management strategies in long run [36, 37].

In the organic farming system, both small and large-scale vegetable farmers have the same goal of investing less time and resources for managing the weed population. In response to this goal, growers should aim to control weed seedlings before they reach reproductive/seeding stage, while other focus should be on depleting the number of weed seeds in the soil [38]. In general, weed seeds are controlled during “critical period of weed control”. The critical period refers to the stage of the crop growth cycle in which weed infestation must be controlled to avoid significant yield losses [39]. However, there will be an increase in weed seedbanks and an increase in weed emergence in subsequent crops if weeds are only controlled during the critical period [36, 37]. If weeds are only controlled in this critical period, later-emerging weeds may proliferate leading to increase in weed seed bank over a period [31, 40]. Thus, for sustainable weed control in organic system, regular scouting, and appropriate control whenever necessary should be considered by the growers.

4. Current status of organic farms in the United States

The growth of the organic market and consumers demand has led to the adoption of organic farming by more growers in recent years. Most of the organic growers use combination of physical, cultural, biological practices for weed control. In a nutshell, integrated approaches include those that prevent, avoid, monitor and/or suppress all types of pests, plant diseases, weeds, nematodes [41, 42]. However, it is more difficult to estimate Integrated Pest Management adoption rates, given that there is no single, ongoing program or standardized approach, but rather periodic and often disparate assessments [43]. According to the USDA NASS [44], there are 48 certified organic farms in Oklahoma, this is a small number when compared to total farms in the US 17,445 or when compared to top 5 producing states that are led by California with 3061 farms, New York (1407 farms), Montana (206 farms), Wisconsin (1455 farms), and Texas (258 farms) (Figure 1). There has been a 5% increase in the number of organic farms in the U.S., from 2019 to 2022 [44].

Figure 1.

Number of certified organic farms in the USA. Adapted from Certified Organic Survey, NASS/USDA 2022 [33].

The top three states with the most area under organic production system includes California, followed by New York and Montana (Table 3). Likewise, the top three organic products in the USA are milk, broiler chickens and eggs, followed by apples, grains (for corn) and then the fruits and vegetables, as shown in Tables 4 and 5 respectively.

Top states: Certified organic acres
State	Thousand acres
California	814
New York	331
Montana	320
Wisconsin	245
Texas	241
Oregon	228
Idaho	216
Vermont	203
Colorado	191
Iowa	169

Table 3.

States with highest number of certified organic farms in the USA.

Adapted from NASS/USDA 2022 [33].

Commodity	$ million
Milk	1633
Broiler Chicken	1509
Eggs	1221
Apple	629
Corn for grain	424
Strawberries	336
Cattle	316
Grapes	309
Lettuce	276
Soybean	242

Table 4.

Major organic commodities in united states, dollar value.

Adapted from NASS/USDA 2022 [33].

Vegetable	Harvested
	Farms	Acres
Tomato	10	8
Broccoli	4	2
Green Onion	5	2
Lettuce	7	5
Onion	5	1
Potato	7	4
Squash	5	5
Sweet Potato	4	142
Other Vegetables	10	28

Table 5.

Major organic commodities in united states, production acreage.

Adapted from NASS/USDA 2022 [33].

Demand for organic produce and meat has been on the rise in the USA. For instance, from 2016 through 2019 organic product sales doubled in the state of Oklahoma [44]. The latest USDA data on organic crops show continued growth in the number of certified organic farms and acres nationwide [44]. Even though, some of the U.S. states have adopted organic farming more openly, some other states have lower number of organic farms. For example, Oklahoma has about 3.8% of both the farms and the agricultural land in the U.S., and nearly 2% of the agricultural sales. In the organic sector, though, Oklahoma has just 0.2% of the farms, and less than 0.02% of the sales, nationwide (Figure 2).

Figure 2.

Organic farms in Oklahoma, Source: NASS/ USDA 2022, Organic Survey.

5. Major weeds in organic vegetable production and their management

In vegetable cropping systems according to Baker and Mohler, 2015, organic vegetable farmers considered common purslane (Portulaca oleracea), common chickweed (Stellaria media), and hairy galinsoga (Galinsoga quadriradiata) as weed species that are most problematic for production [45]. In a survey by Jabbour et al. 2014, 23 organic farmers were interviewed in New England (19 of whom grew primarily vegetables), and they reported crabgrass (Digitaria spp.) and G. quadriradiata as their most problematic weed species. In that analysis of seedbanks, Digitaria spp. and G. quadriradiata were more abundant (mean 2456 and 840 seeds m⁻², respectively) than P. oleracea and S. media (mean 284 and 239 seeds m⁻², respectively) [38]. All these species tend to set seed rapidly after emerging. For example, G. quadriradiata can produce viable seeds within 35 to 40 days of plant emergence [46, 47].

Organic crop production has different ways of controlling weeds and some of them are discussed herewith. Crop rotation is among the most popular cultural practices for weed suppression in organic farms. This cultural technique reduces weed survival in the soil seed bank, suppresses weed seedling emergence, and minimizes seed production of weeds that escape the control method [48]. Effective crop rotations are the foundation of cropping systems [49]. Rotations are designed to contain crops with varying life cycles. Weeds that are common during the cool season, such as downy brome (Bromus tectorum), are easily handled throughout the growing season of warm season crops, eliminating downy brome seed production in the first cropping [48]. Crop selection for an organic grower is complicated by the requirement to consider soil fertility levels within the cropping sequence and to incorporate fertility-building times in the rotation. Variations in crop and weed responses to soil nutrient levels can also play an important part in weed management [30]. In vegetable production, it was traditionally recommended to include potato (Solanum tuberosum) in the rotation of crops to reduce weed problems before a less competitive crop was grown [10]. Vegetables having weak competitive abilities are onions (Allium cepa), carrots (Daucus carota), and leeks (Allium porrum) [50].

Another way of controlling weed density is through cover crops. The inclusion of cover crops in the cropping system helps produce an environmentally friendly vegetable production by providing agroecological services in the field like maintaining soil fertility and preventing soil erosion [30]. The most common cover crops used in organic production are legume crops that contribute to soil fertility through nitrogen fixation. Casini and Olivero reported that the allelopathic effect of cover crops like jackbean (Canavalia ensiformis) and velvetbean (Mucuna pruriens var. utilis) can suppress the growth of cogon grass (Imperata brasilinsis) [51]. Moreover, Grundy et al. noted that allelopathic ability may also play a role in reducing weed development, but it is the weed suppression caused by competition for growth factors that is the main effect of a cover crop [52].

Biological control is a form of “ecologically based pest management that uses some kind of organism (the natural enemies, for example: nematodes, plant pathogens, fishes, goats) to control pest species [53]. This kind of control in organic farm can be naturally occurring, or foreign agents classically introduced and established for ecosystem benefits. Other biological control options used in organic farms include pheromones used for monitoring pest populations and to disrupt mating, sterile insect releases, biopesticides which are pesticide formulations made from living organisms or the products of living organisms [54]. Additionally, bio stimulants which are biological products can also reduce the impact of pest activity because of a complex constituents or indirect mode of action that improves plant tolerance to abiotic stresses, and not because of the sole presence of a known plant protective compound acting directly on pests [55, 56, 57].

In summary, Integrated Pest Management helps the farmers in several ways to control, manage and avoid the multiplication of weed seeds. Even with IWM resulting in excellent results, it is necessary to define a hierarchy in this management to develop the best possible way in a most economical way.

To achieve the best weed management for the least cost, the focus of weed management must be on the components in the top half of the table (Table 6). The first five are considered to achieve most of the weed management, especially in vegetables systems. The crop rotation is the most effective means yet devised for keeping land free of weeds. No other method of weed control, mechanical, chemical, or biological, is so economical or so easily practiced as a well-arranged sequence of tillage and cropping [58]. Another alternative to control weeds in a sustainable agricultural system is using synthetic materials or plant residues/waste on the soil, also known as mulching [59]. Mulch film improves soil temperature and moisture, providing a suitable environment for enzymes produced by the microorganism community and improving soil productivity. The additional advantage of mulching is improved weed management by preventing weed seed germination and blocking emerging seedlings’ growth [60].

Ranking	Management
1	Minimizing weed seed rain and the seedbank
2	Rotations
3	Plants versus weeds
4	Pre-crop emergence weeding (tillage)
5	Post-crop emergence weeding
6	Crop, pasture, and livestock choice
7	Sowing, planting, and related techniques
8	Soil conditions
9	Hand weeding

Table 6.

Ranking of hierarchy in integrated weed management (IWM).

Adapted from Gage and Schwartz-Lazaro [58].

6. Unmanned aerial vehicles (UAVs) for organic weed management

In the realm of organic farming, effective weed management poses a unique challenge that requires sustainable and innovative solutions. UAVs have emerged as a transformative tool in this domain, providing organic farmers with precise, efficient, and environmentally conscious methods for weed control. With the advancement in UAV sensors and machine learning techniques in the recent years, use of remote sensing and drones for weed detection and management in organic farms have gained popularity among growers. The UAVs equipped with RGB, multispectral, thermal, and hyperspectral sensors offer organic farmers a powerful means to assess various vegetation parameters crucial for weed management at multiple temporal and spatial scales [61]. These sensors provide high spatial resolutions, enabling the creation of detailed weed maps [62]. Remote sensing technology facilitates a comprehensive analysis of spatial variations in weed populations, allowing for targeted interventions.

The affordability and enhanced flight efficiency of consumer drones make them particularly well-suited for organic farming practices. While advanced sensors such as hyperspectral and LiDAR can be valuable, cost-effective options like RGB cameras are practical for smaller-scale organic operations with limited budgets [63]. Drones play a pivotal role in enabling precise crop monitoring, biomass observation, and weed condition assessment in an organic farm setting. One of the key advantages of UAVs in organic farming lies in their ability to facilitate real-time Site-Specific Weed Management (SSWM) [64]. By employing two-dimensional and three-dimensional sensors coupled with efficient data processing, drones equipped with hyperspectral, multi-spectral, or thermal sensors significantly reduce operational time compared to traditional methods. This efficiency is crucial for organic farmers seeking sustainable alternatives to conventional herbicide spraying. SSWM allows farmers to customize weed management strategies based on specific weed populations within a field [65]. UAVs with precision spraying systems enable targeted weed management in organic agriculture, aligning with principles of minimizing blanket spraying. This approach reduces resource usage and environmental impact, as drones precisely apply site specific organic herbicides. Mattivi et al. presents a noteworthy case study for smart weed control in organic farms. This experimental research used UAVs to autonomously identify the presence of Sorghum halepense, Chenopodium album, and Amaranthus retroflexus in a corn field [66]. Zhang et al. designed a hyperspectral imaging system integrated with a micro-spray heated oil application system to address weed control in young tomato plants. The authors reported that the hyperspectral imaging system accurately recognized 95% of tomatoes, 94% of black nightshade, and 99% of pigweed [67].

The fundamental approach to weed discrimination using UAVs involves identifying spectral regions or vegetation indices maximizing the distinctions between weed and crop plants based on reflectance values obtained from aerial images [68]. Environmental conditions, textural phenotype, and spectral signatures of weeds influence detection and identification. Spectral signatures, representing chemical content in leaves or plants, play a crucial role in evaluating weed identification capacity [69]. For instance, Feyaerts and Van Gool employed an UAV equipped with an RGB sensor and utilized RGB values to calculate a normalized difference vegetation index, enabling the differentiation between crops, such as sugar beets, and weeds [70]. In Perez et al. study, images of cereal fields were analyzed to compute the Normalized Difference Index (NDI) and leaf shape, subsequently employed by Bayesian rule classification and k-Nearest Neighbor algorithms to distinguish cereal plants from weeds [71]. Vrindts et al. took a different approach by using hyperspectral cameras to identify the hyperspectral signatures of crops (sugar beets and maize) and common weeds. They selected specific wavelengths proven significant for crop and weed identification, using these wavelengths as classifiers [72]. These examples suggest that advancement in sensors technologies and next-generation computer vision models for image processing can be exploited for the development of smart weed management techniques in future organic production system.

7. Conclusion

Weed management in organic farming necessitates a holistic and multifaceted approach, considering the diverse strategies available. Tillage practices, mulching, cover crops, crop rotation, and innovative techniques such as allelopathy and biological weed control collectively contribute to sustainable weed control while promoting soil health and biodiversity. It is crucial for farmers to balance the benefits of these practices with potential challenges, ensuring a tailored and integrated weed management plan. Furthermore, the integration of modern technology, such as Unmanned Aerial Vehicles (UAVs), can provide valuable insights into crop health and weed distribution, facilitating precision farming, and enhancing the efficiency of weed management strategies. The ongoing exploration of advanced tools and techniques alongside traditional weed control practices is essential for developing resilient and environmentally friendly weed management systems in organic agriculture.