Title: Ecological Understanding of Weeds

An ecological understanding of weeds is the foundation of an effective Bio-Holistic ChemFree weed management program that can make the difference between success and failure. If it weren’t for weeds, the world would have lost more topsoil than it has to date, and humankind might have suffered mass starvation by now. The plants we call weeds do a vital job in ecosystems: they quickly establish in, protect, and restore soil that has been left exposed by natural and human-caused disturbances. Watch any logged-over area and you will see precious topsoil washing away in each heavy rain until weeds cover the ground. Regions have their own characteristic guilds of pioneer plants that start the restoration process after timber harvest, natural disaster, or other disturbance has stripped the soil of vegetation. These pioneer plants initiate the process of ecological succession that, if left uninterrupted, will eventually restore the climax plant community native to the region: forest, savannah, prairie, chaparral, and so forth.

In agriculture and horticulture, humans replace the native climax vegetation with a suite of domesticated plant species chosen for their value as food, fodder, fibre, and fuel. Most agricultural systems severely reduce the diversity of the plant community and impose some form of repeated disturbance designed to maintain conditions favourable to growth of the chosen crop species. This disturbance inevitably elicits a "weed response" from nature, especially in annual cropping systems such as vegetables, in which the soil is frequently tilled or otherwise prepared for planting. Successful ChemFree weed control—managing the land’s natural weed response to cultivation—begins with an ecological understanding of weeds and their roles in the farm or garden ecosystem (Altieri, 1995; Sullivan, 2003).

What is a Weed?–A Working Definition

The word “weed” has been defined as a plant out of place, an unwanted plant, or a plant that is a pest in that it interferes with crop or livestock production. The term is typically applied to any plant species considered a pest. However, weed manuals and herbicide promotional literature also list as weeds species such as clovers (Trifolium spp.), orchardgrass (Dactylis glomerata), tall fescue (Festuca arundinacea), hairy vetch (Vicia villosa), and Jerusalem artichoke (Helianthus tuberosus)—plants that many farmers value as forage, cover, or food crops when grown in the right context. Indeed, “volunteer crops” such as buckwheat (Fagopyrum esculentum), winter rye (Secale cereale), corn (Zea mays), or even forage soybeans (Glycine max) can become weeds when they self-seed and emerge in another part of the crop rotation when they are no longer wanted.

From these examples, it becomes apparent that the term “weed” is, in part, a human value judgmentof certain plants or plant species as interfering with the desired use of a particular field or tract of land at a particular time. Even alfalfa (Medicago sativa), one of the most highly valued forage or hay crops, can be a weed in a vegetable garden.

Weeds are also, in large part, a human creation. Human activities can turn plant species into weeds in two ways:



Weeds are evidence of nature struggling to bring about ecological succession. When we clear native vegetation and establish annual crops, we are holding back natural plant succession, at great cost in weed control.... Modern crop agriculture is typified by large acreages of a single plant type, accompanied by a high percentage of bare ground—the ideal environment for annual weeds to prosper in the first stage of succession. Holding succession at bay in these unstable agroecosystems can be managed only with big investments in cultivation, mowing, herbicides, and fertilisers (Sullivan, 2003)

Where soil fertility is kept high through regular fertiliser inputs, those pioneer species adapted to high available nutrient levels become our main “weeds of cultivation.” Liebman et al. (2001) define agricultural weeds as “plants that are especially successful at colonising disturbed, but potentially productive sites, and at maintaining their abundance under conditions of repeated disturbance.”

In addition, exotic plant species can create unexpectedly severe weed problems when these plants grow amok in new habitats in the absence of their natural checks and balances.

In practice, any vegetation that comes up in a garden or field that the grower did not plant is often collectively called “weeds,” whether or not it is causing a problem. However, trying to eradicate all volunteer vegetation is hard on farm budgets, gardeners’ backs, soils, agroecosystems, fuel supplies, and the wider environment. Managing a weed species to protect the desired crops usually does not require exterminating that weed altogether. Based on these considerations, a weed might be defined as any plant not intentionally sown or propagated by the grower that requires management to prevent it from interfering with crop or livestock production. In this definition, a weed is a non-crop plant that can become a pest if not managed adequately. However its presence may not always be harmful and does not automatically warrant its immediate eradication. Weeds can perform vital ecosystem services such as protecting and restoring exposed or degraded soils. In addition, some weeds provide habitat for beneficial organisms, and thereby contribute significantly to natural and biological control of some insect pests. Certain weeds also make nutritious food or fodder. For example, a flush of common lambsquarters coming up in a fallow bed in late spring can take up nutrients that would otherwise leach, protect the soil surface from crusting and erosion, and provide highly nutritious greens for human or livestock consumption. Wild carrot (Daucus carota) flowering in field margins provides nectar and pollen for the adult phases of important natural enemies of vegetable pests. On the other hand, both lambsquarters and wild carrot can seriously interfere with vegetable production if they emerge in force with the crop, and allowing them to set seed in the field will intensify weed competition against future crops. Thus, managing weedy species like lambsquarters and wild carrot can consist of letting them grow and utilising them when and where their presence is mainly beneficial; removing them promptly when they threaten to interfere with the crop; and mowing or pulling before they can cast seed onto crop fields.

The goal of sustainable ChemFree weed management is to minimise the adverse impacts of weeds on crops, and sometimes to reap the benefits of volunteer vegetation when the benefits outweigh the costs of allowing it to remain.

“What are Weeds Doing in My Field?”

Weeds are Nature’s way of covering soil that has become exposed by fire, flood, landslide, clear-cutting, clean tillage, or other disturbance. Bare soil is hungry and at risk. The soil life, so vital to soil fertility, goes hungry because the normal influx of nourishing organic compounds from living plant roots has been cut off for the time being. The exposed soil surface is at risk of erosion by rain or wind, especially if root systems have also been removed or disrupted. Pioneer plants—what we call weeds—are those species that can rapidly cover bare soil and begin performing one or more of the following vital ecological functions:



It is important to understand that this flush of weed growth is, in effect, a healing response to land disturbance after either natural disaster or human activity leaves the soil vulnerable to erosion or degradation.

At the same time, agricultural weeds can hurt crop yields or increase costs of production by:




Cultivating land for annual crop production leaves the soil surface exposed for parts of the season, and thereby creates empty ecological niches. Open niches (bare, unoccupied soil; unutilised moisture and nutrients) may occur in time (between harvest of one crop and establishment of the next) and in space (between rows or beds until the crop has closed canopy), or among sparsely-growing crop plants like onion family (Allium spp.) vegetables. Weeds emerge, grow, and reproduce in these open niches—until they are stopped by cultivation, pulling, mowing, herbicides, or direct competition from crops.


Weed Management and the Global Carbon Cycle

In this time of growing awareness of the role of human-caused global climate change related to rising atmospheric carbon dioxide concentrations, farmers and agricultural professionals are beginning to talk about carbon sequestration as an added benefit of sustainable and ChemFree farming. Fossil fuel burning is the most widely-understood source of excess carbon dioxide in the air. Less well known are the contributions of deforestation and net losses of soil organic matter to the annual carbon dioxide burden.

All living vegetation, including weeds, absorbs carbon dioxide from the air and converts it into plant tissue. When the plant dies or sheds its leaves, part of this fixed carbon is returned to the atmosphere as carbon dioxide during decay, while part remains behind in the soil as stabilised organic matter, from which additional carbon dioxide is released only slowly. Farming systems that cause a net decrease in soil organic matter also entail a net release of carbon dioxide, whereas systems that achieve a net increase in soil organic matter sequester carbon into the soil.

What does this have to do with weed management? To begin with, a field of bare soil almost always suffers a net loss in organic matter and releases carbon dioxide, whereas a field covered with crops and/or weeds takes up carbon dioxide. Every tillage operation stimulates some degree of organic matter oxidation and carbon dioxide release. The less bare soil in the crop rotation, and the less tillage used for weed control, the less carbon dioxide the farm adds to the air. Weed-preventive measures that increase crop cover and build soil organic matter are beneficial to the global carbon balance. Research has shown that, with best weed and soil management practices, ChemFree and sustainable farms can make a net contribution toward addressing the global climate crisis.

Cover crops are especially effective in sequestering carbon. Fast-growing summer annuals like pearl millet, sorghum–sudangrass, forage soybean, and sunnhemp can generate two to four tons per acre of biomass over a two to three month growth cycle (Schonbeck, 2006; Clark, 2007). Since plant biomass is about 40–45% carbon (dry-weight basis), the growth of one acre of these cover crops can remove 25–35 lb carbon (or 90–130 lb carbon dioxide) from the atmosphere per day. This rivals or exceeds the carbon uptake of solid stands of the most aggressive weeds, and of course the cover crop is far easier to manage! Such a cover crop can also “pay for” the carbon dioxide released by mechanised seedbed preparation, planting, and management within a few days. Growing a grass–legume cover crop biculture to maturity (flowering/heading) maximises the humus formed when the cover crop is returned to the soil, and thus confers the greatest potential net sequestration.

The more a cropping system differs from a region’s native vegetation, the less diverse the cultivated crop community, and the less it keeps the soil surface covered throughout the year, the more it provides open niches for weeds and the greater the effort required for successful weed control (Sullivan, 2003). Thus, in the eastern United States, where natural plant communities consist of mixed hardwood forest, even the best managed vegetable field will have more weeds than a mature orchard with a grass–forbs understory. In the Midwest, a perennial grass–legume pasture or hayfield with several forage species mixed together will likely have fewer weeds than a corn–soybean rotation.

Permaculture, an alternative food production system that aims to mimic local natural plant communities and ecological processes, strives to keep all available niches occupied by useful vegetation year round, thus leaving minimal space for troublesome weeds (Mollison, 1988). Permaculture emphasises the use of perennial food-producing crops, such as fruit- and nut-bearing trees and shrubs, with desired annual vegetables intercropped with perennials or grown in relatively small patches. Agroforestry, the practice of integrating tree crops (fruits, nuts, timber, wildlife habitat) with pasture or hayfields, simulates the savanna ecosystems of subhumid climates. Traditional food gardening systems in Mexico and other developing countries may include up to 75 useful plant species (including some that American farmers might call “weeds”) growing in multitiered plant communities of trees, shrubs, grasses, vegetables, and herbs that provide a wide diversity of foods, fodder, and even medicines, and leave little room for harmful weeds to grow.

The permaculture approach may be impractical for farms whose main cash crops are annual vegetables that require a prepared seedbed to get established. However, the grower can close off some weed niches through preventive measures based on an ecological understanding of weed–crop interactions. A diverse crop rotation tends to grow fewer weeds than continuous corn or other monoculture. Cover crops, mulches, high density crop planting, reduced or strip till, intercropping, and relay cropping all reduce the “window of opportunity” for troublesome weeds (Yandoc et al., 2004).

Two permaculture concepts that are relevant to annual cropping systems are multifunctional components (i.e., components that perform several functions at once), and the use of multiple tactics to accomplish a particular objective. Cover crops that protect and restore soil, harbor desirable insects, and suppress weeds are an example of a multifunctional component. Managing a problem weed with cover crops and livestock grazing as well as timely cultivation is an example of multiple tactics.

Understanding what Makes Weeds Tick

Most weeds are pioneer plant species that have evolved various traits that adapt them to thrive and reproduce successfully in recently-disturbed habitats. Each kind of disturbance presents certain stresses (temperature extremes, exposed subsoil, poor retention of moisture, or—in the case of agriculture—the likelihood of frequent habitat disruptions) and certain opportunities (removal of shade and competing vegetation, release of soluble plant nutrients). Weeds are plants that have evolved mechanisms to cope with stresses and exploit the opportunities of disturbance including:



Few plant species possess all of the above traits, and most weeds display combinations of several traits that equip them to thrive in certain kinds of disturbed habitats. The “weeds of cultivation,” such as pigweeds, lambs quarters, and crabgrass, have evolved strategies to compete with annual crops (rapid growth, strong response to soluble nutrients), and maintain their populations through repeated disturbance (rapid maturation, prolific seed set). Weed floras that develop in frequently-tilled fields differ from those that develop in pastures or rangeland, while a no-till row cropping system elicits yet another weed flora. In addition, long-term use of certain herbicides such as atrazine (not an option for ChemFree farmers) has elicited yet another evolutionary response from the weed community: herbicide resistance.

ChemFree vegetable systems often entail frequent rotary tillage and shallow cultivation, and certain annual weeds tend to multiply even when a diverse rotation is practiced. Most of these “weeds of cultivation” are prolific producers of very small seeds (from 0.1 to 2 mg; for comparison, a lettuce seed is about 1 mg) that germinate from near the soil surface. These seeds often germinate in response to light, even a brief flash of light, which serves as a signal that the soil has been tilled or otherwise disturbed, and that competing vegetation has been cleared away. The seeds of many weed species also respond to wide fluctuations in soil temperature or moisture (indicating proximity to an exposed soil surface), a sudden increase in soil aeration (a signal of soil disturbance), or high levels of nitrate N and/or other soluble plant nutrients (which can result from the rapid mineralisation that often follows tillage). All of these responses help explain why tillage often leads to a flush of weed emergence.

One interesting and useful fact of weed ecology is the importance of light colour or light quality for many small weed seeds. Whereas direct sunlight (or even direct moonlight!) stimulates germination, the green light under a dense plant canopy—a signal that the soil is already occupied by other vegetation—can often inhibit germination and induce dormancy (Egley, 1986). A closed crop canopy can thus shut off weed seed germination as well as retard growth of those weeds that do emerge.

Farmers who moldboard-plow their fields regularly may encounter a different weed flora from those who rely mainly on the rototiller (Mohler, 2001b). These include large-seeded annual weeds whose seeds can emerge from a couple inches depth and survive in a dormant condition for many years when buried more deeply. Reduced till and no-till cropping systems may elicit an increase in weedy grasses that can emerge from seeds lying on the soil surface, and perennial weeds that develop from overwintering rhizomes and tubers. Grass seedlings and stout shoots from vegetative propagules can more easily emerge through the surface residues of no-till fields than newly-germinating broadleaf seedlings.

Overgrazing of pastures imposes a different set of stresses that elicit a different collection of weeds—those adapted to occupying bare or nearly-bare soil that has undergone removal of aboveground vegetation and compaction by livestock, but has not been loosened and aerated by tillage.

Many weeds have wide ranges of tolerances to soil conditions including nutrient levels, textures, and pH, and/or considerable tolerance to drought, temperature extremes, waterlogging, or repeated disturbance. These weeds tend to become troublesome to farmers over wide geographic ranges, sometimes around the world. Others are adapted to specific stresses, such as the extreme soil acidity or toxic excesses of trace elements characteristic of mining wastes (these pioneer plants may do more good than harm!), or frequent flooding, such as barnyard grass (Echinocloa crus-galli), which is a major weed of rice paddies.

Knowing what conditions tend to favour a particular weed species, and how that weed functions in relation to the plant community and ecosystem, can help the ChemFree grower identify and change management practices that may be giving that weed an advantage over crops. Some weeds respond dramatically to increases in soluble soil nutrient levels (especially N, P, and/or K), and their growth rates continue to rise with nutrient concentrations well beyond the “saturation point” at which growth rates of corn and other heavy-feeding crops level off. In addition, small seeded weeds require and respond to available nutrients immediately after emergence, while larger-seeded crops like corn utilise them somewhat later. Therefore, over fertilising, or fertilising too early in the season, can give the weeds a jump on the crop (Mohler).

Part of understanding weed ecology is knowing the weaknesses of particular weed species, which can become opportunities for more effective control. Because pioneer plant species are adapted to establish in empty niches, many of them do not tolerate competition early in their life cycle. Thus, a vigorous crop that rapidly closes its canopy can shut out many newly-emerging weeds.

Weed-Crop Interactions: Competition and Allelopathy

Weeds most commonly retard crop growth by competing directly for resources, including light, space, soil moisture and/or nutrients. Weed-crop competition can be likened to a race, the outcome of which can range from essentially no impact on crop yield (weed growth is minor compared to that of the crop) to complete crop loss (the weeds overwhelm the crop). Factors determining the competitive balance include weed density (numbers of weeds per unit area), density and planting pattern of the crop, growth rates and mature heights of weed and crop plants, and relative times of emergence of weed and crop (Liebman and Gallandt, 1997; Mohler, 2001a; Mohler 2001b).

Seedlings of small-seeded annual weeds tend to have a higher relative growth rate (how quickly they double in weight or in leaf area) than most crop seedlings. However, because these weed seedlings are so small when they emerge, their absolute growth rate (weight gain or leaf area increase per plant per day) is initially 10 to 100 times slower than seedlings of most crops (Mohler, undated slide presentation). Thus, fast-growing vegetables like winter squash, Irish potato, and sweet potato that rapidly form a closed canopy can outcompete many weed seedlings by appropriating most of the available light. Slower growing vegetables like onions, and especially small-seeded direct-sown crops like carrots and parsnip, are highly susceptible to weed competition. Many small-seeded crops, such as brassicas, lettuce, tomato, and pepper, are started in the greenhouse and transplanted to the field as vigorous “starts,” which gives them a substantial head start on the weeds.

Weeds that emerge before the crop have the most severe impacts on crops, which is why a “clean seedbed” is essential for planting most vegetables. Weeds that emerge with or shortly after the crop can substantially reduce yield unless controlled through cultivation or other means. This competition does not set in immediately after emergence, but at some later point in time (called the “maximum weed-infested period”). If the initial flush of weeds is removed by cultivation before the end of this period, it does not affect crop production. Additional weeds usually emerge after cultivation, and may still affect the crop through competition unless they are removed. However, the later the weeds emerge relative to the crop, the less their impact becomes. At some point (called the “minimum weed-free period”) newly emerging weeds will no longer significantly reduce the yield of the current crop. For most vegetable crops, this minimum weed free period is the first one-third to one-half of the crop’s growing cycle—about four to six weeks for vigorous summer vegetables like squash, cucumber, snap beans, and transplanted tomato.

The crop–weed interaction is not quite this simple for several reasons. First, the weed flora in most fields is composed of several or many weed species, which compete to different degrees and in different ways with the crop.

Second, weeds affect crops in other ways as well, such as releasing substances that are toxic to the crop, a phenomenon called allelopathy. Allelopathic interactions can be quite species-specific, and can go both ways (Putnam and Tang, 1986; Rice, 1995). Some weeds are known to release allelochemicals toxic to many crops. On the other hand, some cultivated plants, especially cover crops like winter rye, mustards (Brassica spp.), forage radish (Raphanus sativus), and sorghum–sudangrass (Sorghum bicolor X sudanense), can suppress many weeds through allelopathy (Haramoto and Gallandt, 2004; Putnam and DeFrank, 1983; Rice, 1995). Most of the active allelochemicals (which can be considered nature’s herbicides) are most toxic to newly-germinating seeds and seedlings. Larger plants, and shoots emerging from rootstocks, rhizomes, tubers, bulbs, or other underground storage and reproductive structures of perennial weeds, are far less susceptible.

Third, weeds and crops interact indirectly through their effects on soil microbial communities. One plant species may harbor particular soil bacteria and fungi that either favor or hinder other plants to varying degrees. For example, the majority of crops and some weeds form symbiotic associations with mycorrhizal fungi that greatly benefit the plant by enhancing nutrient and moisture uptake, and protecting against soil-borne diseases. Certain plant families, including the brassica (mustard–cabbage), amaranth (pigweed), buckwheat, chenopod (spinach, beet, do not benefit from mycorrhizae ("fungus–root" symbiosis), but are instead somewhat weakened by the fungal infection (Francis and Read, 1995; Vatovec et al., 2005). Thus, a soil rich in mycorrhizal fungi may give grain, legume, allium, and solanaceous crops (all “strong hosts” that benefit from mycorrhizae) a competitive edge over "non-host" weeds.

One "cutting edge” in ChemFree weed management research is to learn more about specific weed–crop–soil–microbe interactions, including but not limited to these mycorrhizal relationships, and their practical significance in field conditions (Yandoc et al., 2004). Such information can lead to more precise soil management and crop rotation strategies to give crops an edge over particular weed species.

Reflections on the “War on Weeds”

The fact that weeds cost farmers more than any other major pest category has understandably engendered a “war mentality” about weeds. Annual crop production almost inevitably gives rise to a “weed response” from any but the most “dead” soils. Due vigilance to keep weeds from restricting crop production is essential, ChemFree weed management can be approached as a “dance with nature” in which the farmer works with natural processes to develop better strategies to fulfil the ecological functions of pioneer vegetation. Examples of such strategies can include cover cropping, relay intercropping, and even weedy fallow managed to allow weeds to cover the soil and add organic matter, but not to set seed.

Declaring all-out war on weeds with either chemicals or steel can yield clean fields and good harvests—for a short time. It can also backfire in the overuse of herbicides and development of herbicide resistance on conventional farms, and in over cultivation and soil degradation on both ChemFree and conventional farms. In addition, agroecologist Miguel Altieri (1993) notes that eliminating all weeds from the farm ecosystem can destroy valuable habitat for natural enemies of insect pests, and thereby increase costs for insect pest control.

Stamping out weeds can even contribute to human malnutrition! In developing countries, replacing traditional polycultures that allow some volunteer plant growth with large scale monocultures and near-100% weed control has often undermined food security in rural communities (Altieri, 1993; Altieri, 1995). In addition to curtailing local food production in general, these industrial farming systems eliminate palatable, nutritious weeds from farmers’ fields, and can thus rob low-income communities of an important source of dietary vitamins and minerals. For example, the Tarahumara Indians of the Mexican Sierra depend on edible weeds for food before their traditional crops mature, and especially in the event of crop failure—a clear example of a food system better served by traditional weed management than by modern industrial weed control.

Traditional farmers in the lowland tropics of Tabasco, Mexico separate weeds into “good” and “bad” categories based on centuries of observation of their impacts on garden and farm ecosystems, including intensity of competition against crops, effects on soil tilth, harbouring of beneficial or pest insects, and their potential use for food or medicine. Farmers in this and many other parts of Mexico have developed management systems that permit moderate populations of the more desirable weed species to grow with their crops, while removing the more harmful species (Altieri, 1995).

When do Weeds Become a Problem?

As discussed above, weeds are a normal part of most agroecosystems and will always occur in a healthy, ChemFree farm or garden in which annual crops are grown. If you encounter a truly weed-free field, you will very likely be looking at a dead soil or a chemically-fumigated soil, on which ChemFree horticulture would be very difficult until soil life is restored. However, weed problems are not inevitable in ChemFree production. Costly weed problems develop when three conditions occur together:



On ChemFree farms, weed problems are minimised through an integrated combination of approaches that:



The severity of weed competition against crops is related to weed population density (numbers per unit area), timing of weed emergence relative to the crop, and proportion of resources (light, water, nutrients) consumed by the weeds. Therefore, ecological weed management aims to reduce weed density, delay weed emergence relative to the crop, limit production and dispersal of weed seed and other propagules, and maximise utilisation of available resources by crops (Liebman and Gallandt, 1997).

Weed management strategies that are based on an understanding of the ecology of agricultural weeds, particularly the major weed species present on the farm at hand, are most likely to yield the best results and to reduce the need for cultivation or herbicides. Such an ecological approach is essential for successful ChemFree vegetable production and measures to protect and improve soil quality are required.

Original Source: The Extension Foundation



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Date: 16 July 2014; 12:47:04 PM AEST

Author Name: Zheljana Peric
Author ID: zper12