In this final module, you’ll learn about agroecological methods of weed management. According to the Cooperative Extension System, organic farmers consider weeds “their most serious barrier to successful organic production, and effective organic weed control a top research priority (Schonbeck, 2013). Choices around weed management can easily dictate success or failure in a market garden. However, as you’ll read below from Altieri and Nicholls, agroecology is about recovering balance within agroecosystems and larger social systems. This module explores this perspective, and then gives you some very practical approaches around applying this type of weed management in your system. You’ll finish with a case-study about an innovative whole systems approach to weed management in the Philippines.
Excerpt 1: Ecologically based pest management: a key pathway to achieving agroecosystem health
by Altieri, M. and Nicholls, C
“Agroecology provides the guidelines to restore and enhance the resiliency, sustainability and health of agroecosystems. Biotic constraints stressing agroecosystems are understood as imbalances; therefore the goal of the agroecological treatments is to recover balance and enhance the “immunity”of the agricultural system. Agroecologists contend that the links between healthy soils and healthy plants is fundamental to ecologically based pest management. Also agroecologists promote biodiversification as the primary technique to evoke self-regulation and sustainability. However, ecological health and sustainability is not possible without preserving the cultural diversity that nurtures local agricultures. In addition to a proper balance of crops, soils, nutrients, arthropods, etc, stable production must take place in the context of social organization that protects the integrity of natural resources and encourages the harmonious interaction of humans, the agroecosystem and the overall environment.
The agroecosystem is productive and healthy when this balance and rich growing conditions prevail, and when crop plants remain resilient to tolerate stress and adversity. Occasional disturbances can be overcome by vigorous agroecosystems, which are adaptable, and diverse enough to recover once the stress has passed (Altieri and Rosset, 1995). If the cause of disease, pests, soil degradation, etc. is understood as imbalance, the goal of agroecological treatment is to recover balance: the agroecosystems natural tendency toward repairing itself. This tendency is known in ecology as homeostasis, the maintenance of the system’s internal functions and defense to compensate for external stress factors. But achieving and maintaining such state of homeostasis requires a deep understanding of the nature of agroecosystems and the principles by which they function.”
Excerpt 2: Weed Biology for Weed Management
by Brown, M., J. Perez, and A. Miles | from Teaching organic farming and gardening: Resources for instructors, 3rd edition., 2015
Weed Prevention Mechanisms
I. Restrict Seed Dispersal
- Clean equipment when moving from one field to another to avoid transporting weed seeds from infested fields
Bucket of sand with mineral oil for cleaning tools
- Filter any irrigation water to avoid importing weed seeds
- Thoroughly compost all materials, especially imported animal manures to insure destruction of viable weed seeds. Aerobically composted manures and plant materials in which temperatures are sustained at 131 degrees for 15 or more days should destroy all viable weed seeds.
- Since weeds are a pioneer species, they thrive in a disturbed habitat, such as a frequently tilled soil. One way to prevent weed growth is to reduce tillage. Every square inch of your garden contains weed seeds, but only those in the top inch or two of soil get enough light to trigger germination. Digging and cultivating brings hidden weed seeds to the surface, so assume weed seeds are there ready to erupt, like ants from an upset anthill, every time you open a patch of ground.
- Do not allow weeds to form seed heads and/or perennial root structures in the cropping system. A single season of allowing weeds to set seed may create years of weed management problems. Annual preventative clean cultivation will exhaust the existing seed bank.
- Cultivation doesn’t do any good if weeds have already gone to seed- make sure hoeing or clipping happens before seeds set on the weeds.
II. Prevent Seed Germination
To germinate, seeds need sunlight and water. This applies to weed seeds as well. By limiting sunlight and water, you can prevent the germination of weed seeds.
- Keep plants covering the soil surface, by transplanting larger seedlings. By transplanting crop seedlings, these seedlings can get an edge over the weeds and outcompete them.
- Intercropping- by densely intercropping, light contact with the soil will be reduced, and there will be less germination.
Limiting Water- Water only the plants you want to grow.
- Drip tape can help you avoid wetting the entire soil surface. Reducing the soil surface area exposed to moisture will reduce the surface area of land able to support weed populations.
- You can pre-irrigate beds and lightly cultivate prior to planting to destroy newly germinated weeds, and doing this multiple time before planting a crop can exhaust a seed bank in known weedy areas.
- Irrigate deeply and infrequently to allow crops to establish deep roots, and to prevent surface wetting leading to weed outbreaks.
Weed growth in mid-summer under drip (left) and overhead sprinkler winter squash plots in 2003. Weed samples were collected in early July and plots were then thoroughly weeded.
III. Manage Weedy Areas
- Fallow periods can control problem perennial weeds.
- Weed suppressive cover crops can be used to exhaust the weed seed bank- examples of weed-suppressive cover crops:
a. Sudangrass (Sorghum bicolor): heat-loving summer cover crop quickly grows to 8 ft, shades other weedy plants. Prevents successful weed reproduction, exhausting seed bank.
b. Buckwheat (Fagopyrum esculentum)
c. Sesbania (Sesbania macrocarpa): Vigorous growth in hot summer areas, outcompetes and shades weeds
d. Annual ryegrass (Lolium multiflorum)
e. Perennial ryegrass (Lolium perenne): Dense growth and allelopathic chemicals suppress germination and growth of weedy seeds.
IV. Other Weed Prevention Techniques
- Keep weed cultivations shallow. This will avoid destroying soil structure, and avoid bringing up new weed seeds from lower soil horizons.
- Work soil at optimal soil moisture content (~50 % field capacity) to avoid compaction.
- Soil solarization uses 2ml clear plastic tarp over the soil during the warmest part of the year. Plastic is left on the soil for 4-6 weeks, and can be effective in controlling winter annuals, but can be cost-prohibitive.
- Flame weeders can be used on beds of slow-germinating crops such as garlic and carrots after irrigation and before crop emergence to kill newly germinated broadleaf weeds.
- Dark plastic mulches can suppress weeds while retaining moisture and keeping soil warm
- Organic mulches such as straw and sawdust can maintain temperature and moisture while preventing weed growth
- Living mulches: Intercropping with a cover crop in the main season crop can prevent erosion and limit weed growth, especially in no-till systems, but can often result in increased water usage
Excerpt 3: “Allelopathy and Weeds”
by Liebman | from Weed Control, 2000
Allelopathy: Allelopathic effects of several non-leguminous cover crops, such as rye (Secale cereale L.) and certain crucifer species, have been studied intensively. Although rye can suppress weeds by reducing light and nutrient availability and altering soil physical conditions, significant weed suppression under field conditions has been attributed to the allelochemicals it releases (Weston,1996), including b-phenyllactic acid and b-hydroxybutyric acid (Shilling et al., 1986) and various benzoxazolinone compounds (Barnes & Putnam, 1987). Glucosinolate compounds contained within crucifer cover crops can also contribute to weed management (Vaughn & Boydston, 1997). Boydston & Hang (1995) found that residues of Brassica napus incorporated into field soil before planting potato reduced weed density by 73±85% and reduced weed biomass by 50±96%. Al-Khatib et al. (1997) reported that soil incorporation of B. napus residues also reduced weed density and biomass in a pea (Pisum sativum L.) production system.
Three factors appear to make it possible to use allelopathy to suppress weeds but not crops. First, the toxicity of plant residues can decline substantially after several weeks of decomposition (Dabney et al., 1996). Waiting several weeks between residue incorporation and seeding a sensitive crop may increase crop safety, while reducing weed establishment before planting. To prevent subsequent stimulation of weed emergence by secondary tillage, this method may require the foregoing of soil disturbance after residue incorporation. Secondly, ridge-tillage equipment and other specialized machinery can be used to clear allelopathic residues from bands where crops seeds are being sown (Exner et al., 1996). Finally, small-seeded weed and crop species appear to be especially susceptible to allelochemicals, whereas large-seeded species appear to be relatively insensitive (Putnam & DeFrank, 1983). Transplanting seedlings of small-seeded crops may reduce susceptibility to allelochemicals.
Excerpt 4: “Wild Greens in Integrated Duck/Rice Farming Systems in China”
A great example of a balanced, diverse agroecosystem is the integrated duck/rice farming systems in China. The Rice/Duck farming systems cycles nutrients, keeps pests populations down, and provides a diversified income and diet for farmers. Farmers can harvest the rice, wild greens, duck eggs and meat from the system, providing a complete diet from a single field. “Weeds” are not a problem in these systems- wild greens are harvested by the farmer and consumed as part of a balanced diet, and other are eaten by the ducks.
This woman-powered integrated rice-duck farming is in Dimataling, Zamboanga del Sur, Philippines.
“The integrated duck and rice farming system technology is about growing rice and ducks together in an irrigated paddy field. The paddling movement of the ducklings/ducks stimulates the rice plants to produce massive tillers. Duck manure fertilizes the soil and eliminates the need for synthetic fertilizers. The ducks also eat the harmful insects and weeds, thus eliminating the need for pesticides and herbicides.” -Sef Alba Carandang of Women in Agriculture – Zamboanga del Sur.
Film 1: “Rice-duck farming”
Schonbeck, Mark. “An Ecological Understanding of Weeds.” EXtension. Cooperative Extension System, 20 Aug. 2013. Web. 01 June 2017.