Answer: There are several tools one can consider for controlling weeds organically. No matter what tool(s) are used, it is important to recognize weeds as a symptom of land management. Individual weed species are indicators of specific nutrients lacking in the soil. For example, dandelions are an indicator species of low organic matter in the soil.
Organic weed management should consider both proactive and reactive strategies. Proactive approaches to weed management include mulching, crop rotations, cover crops, and the use of flame weeding in which weeds are “seared” with a flame weeder prior to planting. Flame weeding can also be considered a reactive approach as is hand or mechanical cultivation and the use of organic herbicides.
Due to the scale of your garden and the fact that you are targeting dandelions and thistle, both of which have taproots, using a spade to dig out the weeds may be a viable option. Goats will probably not have a big impact in controlling these weeds due to their taproots; however, pigs can do a great job plowing/rooting through the soil.
Another suggestion would be to cover the area with tarps – a process known as occultation. Occultation is the use of heavy tarps or black landscape fabric to cover moist soil. This process utilizes the tarps to first warm the soil in order to promote the germination of weed seeds in the soil that then die-back due to a lack of light. Removing the tarps after three to four weeks results in a bare soil that is ready for planting. While some growers use permeable black landscape fabric, others are finding great success in using heavy, UV-stabilized silage tarps. These tarps can often be reused for years but can be heavy and difficult to move around. Whatever the material may be, it is important that the cover is tightly anchored to the ground in order to prevent any light from getting through.
There are several naturally-based herbicides that are effective in managing weeds. As with any type of herbicide, the timing of the application is critical. And while all-natural herbicides are effective, the downfall is that they often require multiple applications for a sufficient kill rate. Here is a list of the active ingredients found in many common least-toxic herbicides. Please note that for use in organic certification, the active ingredients must be approved for use by the USDA National Organic Program (NOP).
• Corn gluten meal – pre-emergent herbicide that inhibits growth and root development. It is non-selective and must be applied just prior to weed-seed germination to be effective.
• Vinegar (Acetic Acid) – 5%-30% acetic acid as post-emergent herbicide. It is a post-emergent herbicide used to burn off top growth (often referred to as burndown). Acetic acid is most effective on small annual weeds and less effective on grasses than it is on broadleaf weeds. *Please note that vinegar can be corrosive to metal sprayer parts and plastic equipment is recommended.
• Herbicidal soaps are fast-acting, broad spectrum herbicides made from fatty acids. They are used as post-emergent and are most effective on annual broadleaf weeds and grasses.
• Clove oil is an active ingredient in post-emergent, non-selective organic herbicides. Research has shown that is can be as effective as acetic acid in controlling broadleaf weeds but at a lower application rate.
Once the weed situation is under control, it is highly recommend to plant cover crops, unless you are planning to plant your garden immediately. Cover crops are incorporated in to cropping systems for many reasons, one being for weed control. By covering the soil, cover crops reduce the opportunity for weeds to establish themselves. Cover crops species are selected for various attributes that they provide to the garden crops, as well as how they fit in to a crop rotation. For example, rye is a very common cover crop due to its allelopathic qualities. When rye is terminated and left on the soil surface, it releases allelopathic chemicals that inhibit the germination of small weed seeds.
I recommend that you read the ATTRA publication Sustainable Weed Management for Small and Medium-Scale Farms. This publication discusses weed-control several strategies, both proactive and reactive, as alternatives to conventional tillage systems. Options include mulching, competition, crop rotations, and low-toxicity control alternatives.
In addition, ATTRA produced a short video titled Organic Weed Removal Technique that you should find useful.
There are many great resources on cover crops, including the following:
Using Cover Crops and Green Manures in the Home Vegetable Garden, by Doug Higgins and Kristin Krokowski, UW-Extension Waukesha County, and Erin Silva, UW-Agronomy
Cover Crops in Wisconsin, by the University of Wisconsin-Extension
Midwest Cover Crops Field Guide, by Purdue University, College of Agriculture
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