Answer: I am pleased to provide you with information regarding weed management tools for a 4-acre planting of peonies, other than manual cultivation and mulch.
My recommendation for weed management in peonies is based on my own experience and visits to another local grower. I have only a few hundred plants and he has a few thousand, so these ideas may not work for you. I would suggest a perennial cover crop in the walkways, one that can be mowed and will not creep into the peony beds or rows. Your local Extension agent may be able to recommend an appropriate cover crop. I suspect your biggest concern is the weeds between the plants in the bed/row. Can you get low-cost or free organic mulching materials, such as chopped leaves from a landscape maintenance business? The time involved in spreading them would be your only cost. Leaves break down over time, so this task would need to be repeated, but they do increase the organic matter in the soil as they decompose.
Landscape fabric is another option worth considering. Although the initial cost may seem high, the material lasts for years. And by comparison, the cost of labor for hand weeding is even higher. Unlike plastic film mulch, landscape fabric is somewhat porous and rain or irrigation water can soak through it. You can purchase landscape fabric in rolls of various lengths and widths. Contact a horticultural supply company for more information if you are interested.
A University of Nebraska-Institute for Agriculture and Natural Resources publication on peonies says this about weed management:
Weeds can be removed by hand or by hoeing, but peonies have shallow feeder roots, so cultivating for weed control should be done with care. If mulches are used, 2 to 3 inches of an organic mulch (bark, wood chips, etc.) may be applied in early July. Mulching also will conserve moisture and maintain even soil temperatures.
You can find the entire publication at:
A University of California publication Integrated Pest Management for Landscape Professionals and Home Gardeners: Weed Management in Landscapes recommends:
Control perennial weeds before planting; use geotextiles where possible; use mulches with a preemergent herbicide; and supplement with hand-weeding.
For the complete publication, go to:
Another option that you might think about are herbicides that are approved for use in organic production.
Acetic acid in vinegar kills plant tissue by dissolving the cell membrane, which causes the plant to dry out. David Chinery, Cornell University Extension, tested the use of vinegar as an herbicide on different plant species and at different application timings. His study showed that acetic acid is a useful herbicide. Acetic acid at 5% concentration (as would be found on the supermarket shelf) provided only short-term control of most perennial weeds, but did effectively control crabgrass and plantain (Chinery, 2002). Three applications of acetic acid were seen to be much more effective than one application in most cases, suggesting that repeated applications may be necessary. The highest concentration of acetic acid (20%) and the commercial formulations provided some good control, but were not as effective as glyphosate.
Several vinegar-based herbicides have been developed, including St. Gabriel Labs’ BurnOut Weed and Grass Killer concentrate and Nature’s Glory Weed and Grass Killer concentrate (Sullivan, 2004). These two come as 25% concentrates with instructions to dilute them to 6.25% and use on non-crop, right-of-ways, and industrial lands. More dilute products that come ready-to-use include Fast-Acting BurnOut RTU, Nature’s Glory Weed and Grass Killer RTU, and Greenergy’s Blackberry and Brush Block, with acetic acid concentrations from 6.25% to 7%. The Greenergy product lists acetic acid as an inert ingredient and citric acid (at 7% concentration) as the active ingredient. SummerSet Products sells a vinegar-based herbicide called Alldown Green Chemistry Herbicide that has been approved for use in organic farming by the Organic Materials Review Institute (OMRI). Alldown contains vinegar, garlic, and citric acid.
Nature’s Glory Weed and Grass Killer RTU and Fast Acting Burn Out RTU are registered with EPA. Greenergy’s Blackberry and Brush Block and Alldown Green Chemistry herbicide have a 25(b), “minimum risk pesticide,” exempt status with EPA, and so do not have EPA registration numbers. The Greenergy product lists acetic acid as an inert ingredient and citric acid (at 7% concentration) as the active ingredient. If an herbicide has an EPA registration number, it has been approved for sale “at the Federal level.” However, companies must still register their products with the individual states to sell.
Vinegar concentrates are applied in two ways: They are either sprayed directly on the plant (as a contact herbicide) or they are applied to the soil as a soil drench (Lovejoy, 2003). Spraying the product directly on plants strips off the waxy, protective coating on the foliage, making the plants vulnerable to desiccation. Typically, the plant will dry out all the way to the root.
Vinegar concentrates that are sprayed onto the soil, on the other hand, work by lowering the soil pH to a level where the plants can't survive. This acidifying effect lasts from several months to a year, depending on the soil type and the weather. Once the soil pH is lowered, taproots will eventually starve, but before dying they will often have one last flush of growth from nutrients stored in the roots. Once the weeds are dead, which may take up to six months for some woody weeds, the soil pH will need to be brought back to a neutral level. To do this, add some lime at the rate of 5 pounds per 100 square feet and add a 3- to 4-inch blanket of compost to restore microorganisms. Some soil organisms will have been killed by the application of vinegar to the soil, but as long as the treated area is small, these critters will repopulate quickly from the surrounding unharmed soil when the pH is restored.
Vinegar weed-control products work best on warm, dry days when the temperature is above 65°F. Don't use the spray directly on plants just before watering or before a rain, because it will be washed off. In this case, not only will the vinegar not kill the plant, but it may act as a fertilizer, releasing nitrogen, among other things, into the soil.
Wear goggles, very dangerous to your eyes. Even the mist from spraying could cause injury. Wear a respirator; caustic fumes from the vinegar combined with oils (?) volatilizing from plants are hazards. FLAMMABLE as a mist or spray-avoid ignition sources. Avoid spraying directly onto the trunk, bark, shoots, stems, fruit or leaves of crop plants. Caution: Lethal or dangerous to small animals, reptiles and arthropods. Avoid spraying non-targets with this caustic material. Containers should be marked as hazards and stored as a dangerous material. The material could be mistaken as regular 5% table vinegar, and should be clearly marked as NON EDIBLE.
30% vinegar and its inherent low pH can be destructive to sprayer parts made of copper, brass and certain plastics. Rinsing equipment after use extended the life of sprayer parts, but did not prevent damage completely.
Chinery, David. 2002. Using Acetic Acid (Vinegar) as a Broad-Spectrum Herbicide. Rensselaer Horticulture. Cornell Cooperative Extension.
Lovejoy, Ann. 2003. Vinegar-Based and Citrus-Based Weed Killers. Fine Gardening (91). May-June. p.88, 90.
Shaffer, Bob. 2003. Vinegar Coffee, Grapes, and Ginger. SANET. Sustainable Agriculture Research and Education.
Sullivan, Preston. 2004. Thistle Control. ATTRA Publication. National Center for Appropriate Technology, Butte, MT.
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