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Home  > Question of the Week

Question of the Week

Permalink What information can you give me on rooftop gardening?


Answer: I am pleased to provide you with information on rooftop gardening.

Rooftop gardens are a unique way to use unused and sterile spaces for food production. There are many different types of rooftop garden systems that can include growing crops in containers, in raised beds, or even greenhouses. Rooftop gardens not only provide food, but also help cool buildings and can improve air quality. Below is an excerpt from an article that was written by two rooftop gardeners in Canada. The garden is part of a university and is located on the roof of an academic building. The authors discuss the many benefits of producing food on roofs, but also share many of the problems and mistakes that they experienced during their growing season. The section of the article that is cited below begins by comparing the differences between traditional food production systems and growing crops on roofs.

The main differences can be summed up in two words: sun and wind. The resulting growing conditions tend to be more extreme (1). Even after a good rain, it takes very little time for the beds to dry out; our solution is mulch, mulch and more mulch. Even so, not everything grows well on the roof. In particular, we have difficulty with spinach, peas and beans. Other heat-loving plants, however, do very well including tomatoes, peppers and basil.

Another significant limitation on the roof is soil fertility. In the spring we recruit unsuspecting (or very generous) volunteers to help us haul compost from The Spoon. We further amend the soil with sheep manure from Tom's farm, green manure and compost tea. In particular using green manures or compost tea is labour-saving, because it precludes the need to bring more materials up to the roof through the Environmental Sciences Boardroom (the only access to the roof).

Despite these challenges, rooftop gardening provides a number of incentives. We need not worry about pests such as deer. Furthermore, what is a challenge in the summer - namely the warmer, dryer conditions - is an advantage in the spring when we're able to start gardening a few weeks earlier than the surrounding area. Thus the rooftop climate acts as a season extension.

Fortunately for us, the rooftop garden was a part of the initial building design. Thus, not only is there proper irrigation and drainage, but the building has sufficient load-bearing capabilities to support eighteen inches of saturated soil. To prevent water and roots from compromising the roof there is an impermeable membrane beneath the soil. The garden acts as a temperature moderator for the building below, cooling it down in the summer and insulating it in the winter.

On a larger scale rooftop gardens and sod roofs can do the same in a city. A recent study prepared by Ryerson University for the City of Toronto, found that green roofs significantly reduce stormwater runoff, reduce energy consumption and the reduce the heat island effect. Furthermore, they help to beautify the city and create more natural green spaces in urban areas - for everyone, including the black swallowtails.

Tips for Rooftop Gardening
• Mulch everything.
• Water deeply and often.
• Choose vegetables that suit the environment.
• Use compost tea and green manures to ammend the soil.
• Attempt to create shade, with trellises for example.
• If you don't have the means for an intensive rooftop garden consider using containers.

Referenced below are several articles on rooftop gardening. There are also a few websites posted below that provide further information. One website posted below ( is for the Rooftop Garden Project. This collaborative organization has recently published a manual titled “Guide to Setting Up Your Own Edible Rooftop Garden.” (PDF/2.28MB) This publication contains over 80 pages and is available to download from the website. If you are unable to download the publication, it can be obtained by contacting the organization Alternatives by telephone at (514)982-6606 x2230.


(1). Blyth, Aimee and Leslie Menagh. 2006. From Rooftop to Restaurant – A University Café fed by a Rooftop Garden. British Columbia, Canada: City Farmer.


Price, Martin. 1992. The Eave Trough Garden. North Fort Meyers, FL: ECHO.

Price, Martin. 1992. The “Wick” Method of Rooftop Gardening. North Fort Meyers, FL:

Voelz, Jan. 2006. The Characteristics & Benefits of Green Roofs in Urban Environments. Davis, CA: UC Davis Extension.

Wilson, Geoff. Date unknown. Can Urban Rooftop Microfarms be Profitable? Netherlands: RUAF.

Web Resources:

Community Food Security Coalition

The Rooftop Garden Project

Green Roofs for Healthy Cities



Permalink What are some weed management options for a perennial plant operation?


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.

Maintenance Issues
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.



Permalink How can we eradicate absinth wormwood without herbicide?


Answer: Historically, research in absinth wormwood (Artemisia absinthium) control has predominantly been oriented toward chemical control. However, using an integrated, non-chemical approach including mechanical and biological controls is gaining precedence in the western states. Mechanical controls include the use of fire and mowing, and biological controls include multi-species grazing and targeted beneficial insect release. I will address each of these approaches in turn and will conclude by offering some resources for further reading.

Any integrated weed control program should be oriented toward the facilitation of vigorous perennial grass growth in order to out-compete the targeted weed population. This is particularly important with absinth wormwood control, as a vibrant perennial grass stand can out-compete it (1). For detailed information on grass management see the ATTRA publication Pasture, Rangeland, and Grazing Management and the publication Revegetation Guidelines for Western Montana: Considering Invasive Weeds.

There is not much research on grazing absinth wormwood, although sheep and goats are known to eat it. Research on other plant species such as knapweed and leafy spurge suggest that targeted grazing can be a useful tool for control if the grazing animal’s diet is sufficiently diverse. I recommend the book (available online) Targeted Grazing: A Natural Approach to Vegetation Management and Landscape Enhancement.

Beneficial insects are those that are known to target a particular plant species, and for some, can provide significant control. Research has shown that this method is useful if the target weed population is large enough to allow the insect population to reach a critical mass and begin to defoliate leaves, mine the stems, or populate the seed heads or flowers. This method usually requires several years to allow the insect population to reach critical mass. The pyralid moth Euzophera cinerosella is the only species I know of that targets wormwood; however the insect could also target other Artemisia species, making its usefulness problematic. Contact your county weed board for information on insectaries and release programs in your area.

For cultural control, a spring burn followed by periodic mowing has shown some success. Mowing in the spring can help prevent seed production, but burning alone has not shown effective for wormwood control (3). For a mowing regime, periodic mowing during the vegetative stage of growth through flowering can significantly impact seed production and weaken the plant, enabling perennial grasses to establish and take over the stand.

References and Resources

1. Absinth Wormwood: Options for Control. Lincoln County, WA Noxious Weed Control Board.

2. American Sheep Industry Association. 2006. Targeted Grazing: A Natural Approach to Vegetation Management and Landscape Enhancement. A handbook on grazing as a new ecological service. Karen Launchbaugh, Editor.

This handbook represents a compilation of the latest research on harnessing livestock to graze targeted vegetation in ways that improve the function and appearance of a wide variety of landscapes.

3. Evans, J.E. and Nancy Eckardt. 1987. Element Stewardship Abstract for Artemisia absinthium. The Nature Conservancy.

4. Owsley, Cindy. 1999. Biological Weed Control – Grazing. Boulder County Parks and Open Space.

Livestock can be a valuable tool within an Integrated Weed Management system.



Permalink What information can you give me on the production and marketing of medicinal herbs?


Answer: Thank you for requesting information from ATTRA on production and marketing of medicinal herbs. Please refer to the ATTRA publication Herb Production in Organic Systems, which provides a detailed overview of medicinal herb production and marketing as of 2005. It includes research summaries. Also see the publications Ginseng, Goldenseal, and Other Native Roots.

Production/Marketing of Medicinal Herbs

The latest figures we have on sales of specific types of herbs are from 2005, published in a 2006 Market Report in the American Botanical Council’s trade publication, HerbalGram.

You will note that mass market sales are down for all types of medicinal herbs—with four exceptions: cranberry (a functional food), milk thistle, ginger, and green tea. Table 1, [Medicinal] Herb Sales in All Channels (including Internet, direct marketing, health food stores, etc., as well as mass marketed products) shows a sudden leveling off between 1998 and 1999, with greatly diminished market expansion after that. Even sales of garlic, the top herb seller since 2000, have been diminishing. Sales of ephedra (Ma Huang)—formerly the number one herbal best seller—evaporated after it was banned by both USDA and FDA around the year 2000. Results of a Stanford University garlic trial, published in Archives of Internal Medicine and reported in HerbalGram (February 2007, 74:6) provide “significant evidence” that “commercial garlic dietary supplements do not have a clinically significant cholesterol-lowing effect.”(1) Publication of such findings usually has a negative effect on sales of such products in subsequent years.

Other recent HerbalGram reports include the following:

• Appeals Court Sides with FDA in Ephedra Ban Case
• UK Expert Committee Upholds Kava Ban
• FDA Denies Medicinal Value of Smoked Marijuana
• FDA Rejects Proposed Health Claim for Cardiovascular Benefits of Green Tea
• Australian TGA Publishes Liver Warning Policy for Black Cohosh
• European Health Agencies Recommend Liver Warnings on Black Cohosh Products
• Canada Issues Advisory on Black Cohosh

• FDA Approves Special Green Tea Extract as a New Topical [external] Treatment
• Intravenous Milk Thistle Compound Used to Save Victims of Poisonous Mushrooms

While scattered local niche markets may exist, the dream of a decentralized network of U.S. herbal practitioners dispensing locally raised botanicals remains largely unrealized. Mass marketing of nutriceuticals has raised huge problems of safety, efficacy, quality control, environmental concerns, and government regulation. Most raw materials for such products are now wild-gathered outside the U.S. or raised cheaply in low-wage countries.

Listed below is an article describing difficulties that long-established Marathon County, Wisconsin, ginseng growers are experiencing due to foreign competition.

For a membership in the American Botanical Society, including a subscription to HerbalGram, call the toll-free number 800-373-7105 or 512-926-4900. Personal membership is $50/year.


1) Oliff, Heather S. 2007. Trial Finds No Benefit in Raw Garlic or Garlic
Supplements for Hypercholesterolemia. [Review: Gardner, CD, Lawson,
LD, Block, E., et al. Effect of raw garlic vs. commercial garlic supplements
on plasma lipid concentrations in adults, with moderate
hypercholesterolemia; a randomized clinical trial. Arch. Int. Med.
2007;167:346–353.] HerbalGram No. 75, p. 27.


Blumenthal, Mark, Grant K.L. Ferrier, and Courtney Cavaliere. 2006. Market Report: Total Sales of Herbal Supplements in United States Show Steady Growth: Sales in Mass Market Channel Show Continued Decline. HerbalGram No. 71. p. 64–66.

Cavaliere, Courtney, and Mark Blumenthal. 2007. Wisconsin Ginseng Farmers Fight to Protect Product Reputation. HerbalGram No. 75. p. 54–59.



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