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Weed Control: Mulching Questions Answered

By Anne Van Nest
From the May 2011 issue of Acres U.S.A. magazine

Weed control through mulching makes sense for many growers, but there are often questions about sourcing, safety and sustainability. May is a critical time for many gardeners to deal with existing or imminent weed issues before problems get totally out of control. Mulching is a key component of a multi-pronged approach to gaining the upper hand in the weed control battle.

Is Cypress Mulch Sustainable?

Shredded cypress is a popular mulching material for weed control because it is slow to decompose and the long strands lock together and don’t blow or float away easily. Attractive and natural looking, cypress mulch has many fans in the garden. The majestic swampy cypress forests across the Southeast, where most of the cypress mulch is harvested, are a true ecological sanctuary and face encroaching building development as more and more people flock to the Sunbelt. Gardeners are  now beginning to look at the source of their mulch and question the sustainability of cypress mulch harvesting.

A group called Save Our Cypress  is drawing attention to the harvesting practices of cypress  for  use as lumber or mulch. Their website says that “in the past cypress mulch used to be a by-product of lumber mills. This is no longer true. The mulch purchased today comes from widespread clear cutting of entire ecosystems.” The Save our Cypress website lists several alternatives to cypress mulch including leaves, pine straw, recycled yard waste, pine bark mulch  and  eucalyptus mulch.

Supporting the harvesting practices of the cypress mulch industry, the Mulch and Soil Council — a mulch certifying industry group — reports that in Louisiana,“cypress forests are growing six times faster than trees are being harvested with 400,000 new cypress trees planted each year and even more naturally regenerated.Cypress trees are not being cut in areas such as flooded swamps where they will not re-grow. Most harvested cypress is sold to sawmills. The by-products from these sawmills are sold to the mulch industry to deal with the disposal of their debris.” Using a waste by-product for mulch reduces the threat of this debris being a fire hazard or ending up in over-burdened landfills.

Whether you think that cypress mulch should be avoided because there are more sustainable alternatives or not, there are many alternatives to consider. But should colored mulches be your alternative to cypress mulch for weed control?

Are Mulch Colorants Safe?

Setting aside the fact of whether you think mulch colorants look natural or not, the aesthetically pleasing aspect should take a backseat to the health issue of whether the mulch colorants are safe for pets, plants, people and the environment. It’s a standard of good stockmanship. A little homework or a phone call before purchasing or contracting for your mulch delivery should be able to give you the answers you seek. Many of the bigger mulching  companies, such as Mobile Mulch Systems in St. Paul, Minnesota, use iron oxide pigments in their mulch that come from natural sources. They say that these colorants have proven to be completely safe for the environment. Another large mulch company, Amerimulch, uses water-based formulations, rather than solvent-based colors, which they say produces a safer colored mulch product.

The Mulch and Soil Council have investigated the use of colorants in mulch products and found that the red colorants from iron oxide have been used for centuries and are currently used extensively for many other products such as facial cosmetics and paints that result in close, frequent human interaction. They found no specific concerns with the red color used to dye mulch.

They also investigated the black used in mulch coloring. They found that carbon black “is virtually pure elemental carbon and is used in many consumer and industrial  products  such  as  tires, belts, virtually all other rubber goods, video and audiotapes, nearly all electric motors as the brush contacts, insulators, and dry cell batteries. As a pigment, it is used as a toner for paper copiers and printers, inks for newspaper, and in most dark-colored paints and coatings. Given the wide manufacture of both pure carbon black and products containing carbon black, there is a wealth of information published on the human health aspects of this material. Occupational studies over 60 years do not show any increased health risk to workers exposed to carbon black compared to the general public.”

The Mulch and Soil Council have investigated the use of colorants in mulch products.

The Mulch and Soil Council did not find any evidence that any of the components of colorants were an environmental concern when used according to label directions and rates on mulch. Dyed mulch aside, some people are not concerned so much with the colorants used for mulch. Instead, people like David Beaulieu are concerned with what is used for the mulch. He says on the About.com Landscaping website, “The source of most dyed mulch is recycled wood. So far, so good. But the problem is that some of that recycled wood may be (chromated copper arsenate) CCA-treated wood, which, used as a mulch, can raise the arsenic level in your soil. Although the use of arsenic in making pressure-treated lumber was largely banned after 2002, who’s to say part of the source of the dyed mulch you’re buying isn’t old, leftover CCA-treated wood?” Beaulieu recommends looking for Mulch and Soil Council (MSC) certified mulch that is free of CCA-treated wood.

A good suggestion from a mulch industry worker at Amerimulch.com is to look at the type of company supplying the dyed mulch. The industry has two types of suppliers: those who are land clearing companies and take the logs to a lumberyard and grind up the smaller  pieces into mulch; and those that are wood waste recycling companies that take used wood and grind up whatever they can get for mulch. The suggestion is that the land clearing companies will have products that are CCA-free and the wood recycling companies’ products have the chance that CCA-treated wood could have made it into their batches.

Is Cocoa Bean Mulch Toxic to Dogs and Cats?

The sweet smelling cocoa bean shell mulch that is popular in some parts of the country could be very hazardous to your pet. Chocolate can be toxic to your pets. The ASPCA, on its website, addresses reports that “dogs who consume enough cocoa bean shell mulch could potentially develop signs similar to that of chocolate poisoning, including vomiting and diarrhea.”

It is the residual quantities of theobromine found in chocolate that causes the trouble for dogs and cats. “We advise pet parents not to use cocoa mulch in areas where dogs can be exposed unobserved, particularly dogs who have indiscriminate eating habits,” says Dr. Steve Hansen, ASPCA Senior Vice President of Animal Health Services. He recommends shredded pine, cedar or hemlock bark as alternatives. The ASPCA website also mentions that no dogs have died from ingesting cocoa mulch and a large quantity is needed to make them sick.

On the About.com website, the Merck Veterinary Manual is referenced and approximate levels of theobromine in different types of chocolate are listed as: dry cocoa powder = 800 mg/oz, unsweetened (Baker’s) chocolate = 450 mg/oz, cocoa bean mulch = 225 mg/oz, milk chocolate, 44-64 mg/oz.

According to Dr. Rhea Morgan in The Handbook of Small Animal Practice, a toxic dose of theobromine is 100-150 mg/kg for dogs.

As an example, a 20-pound dog (9 kg) would have to eat 900-1,350 mg of theobromine, which would be 4-6 oz (.25-.4 lb) of cocoa bean mulch to receive a toxic dose.

Does Using Mulch Attract Termites and Ants to Buildings?

Donald Lewis from the Department of Entomology at the Iowa State University Extension admits that subterranean termites can be routinely found in wood chip mulch. He says that worker termites come to the soil surface to feed on wood and other cellulose materials and carry this back to share with other colony members. A moist environment such as that created under mulch is an inviting place for termites, and they will search it out. Tests done in Iowa with different types of mulches to see if one type favored termites found that mulch type had little effect and that surprisingly gravel mulches proved to be a magnet for termites.

One valuable pointer that Donald Lewis mentions is to keep mulch several inches from a house foundation, windowsills or house siding. Also watch wood chip mulch for signs of termite activity and call in a professional for an inspection and treatment estimate.

Are Newspapers Safe to use as Mulch for Weed Control?

In 1977 the U.S. Consumer Product Safety Commission banned the overall use of lead paint. This was followed by an EPA ban of lead in printing ink in 1985. Today’s newspaper inks contain mostly carbon black, varnish, polyethylene wax, china wood oil (tung oil), soybean oil and linseed oil. Ohio State University Extension, reporting on a study looking into the safety of using shredded newsprint bedding with black ink for livestock announced that “There is little threat of dermal absorption of ink or its ingredients once the ink is dry because the ink has achieved its stable state. The ingredients that were potentially absorbable become dry and are no longer able to be absorbed.” Charles Walters, founder of Acres U.S.A. would be the first to say that faulty logic would prevail if we believed in the assumption that newsprint being proclaimed safe for livestock bedding by Extension Agents also should be safe to use as mulch among vegetable plants — especially if non-black ink is used.

In 2005, the NOSB board approved in a formal recommendation to the National Organic Program that newspaper  or other recycled paper, without glossy or colored inks (as well as the use of plastic mulch and covers other then PVC types) continue to be included on the National List of substances used for  crop production.

Are Mulch Volcanoes Around Trees Harmful?

First, what are mulch volcanoes? Mulch volcanoes are the steep piles of excessive mulch banked up against a tree trunk. Piling mulch up like this is a very unhealthy practice and can lead to rodent, insect and disease problems and if excessive and long-term, could even kill the tree. Mulch should be kept away from tree trunks and not placed any higher than the root flare, (the flared out area at the base of the tree trunk) which should still be visible after the mulch is applied.

Gary Bachman at Mississippi State University recommends that gardeners contour the mulch layer to resemble a bowl and make sure the mulch does not touch the tree trunk. This way the bowl shape will collect rainwater or irrigation and direct it toward the tree roots, rather than having it run off of the steep-sided volcano shape. Let’s ban the practice of creating mulch volcanoes!