Bagged Commercial Soils are an Avenue for Regional Dispersal of Weedy Plant Species.
Hundreds of common terrestrial weed species plague farmers and gardeners alike and the numbers are growing (USDA NRCS, 2016). Many weed species are nearly ubiquitous in the United States although different regions host subsets of the common species and other species are associated with certain agricultural crops (Barrett, 1983; Gould, 1991; Benvenuti, 2007). An estimated 75% of terrestrial weed species in the U.S. were introduced in the agricultural sector through unintentional or negligent contamination of imported crop seeds and also through the importation of livestock (Mack, 1991, 2000). Once introduced into agricultural cropping systems, weed seeds germinate under nearly ideal conditions for growth and survival (Mack, 2000) and among crop species to which the weed species may be well adapted (Benvenuti, 2007). Under such favorable conditions, establishment was (and is) very rapid and diffusive movement into natural habitats may follow. Many other species are introduced as ornamental and garden plants and escape through seed dispersal (Mack et al., 2001). For example, many species of Euphorbiaceae have been imported and grown for their potential medicinal properties (e.g., Unander el al, 1990) and have escaped to become nuisance species (e.g., Webster, 1970).
An important question regarding weed movement and spread is how weeds and their seeds are dispersed across the landscape. Some avenues of distribution are very well known (Mack, 1991; Benvenuti, 2007). For example, the commercial distribution of baled hay, grasses, and alfalfa as forage for livestock will move weed species from one location to another, often across regional boundaries, and will create the potential for spread into new environments similar to the ones they already occupy. Seeds can be moved by livestock, by agricultural equipment, and by recreational vehicles. Additionally, crop seeds produced in one region are often exported to other regions and contaminating seeds are moved with them (Mack, 1991; Benvenuti, 2007). Weed management protocols recognize these modes of dispersal and attempt to reduce the movement of unwanted weeds and their seeds, but other modes of dispersal remain unrecognized or poorly controlled.
We suggest commercial producers of common home and garden materials may be one of the unrecognized modes of weed seed dispersal. In particular, inexpensive bagged potting soil and topsoil products sold at many retail outlets are produced in large regional outdoor facilities and are composed of a number of different locally obtained organic and inorganic components. These soils are distributed regionally and are available at most garden supply stores, such as The Home Depot, Lowes, WalMart, and Kmart. The inexpensive soils typically cost less than $2.50 per 40 lb (18 kg) or 1 cu ft (28.3 1) bag and are not intended for use as potting soils for commercial plant production. Indeed, these products are very diverse; many of them are composed mostly of clay, sand, and gravel, others almost entirely of shredded tree bark, and all are of low nutritional quality and low water-holding capacity (pers. obs.; Dyer et al, 2016). For some products the packaging may stipulate that the soils are not recommended for growing plants but for other purposes such as filling holes in driveways (e.g., Earthgro top soil, Scotts Miracle-Gro Co., Marysville, OH).
One likely reason for the contamination of weed seeds in these soil products is composition. The components vary tremendously both within and between the soil "types" and brands; some "potting soils" and "top soils" can be very different or very similar. The components making up the products (as indicated by the packaging labels) include sand, local topsoil, bark, pine and hardwood forest fines, fly (coal) ash, peat moss, humus, and manure. Composition can range from sandy topsoil with <10% organic matter to products with >90% organic matter, usually shredded or composted tree bark. The different components of the soils may not contain weed seeds initially, but the production process may create opportunities for contamination. While most commercial soil producers include a composting process that kills main weed seeds (Gary Trinetti, past-director of the U.S. Compost Association, pers. comm.), the outdoor collection and mixing areas where the bulk piles stand for extended periods can collect wind-borne weed seeds. Soil mixture piles that stand in the open over the course of a growing season may become weed and seed producers.
We investigated the diversity of weed species in commercially available potting soils and lop soils by testing locally available brands and varieties over a period of three years. In the third year of the program, some commonly occurring species obtained from the bags were subjected to 2% glyphosate to test for herbicide resistance.
MATERIALS AND METHODS
We purchased 40 lb (18 kg) or 1 cu ft (28.3 1) bags of low-cost top soils and potting soils (Fig. 1) from local retailers of home-and-garden products (e.g., Home Depot, Lowes, WalMart, Kmart). The cost of the products ranged from $1.50 to $2.50 and represented the least expensive soil products available. These distributors sell a large variety of products, which are produced primarily by four large garden supply companies (Table 1) although other minor brands were occasionally available in our sample area. Most products were purchased in the Aiken, SC - Augusta, GA area although we also tested products from Kansas and Florida.
Each bag was spread into five 30 cm X 45 cm plastic trays (0.67 m J total sampling area) to a depth of 3-5 cm and watered regularly under greenhouse conditions. The trays were monitored for seedling emergence and representative seedlings were transplanted into greenhouse-grade fertilized potting soil (Scotts Miracle-Gro, Marvsville, OH). Plants were grown until they flowered, then identified using Radford el al., 2010 or the USDA-NRGS Plants Database website (http://plants.usda.gov/java). Many specimens did not flower in the greenhouse even after several months and could not be identified. Voucher specimens were collected and pressed as a reference library; additional specimens of phenotypically variable species were also collected. This germination procedure was followed several times throughout the year by purchasing additional bags of some products to account for cool and warm season species that might be present. A total of 49 bags of soil of 19 different types were tested (Table 1).
After the first year, soils were tested to estimate total organic content. Single ~100 g samples of each product that had been dried at 40 C for at least 24 h were combusted in a muffle furnace at >500 C for 2-3 h, then reweighed to express organic content as a percent of the dry mass. We did not determine sand-silt-clay texture because of the highly variable organic composition of these products.
In the third year of the project, we conducted a preliminary survey of glyphosate resistance in the commonly occurring potting soil weeds. We transplanted specimens of several species from as many soil products as possible. After plants were grown to maturity, groups of each species were manually sprayed with commercial 2% glyphosate (Roundup[R], Monsanto Co., St. Louis, MO). We sprayed the plant three times with a hand-held spray applicator and attempted to treat the entire plant. If plants did not die from the initial application, a second application was made. We note controlling the quantity of glyphosate applied was difficult given the mode of application. In some cases the plants lost leaves but regrew from the stems, and considerably variation was seen in the rate al which different species died. Here, we report observational results only.
We identified 80 plants to species or genera in 20 families and eight plants that were not positively identified. The majority of these weeds are common across agricultural regions according to distribution maps maintained by the US Department of Agriculture PLANTS Database (USDA NRCS, 2016) and are therefore listed as "ubiquitous" in Table 2. However, the local and regional distributions may not be "ubiquitous" for a large number of these species nor are they necessarily abundant where they are found.
We recovered weed species in nearly every bag of soil tested. Although the actual numbers of weed seeds could not be quantified, only those products that were nearly 100% organic material (e.g., shredded tree bark) contained very few seeds. The identity of the weeds varied between production lots and as a function of the locality in which they were produced. For example, samples of one brand of potting soil (Scotts Premium Topsoil, Scotts Miracle-Gro Co., Marysville, OH) were purchased in South Carolina, Kansas, and Florida for comparison. Bags purchased in South Carolina were sampled every year and were consistently composed of nearly 100% shredded pine bark and contained very few weed seeds. In contrast, the Florida bags contained very sandy topsoil with living plant parts and numerous seeds of plants typical of Florida. The Kansas soil was composed of very dark and heavy soil with high clay content and very few weed seeds. This one product typified the general nature of bagged soils with their composition of regional components but also indicated the wide range of quality of those components and of weed contaminants.
Other products were variable in other ways. For example, EarthGro Topsoil and EarthGro Potting Soil (Scotts Miracle-Gro Co., Marvsville ,OH) appeared similar in quality and composition and all bags contained large numbers of weed seeds. These products contained small rocks, gravel, sand, lumps of clay, and organic matter in the form of sticks and pieces of bark. Organic content for those products was 20-30% (Table 1). In contrast, Organic Valley Top Soil (Garick Corp., Cleveland, OH) was a low nutrient sand and clay mix that contained very few seeds other than crabgrass (Oigitaria spp.) and no visible organic material (10-20% by weight).
Our preliminary testing for glyphosate resistance revealed a range of responses. Most species were not resistant, some were very slow to die suggesting partial resistance or tolerance, and a few appeared to be resistant. We collected a number of sedge (Cyperus) species and one, C. esculentus (yellow nutsedge), died only after .3-4 wk post-glyphosate application. One individual of white clover (Trifolium repens) died after 4 wk. Yellow sweetclover (Melilotus officinalis) was completely resistant to or tolerant of glyphosate. In addition, Palmer amaranth (Amaranthus palmeri) emerged in very large numbers in some bags and showed a range of susceptibility to the glyphosate treatments we applied.
The large number of weedy species we found in bagged commercial soils is of considerable ecological and economic concern. While these soils are sold primarily to urban and suburban gardeners and unlikely to spread quickly into previously uninvaded areas such as nearby wildlife conservation areas or agricultural fields, we contend the intentional movement of these species is unnecessary and potentially dangerous. Gardening centers in every large city and many small cities distribute thousands of cubic feet of contaminated soils annually and this creates the very real possibility of new weed infestations.
The bagged soil products we tested are produced by mixing a number of components that may contain seeds of plants from wherever the components were obtained. To reduce costs, those components are likely to be local or regional in origin, but some specialty products are moved across regions (Gary Trinetti, pers comm). In any particular region, the weed species found in soil products at the local gardening center may be typical of the areas into which they are being introduced and this was the case for the majority of species identified in this study. For these inexpensive soil products, distribution beyond the regional production areas increases costs for the manufacturer and therefore most weed seeds are likely being relocated within the region (Gary Trinetti, pers. comm.), but there are additional concerns even with regional redistribution.
This study was intended as a preliminary investigation into both the prospect of weed seed redistribution and the potential for the movement of herbicide resistance seeds. The response of our Palmer amaranth samples was not surprising because of the glyphosate resistance problems in southern states (Culpepper el al., 2006), but the apparent variation in resistance among plants of the same species suggests commercial soils may be distributing genetic variation for pesticide resistance.
Introduction of invasive species into uninvaded areas.--From an ecological or economic standpoint, the introduction of new nonnative species is a problematic issue. While most of the species we identified in this study have distributions across the southeast, this was not true for all species. From these commercial sources, the movement of these species is primarily into residential gardens, but the soils containing the most weeds are also recommended for tilling holes in yards and driveways, which may allow them to be distributed outside residential areas. In addition, published species distributions are not detailed enough to assess local abundance; therefore, the movement of some species may represent new regional introductions. Additionally, the repeated introduction of their seeds in urban areas may increase local abundance and create focal points for further spread into previously unoccupied areas.
Increasing the probability of new genotype combinations.--While it is true that new introductions rarely lead immediately to invasion problems (Mack and Lonsdale, 2001; Kolar and Lodge, 2001), the frequent introduction of new seeds of a species can accelerate the process because of the increased likelihood of favorable genetic combinations. Typically, newly introduced species are slow to establish and go through an indefinite period of time (lag phase) wherein acclimation (response plasticity) and possibly adaptation to the new environment takes place (Dietz and Edwards, 2006; Prentis et al., 2008). However, most invasive nonnatives display high levels of phenotypic plasticity and are tolerant of a wide range of abiotic conditions (Davidson et al., 2011). For these species, the need for adaptation to ensure survival in a new environment is lessened and the individuals of the population are essentially buffered from the effects of strong selection by their ability to make plastic adjustments (Dyer et al., 2010).
The time required for adaptation to local conditions may be dependent on the accumulation of genetic diversity in the introduced population, which can come through subsequent introductions of new genotypes and the accumulation of random mutations over time (Ward et al., 2008). As more and more individuals are introduced to an area, the more likely new genetic combinations are to be formed and this increases the likelihood of matching a genotype to the environment. This process of repeated introductions of large numbers of seeds, often referred to as "propagule pressure" (Lockwood et al., 2005), hypothelically reduces the length of the lag phase and leads to more rapid invasions. Therefore, the constant movement of seeds into an area represents repeated introductions of many species and this increases the likelihood of establishment and spread by increasing the genetic variation in local populations (Dlugosh and Parker, 2008).
Introducing chemically-resistant genotypes into new areas.--At the time the current study took place in South Carolina no glyphosate-resistant agricultural plants had been officially listed. Although it is highly unlikely for the stale to have no glyphosate-resistant species while being surrounded by other states with such problems, the prospect of intentionally spreading any chemically resistant weeds is undesirable. The fact that such weeds (e.g., Palmer amaranth) were easily identified in bags of commercial potting soils and that thousands of such bags are sold in the state yearly suggests am species contained in those products will become more abundant within the range of distribution of those products. Noxious weed species often produce very large numbers of very small, easily-dispersed seeds and the likelihood is very high of establishment in new areas once they are introduced. Mov ement of weeds from residential and suburban areas into natural and agricultural settings is likely to be diffusive and slow but also inevitable. Therefore, we suggest these soil products create an avenue for dispersal of weed species, but also of unwanted genotypes of those species, particularly those showing resistance to common herbicides.
Combinations of these concerns.--The development of chemical resistance in species that are not closely related to agricultural crops probably occurs most frequently as a function of genetic diversity and the presence of resistance genes. Such genes are unlikely to be present in small populations and are unlikely to appear as long as those populations remain small and isolated from the larger gene pool. However, the process of multiple and constant introductions increases the risk of spread of such genes or of new genetic combinations that may be more resistant or invasive.
In this study we focused only on glyphosate resistance, but ghphosate represents a proxy for all chemical resistance problems in agriculture. For example, herbicides remain a useful tool as long as weed resistance is restricted to specific regions. Ghphosate resistance has been reported in 17 agricultural weed species in the US and 37 species around the world (Heap, 2017). In the US and elsewhere, the resistant genotypes are usually restricted to specific agricultural regions. However, the usefulness of glyphosate will continue to erode as those resistant genotypes are spread more and more widely. The unintended movement of weeds through the regional distribution of commercial bagged soils will only accelerate that process and this applies to all herbicides and the weeds that are resistant to them.
Although commercial soil producers claim their products have been properly composted to kill weed seeds, we found no support for that contention. In fact, given the components that make up these soils, it is unlikely that some of the products were ever free of weed seeds. If we assume that the least expensive products are subjected to minimal hygiene procedures to avoid increased production costs, then bagged soil products create a potential avenue for the movement of unwanted weed species across the regions in which those products are distributed. This will be more pronounced if the product components include local topsoils and if the regional production facilities allow exposure to seed dispersal vectors such as the wind and birds. Given the rapidly growing problems associated with herbicide resistance, the movement of weed seeds in this manner should be viewed as a threat to natural ecosystems and agriculture. Therefore, our results suggest that greater attention should be given to reducing the presence of viable weed seeds in commercial bagged soil products.
Acknowledgments.--We thank the Department of Biology & Geology at USC Aiken for support of undergraduate research. Thanks to Gary Trinetti, past-director of the U.S. Compost Association, for information regarding the production of commercial soils.
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Submitted 15 November 2016; Accepted 11 July 2017.
ANDREW R. DYER, JESSICA E. COCHRAN, JAMIE M. PHILLIPS, KATHERINE 1. LAYNE, MEGAN E. BERRY, and A. KATHERINE KULE, Department of Biology & Geology, University of South Carolina Aiken, 471 University Parkway, Aiken, South Carolina 29801.
Caption: Fig. 1.--Bag of low-cost commercial soil purchased in South Carolina. In this example a number of Cypmis sp had sprouted and punctured several bags on the same pallet
Table 1.--Brands, products, number of bags sampled, organic content, and components of commercially available bagged soils [40 lb (18 kg) or 1 cu ft (28.3 1)] sampled in this study. Soil organic content was calculated from loss of mass in a muffle furnace of 100g samples and listed categorically by percent (<10, 10-20, 20-30, 40-70, 70-90, >90). Components are typically listed on the product package as "Contains one or more of the following ..." with no information about proportions. Some labels do not provide component information. We assume the actual contents vary with locality, component availability, time of year, and cost. Soil product brands are: Garick Corp. Cleveland OH; Oldcastle, Inc., Atlanta CA; Scotts Miracle-Gro Co., Marysville OH; SIMS Bark Co., Muscle Shoals AL Sampled % Brand Product bags Organic Garick Farmer Green top soil 3 10-20 Organic Valley top soil 6 10-20 Organic Valley humus 1 Moo-Nure 1 Oldcastle Jolly Gardener top soil 1 Timberline garden soil 3 10-20 Timberline organic compost 1 >90 Timberline top soil (FL, NC) 3 >90 Timberline top soil (SC, GA) 2 10-20 Scotts Earthgro organic humus 3 10-20 Earthgro potting soil 4 20-30 Earthgro top soil 3 20-30 Hyponex potting soil 4 10-20 Scotts premium top soil 7 >90 SIMS Garden Plus compost and 1 manure Garden Plus potting soil 1 Garden Plus top soil 2 Evergreen composted manure 1 Evergreen top soil 1 Premium top soil 3 40-70 Brand Components Garick Not listed Not listed Not listed Natural forest products, green waste and composted cow manure Oldcastle Not listed Aged and processed forest products, sphagnum peat moss, reed sedge peat, compost (cow manure, poultry manure, leaf compost, mushroom compost), lime as needed to adjust pH. Cow manure compost Not listed Not listed Scotts 10% manure; 90% peat, compost, forest products, manure, and/or other organic materials Hypnum peat, forest products or compost, sand, perlite. Georgia: 77-87% aged pine bark, sand and perlite Peat, forest products, compost, ash, sand or native topsoil Hypnum peat, forest products or compost, sand, perlite. Georgia: 77-87% aged pine bark, sand and perlite Peat, composted forest products, aged rice hulls or compost, and sphagnum peat moss SIMS Manure, pine fines, hardwood fines, fly ash, sand, and forest products Pine fines, hardwood fines, forest products, sa??d, perlite, and fly ash Pine bark, hardwood bark, forest products, sand, and fly ash Not listed Pine fines, hardwood fines, forest products, sand, and flv ash Sand, hardwood fines, perlite, peat moss and forest products Table 2.--Species identified from 51 bags [40 lb (18 kg) or 1 cu ft (28.3 1)1 of 19 different soil products from four major garden product companies. Each bag was spread in five 0.13 [m.sup.-2] trays at 3-5 cm depth and kept moist in a greenhouse. Soil products were not sampled equally (ice Table 1) and therefore the number of species found per brand does not necessarily reflect the level of weed seed contamination. Abbreviations for the U.S. distributions are: e = eastern, s = southern, w = western, ne = northestern, sw=southwestern, se = southeastern. Native species are indicated by * (USDA NRCS 2016). See Table 1 for soil product brand information Family Species Distribution Amaranthaceae Amaranthus palmeri * ubiquitous Amaranlhus retrqflexus * ubiquitous Amaranthus spinosus * e U.S. Asteraceae Ambrosia artimisiifolia ubiquitous Conyza canadensis * ubiquitous Coreopsis linrtoria * ubiquitous Eupatorium rapillijolium * s U.S. Brassicaceae Lepidium densiflonim ubiquitous Lepidium virginicum * ubiquitous Lepidium sp. Sibara virginira * s U.S. Sisymbrium sp. Thlaspi arvense ubiquitous Chenopodiceae Chenopodium album * ubiquitous Chenopodium berlandieri * ubiquitous Dysphania ambrosioides * ubiquitous Convolvulaceae Convolvulus arvensis ubiquitous Dichondra carolinensis * s U.S. Ipomoea sp. Cvperaceae Carex sp. Cypents croreus * s U.S. Cypenis sp. (7) Scirpus sp. Euphorbiaceae Chamaesyce hyssopifolia * s U.S. Chamaesyre maculata * ubiquitous Phyllanthus urinaria s U.S. Triadica sebifna s U.S. Fabaceae Medicago arabica w, s, ne U.S. Lespedeza runeata e U.S. Melilolus officinalis ubiquitous Sesbania exaltata * s U.S. Trifolium repens ubiquitous Trifolium arvense ubiquitous Vicia saliva ubiquitous Geranaceae Geranium carolinianum * ubiquitous Geranium dissectum w, s, e U.S. Lamiaceae Lamium amplexicaule ubiquitous Molluginaceae Mollugo verticillata * ubiqui tous Oxalidaceae Oxalis strirta * ubiquitous Plantaginaceae Plantago virginira w, s, e U.S. Poaceae Agrostis stolonifna ubiquitous Bromus catharticus ubiquitous Cynodon dactylon ubiquitous Dichanthelium acuminatum * ubiquitous Dichanthelium commutation * sw U.S. Dichanthelium dichotomum * e U.S. Dichanthelium/Panicum sp. (2) Digitaiia ischaemum ubiquitous Digitaria sanguinalis/ciliaris ubiquitous Digitaria violascens se U.S. Echinochloa colona w, s, e U.S. Echinochloa crus-galli ubiquitous Eleusine indica ubiquitous Eragrostis pilosa ubiquitous Hordeum pusillum * ubiquitous Lolium multiflorum ubiquitous Panicum dichotomiflorum * ubiquitous Paspal?m? dilatatum w, s, se U.S. Paspal?m? laeve e U.S. Poa annua ubiquitous Setaria viridis ubiquitous Sorghum half pense ubiquitous Polygon aceae Polygonum lapathafolium * ubiquitous Polygonum persicaria ubiquitous Ranunculaceae Ranunculus sardous w, se, ne U.S. Ranunculus sp. Rosaceae Fragaria virginica * ubiquitous Rubus sp. Rubiaceae Hedyotis cmymbosa s U.S. Galium aparine ubiquitous Solanaceae Solanum ptycanthum * ubiquitous Solatium nigrum w, se, ne U.S. Urticaceae Urtica dioica ubiquitous Unknown forbs Unknown sp. (8) Soil product brand Family Garick Oldcastle Scotts SIMS Amaranthaceae X X X X X Asteraceae X X X X X X Brassicaceae X X X X X X Chenopodiceae X X X X X Convolvulaceae X X X X Cvperaceae X X X X X X X X Euphorbiaceae X X X X X Fabaceae X X X X X X X X X X X Geranaceae X X X X Lamiaceae X Molluginaceae X X Oxalidaceae X X X Plantaginaceae X Poaceae X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X Polygon aceae X X Ranunculaceae X X X Rosaceae X X X Rubiaceae X Solanaceae X X X Urticaceae X X Unknown forbs X X X
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|Title Annotation:||Notes and Discussion Piece|
|Author:||Dyer, Andrew R.; Cochran, Jessica E.; Phillips, Jamie M.; Layne, Katherine I.; Berry, Megan E.; Kule|
|Publication:||The American Midland Naturalist|
|Date:||Oct 1, 2017|
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