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Characterizing Pathways of Invasive Plant Spread to Alaska: I. Propagules fromContainer-Grown OrnamentalsAuthor(s): Jeffery S. Conn, Casie A. Stockdale, Jenny C. MorganSource: Invasive Plant Science and Management, 1(4):331-336. 2008.Published By: Weed Science Society of AmericaDOI: http://dx.doi.org/10.1614/IPSM-08-063.1URL: http://www.bioone.org/doi/full/10.1614/IPSM-08-063.1

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Research

Characterizing Pathways of Invasive PlantSpread to Alaska: I. Propagules from

Container-Grown OrnamentalsJeffery S. Conn, Casie A. Stockdale, and Jenny C. Morgan*

To determine the extent and nature of container-grown plant soil as a pathway for introduction of exotic plant

species to Alaska, soil from container-grown ornamentals was obtained from vendors and was incubated in the

greenhouse. Fifty-four plant species were identified growing in containers or germinating from the soil, and included

Canada thistle—a prohibited weed in Alaska—and nine other species listed as invasive in Alaska. The number of

species and estimated seed bank were very low for soil from vegetable starts/herbs and herbaceous bedding plants

(, 2 seedlings/L soil), but was greater for soil from containers containing woody plants, especially balled and

burlapped ornamentals (20 seedlings/L soil). Container alien plant seed bank size was strongly related to type of soil.

Potting (soil-less) soil contained 1.2 germinating seeds/L, soil-based soil 5.5 seeds/L, and mineral soil 18.7 seeds/L.

Growers and vendors were variables that also influenced the size of the container seed bank, suggesting that weed

management practiced during production and at the point of sale can greatly influence seed banks of ornamental

containers.

Nomenclature: Canada thistle, Cirsium arvense (L.) Scop.

Key words: Pathways, container-grown ornamentals.

Prevention is the most powerful and cost-effective toolin combating the spread of invasive plant species(Radosevich et al. 1997; Reichard 1997). Preventionstrategies include use of weed-free crop and revegetationseed, feed, soil, mulches, and equipment; and creation/enforcement of seed and other plant material laws orquarantines (Holt 2004).

Invading species often take predictable pathways to reachnew areas (Mack 2003). Study of these pathways can helpto increase efficacy of commodity inspections and can focusearly detection surveys on areas where invading species aremost likely to appear. Pathways for invading plants aregenerally known but little-studied. Some of the knownpathways for plant introduction include contaminated cropand wildflower seed; vehicles and equipment; hay and

straw; clothing (Klingman and Ashton 1975); migratorybirds (Proctor 1968) and other animals (Mack 2003); andrecreational equipment such as off-road vehicles, boats, andfloatplanes (Sheley and Petroff 1999). Historically, one ofthe largest pathways for movement of alien plant specieswas rock or soil used as ship ballast. Muhlenbach (1979)documented that 386 alien plant species in New York andNew Jersey could be traced to such ballast. Laws thatrestrict discharge of solid ballast have largely eliminated thispathway.

A study by Rejmanek and Randall (1994) using Hulten’s(1968) Flora of Alaska determined that Alaska’s floracontained a much smaller proportion of alien species thandid floras from warmer areas of North America. However,the proportion of alien species in the Alaska flora hasgreatly increased. Carlson and Shepherd (2007) found thatthe number of alien plant taxa in Alaska had increased from174 in 1968 to 283 in 2006, an increase of 63% in 38years. The number of alien plant species can only beexpected to increase further due to climate change andincreasing tourism and trade, especially given current laxregulation and inspection of plant seed movement into

DOI: 10.1614/IPSM-08-063.1

* First author: Research Agronomist; second and third authors:

Biological Science Technicians, U.S. Department of Agriculture,

Agricultural Research Service, Subarctic Research Unit, 360 O’Neil

Building, University of Alaska Fairbanks, Fairbanks, AK 99775.

Corresponding author’s E-mail: ffjsc1@uaf.edu

Invasive Plant Science and Management 2008 1:331–336

Conn et al.: Invasion pathways N 331

Alaska. Prevention of alien plant species into Alaska shouldbe feasible given its geographical isolation with only elevenpoints of entry for goods into the state by container ship,truck, and airfreight.

Prevention efforts can be maximized by targeting themost important pathways. For this reason we began studiesof pathways for plant seed movement into Alaska todetermine the relative importance of each. This paperreports research conducted on soil from container-grownand balled and burlapped ornamentals as pathways forspread of alien plant species to Alaska. We have observedbull thistle [Cirsium vulgare (Savi) Ten] and Canada thistle[Cirsium arvense (L.) Scop.] growing from such containers,and this pathway is thought to be the cause for largepopulations of the aforementioned species in Anchorage.Though much research has been conducted on nurseryweed control (Case et al. 2005) there has been very littleresearch on weed seed banks in container soil (Cross andSkroch 1992).

The objectives of the study were to determine (1) thenumber of weed seeds and species contained in containersand root balls of ornamental plant species shipped to Alaska;and (2) if the weed species and populations vary withdifferent soil types, container types, growers, and vendors.

Materials and Methods

Greenhouses, nurseries, and stores selling container-grown or balled and burlapped vegetable starts, bedding

plants, and ornamentals in Alaska were identified using theState of Alaska Division of Agriculture Food and FarmProduct Directory (Anonymous 2007). We also contactedCooperative Extension Service agents in Fairbanks, An-chorage, and Juneau to identify additional vendors ofcontainer-grown plants not listed in this directory.

During the summers of 2004 and 2005, we visitedretailers of container-grown plants in the Anchorage/Matanuska Valley area (29), Fairbanks (9) and Juneau(2) that sold container-grown plants. Nurseries weresurveyed to determine (1) the numbers and species ofweeds growing in containers, (2) the species of ornamentalsold, (3) container types and sizes, (4) who the growers orsuppliers are and where they are located, and (5) thenumber of plants of each kind imported from each suppliereach year. Out-of-state ornamentals sold in Alaskannurseries were found to be shipped bare root, in plugs, inpots, or balled and burlapped. Since we are interested inweed propagules in soil we did not include plants that weresold as bare root or in plugs in this study. From the storesurveys we determined that container-grown plants fit intofive categories: (1) vegetable starts/herbs, (2) herbaceousperennials, (3) small woody plants (pots 3.8 to 11.4 L; 1 to3 gal), (4) large woody plants (. 18.9 L), and (5) balledand burlapped trees or shrubs.

From the survey we identified 25 out-of-state suppliersproviding container-grown plants to the Alaska market.These suppliers were located in the western and mid-western United States and Canada. We purchased orsampled 10 vegetable starts/herbs, 5 herbaceous perennials,5 small woody, 3 large woody, and 3 balled and burlappedplants from each of the major growers. Not all categoriescould be filled for each supplier because either the growerdidn’t produce each category of plants or the vendor didn’tcarry them. A small diameter soil core was used to samplelarge, costly plants that were not purchased. Three soilcores (2.5 cm in diameter; 1 in diameter) were taken fromeach plant sampled from different locations and into thecenter of the root ball. We also purchased plants from fourAlaska nurseries that make their own soil mixtures. Wepurchased or took soil samples from a total of 330containers or root balls.

Preliminary work determined that visual separation wasextremely time-consuming, and distinguishing seeds fromthe mainly organic soils was difficult. Instead of separatingseed from soil, we incubated the soil in a greenhouse todetermine the species and numbers of seeds in containersoils. In September 2005, soil was removed fromcontainers, was thoroughly mixed, and total soil volumewas determined. A 2000-ml soil sample was taken fromeach container when there was enough soil, and smallervolumes were obtained from the small containers. Thesamples were then placed in a freezer for 2 wk. Sampleswere then thawed and placed in a 2-cm-thick layer in

Interpretive SummaryInvasive plants are brought to new areas by numerous routes or

pathways. Characterization of invasion pathways can aid in thedesign of effective strategies for prevention and early detection ofinvasive plant infestations by targeting the largest pathways first.This research reports on the pathway resulting from importation ofsoil with container-grown ornamentals containing weed seed. Soilfrom container-grown ornamentals was obtained from vendorsand was incubated in the greenhouse.

Emerging seedlings were identified and counted. Fifty-fourplant species were identified growing in containers or germinatingfrom the soil, and included Canada thistle—a prohibited weed inAlaska—and nine other species listed as invasive in Alaska. Thenumber of species and estimated seed bank were very low for soilfrom vegetable starts/herbs and herbaceous bedding plants (, 2seedlings/L soil) but was greater for soil from containers containingwoody plants, and especially balled and burlapped ornamentals(20 seedlings/L soil). We found that potting soil held very fewweed seeds while mineral soil contained an average of 20 seeds/L ofsoil. Suppliers of ornamentals and vendors also influenced the sizeof the container seed bank, suggesting that weed managementpracticed during production and at the point of sale can greatlyinfluence seed banks of ornamental containers. This pathway foralien plant introduction could be reduced through consistent useof sterilized soil and good weed management. Container-grownornamentals (especially those containing mineral soil) are a largepathway for movement of exotic plant species.

332 N Invasive Plant Science and Management 1, October–December 2008

56 cm 3 41 cm covered trays in a heated greenhouse for3 mo. Temperatures in the greenhouse ranged from 18 to30 C (64 to 86 F) and the day length was 14 h. Trays werekept moist, which usually entailed daily watering with afine spray. Seedlings that emerged were marked withcolored toothpicks and were counted daily at first, and thenevery other day once germination slowed. Samples of eachseedling type were transplanted into pots so that they couldbe grown until they could be identified. The soil was stirredto stimulate additional germination when germinationslowed. This occurred three times over the incubationperiod. Photographs of each tray were taken every fewweeks to aid in tracking development of different seedlingtypes.

There was a wide range of species growing in containers.It was not possible to make statistical comparisons betweencategories of containers, soils, or growers using individualplant species because many of these categories did not sharethe same species. Therefore, we compared these categoriesbased on number of species and number of seedlings. One-way ANOVA (PROC GLM of SAS, version 9.1) was usedto determine if the number of species and the number ofseedlings of alien species was different between containercategories or soil types. We also analyzed for differencesamong growers and vendors within container categories.All species count and seedling number data were squareroot–transformed prior to analysis to satisfy assumptions ofnormality, but data in tables were not transformed.

Results and Discussion

Fifty-four plant species were found in soils fromcontainer-grown ornamentals (Table 1). All but three ofthese species are nonnative in Alaska. Eleven of thesespecies were found only during field surveys of nurseriesand stores and were not present amongst germinants fromthe greenhouse-incubated container soils. It is possible thatgermination requirements of some species may not havebeen met and it is likely that our estimates of container seedbanks are low.

The five most common weeds that germinated fromcontainer soil were La Plata sandspurry [Spergulariaplatensis (Camb.) Fenzl], birch (Betula sp.), stickychickweed (Cerastium glomeratum Thuill.), hairy bittercress(Cardamine hirsuta L.), and common groundsel (Seneciovulgaris L.). Ten species that are listed as highly tomoderately invasive in Alaska (Carlson et al. 2008) werefound. One of these species, Canada thistle, is a prohibitedspecies under Alaska Statute and was found growing from acontainer during a nursery field visit. The nine otherinvasive plant species grew from incubated container soil:perennial sowthistle (Sonchus arvensis L.), common tansy(Tanacetum vulgare L.), creeping bellflower (Campanularapunculoides L.), narrowleaf hawksbeard (Crepis tectorum

L.), common St. Johnswort (Hypericum perforatum L.),common groundsel, dandelion (Taraxacum officinale G.H.Weber ex Wiggers, western salsify (Tragopogon dubiusScop.), and common mullein (Verbascum thapsus L.).

Ten of the species found in this study were also listed byCase et al. (2005) as common weeds of container-grownplants throughout the United States. These species were:hairy bittercress, large crabgrass [Digitaria sanguinalis (L.)Scop.], fringed willowherb (Epilobium ciliatum Raf.),prickly lettuce (Lactuca serriola L.), creeping woodsorrel(Oxalis corniculata L.), annual bluegrass (Poa annua L.),birdseye pearlwort (Sagina procumbens L.), commongroundsel, common chickweed [Stellaria media (L.) Vill.],and dandelion.

The five container size/plant type categories weresignificantly different in the number of species and the sizeof seed banks (Table 2). Soil from vegetable start/herb orherbaceous perennial containers had the fewest weed speciesgerminating (0.6 seedlings/L) while soil from balled andburlapped trees contained significantly more weed seed thanall other container types (20.2 plants/L). Grass species wereless common than broadleaf species and were nearly non-existent in all container types except balled and burlapped.

Soil type had a significant effect on the number ofspecies and total number of germinants (Table 3).Significantly fewer species (0.7) and numbers of seedlingsemerged from soil-less potting mix (1.17/L) than from soil-based mixes (1.38 species, 5.5/L) or mineral soil (1.59species, 18.7/L). The number of grass species and seedlingswas much higher in mineral than other soil types.

The very small seed banks found in soil from thevegetable starts/herbs and herbaceous perennials containercategories may be largely explained by the use of soil-lesspotting mix in these containers (personal observation).Other growing conditions that help to keep seed banks lowin the vegetable starts/herbs and herbaceous perennialscategories are a short, early spring growing season andgrowth in greenhouses. Seed rain into pots is minimizedbecause the containerized plants are grown and sold beforemost weeds outside the greenhouse flower and produceseed. Larger seed banks in the small and large woody plantcategories can be attributed to the use of soil mixed fromnonsterilized mineral and organic soil materials or use ofmineral soil containing weed seed. These plants areperennials grown outside for several years where they aresubject to seed rain from distant wind-dispersed species oradjacent larger-seeded species. Also, because these plants aregrown over several years prior to shipment there issufficient time for seeds in the containers to completetheir life cycle and produce more seed. Plants sold as balledand burlapped are generally the largest and oldestornamentals and have experienced the greatest seed rain.Moreover, they are generally planted in mineral soil(personal observation) with an existing weed seed bank.

Conn et al.: Invasion pathways N 333

Table 1. The percentage of pots within container size/plant categories with weeds of various species.

Scientific nameCommon

nameField

surveyaVegetablestart/Herb

Herbaceousperennial

Smallwoody

Largewoody

Balled &burlapped

----------------------- Percent of containers with weed -----------------------

Amaranthus albus L. Tumble pigweed XAnethum graveolens L. Dill 1.3Betula spp. Birch 2.6 14.5 9.8 19.3Brassica sp. Mustard 3.9 8.2 2.9Campanula rapunculoides L. Creeping bellflower XCapsella bursa-pastoris (L.) Medik. Shepherd’s-purse 3.9 1.6 8.8Cardamine hirsuta L. Hairy bittercress 1.3 14.8 6.6 3.8 2.9Cerastium glomeratum Thuill. Sticky chickweed 0 4.5 13.2 11.5 11.8Chenopodium album L. Common lambsquarters 1.3 1.3 3.8 8.8Chenopodium glaucum L. Oakleaf goosefoot 1.6Cirsium arvense (L.) Scop. Canada thistle XCrepis tectorum L. Narrowleaf hawksbeard XDescurainia sp. Flixweed 7.6Digitaria sanguinalis (L.) Scop. Large crabgrass 3.8Echinochloa crus-galli (L.) Beauv. Barnyardgrass 3.8Elymus trachycaulus (Link) Gould

ex ShinnersSlender wheatgrass 3.8

Epilobium ciliatum Raf. Fringed willowherb 3.9 3.2 11.5 2.9Erodium cicutarium (L.) L’Her.

ex Ait.Redstem filaree 3.6

Galeopsis tetrahit L. Common hempnettle XGnaphalium sp. Cudweed X 3.6 2.9Hordeum jubatum L. Foxtail barley XHypericum perforatum L. Common St. Johnswort XKickxia elatine (L.) Dumort. Sharppoint fluvellin 3.6Lactuca serriola L. Prickly lettuce X 3.2Lamium maculatum L. Spotted deadnettle 1.3Matricaria discoidea DC. Pineapple-weed 1.3 2.6 2.9Misopates orontium (L.) Raf. Linearleaf snapdragon 3.6Oxalis corniculata L. Creeping woodsorrel 1.3 3.2 3.6Panicum capillare L. Witchgrass XPlantago major L. Broadleaf plantain X 2.6 3.2 7.2 5.8Poa annua L. Annual bluegrass 1.3 3.6 8.8Polygonum sp. Smartweed, knotweed 1.6Polygonum lapathifolium L. Pale smartweed XPotentilla norvegica L. Rough cinquefoil 1.3 3.9 2.9Rheum sp. Rhubarb 3.6Rorippa islandica (Oeder) Borbas Yellow marshcress 1.3 6.6 1.6 2.9Sagina procumbens L. Birdseye pearlwort 3.6Saponaria officinalis L. Bouncingbet XSenecio vulgaris L. Common groundsel 3.9 9.8 15.4Solanum physalifolium Rusby Hairy nightshade X 3.6Sonchus arvensis L. Perennial sowthistle XSonchus asper (L.) Hill Spiny sowthistle X 7.2Spergularia canadensis (Pers.) G. Don Canadian sandspurry 2.9Spergularia platensis (Camb.) Fenzl La Plata sandspurry 3.9 26.2 38.5 8.8Stellaria media (L.) Vill. Common chickweed 5.2 9.8 2.9Tanacetum vulgare L. Common tansy X

334 N Invasive Plant Science and Management 1, October–December 2008

The weed control practices of growers and vendors arealso important factors influencing the seed banks ofcontainer-grown ornamentals (Case et al. 2005). Thoughwe did not find significant seed bank differences betweengrowers or vendors in vegetable start/herb or herbaceousperennial plant containers, we found that both wereimportant influences on container soil–seed bank size forwoody perennials. Several growers’ containers were signif-icantly more weed-free than others (One-way ANOVA, P# 0.05). For the large woody plant category, containersfrom eight of the growers had # 3.5 seeds/L, while a ninthgrower’s containers had 17 seeds/L. In the balled andburlapped category, seed banks from root balls fromdifferent growers ranged from 3 to 80 seeds/L. Differencesamong growers in container seed bank size could be due toweed management at the site of production, or could be aresult of inadequate weed management after the containersleave the grower. The influence of nursery weed manage-ment on container weeds was studied by Cross and Skroch(1992). They placed containers with four different types ofsterilized and nonsterilized growing media in seven NorthCarolina nurseries and counted and removed weeds in thecontainers monthly over a year. They found significantdifferences in numbers of weeds between nurseries but notbetween container media.

Vendor-related differences were found in weed seedbanks (one-way ANOVA, P # 0.05). Soil seed banks fromthe small woody category were # 5 seeds/L except for onevendor with 81 seeds/L. A similar result was found in thelarge woody category with seed banks from four vendors of# 2 seeds/L and a fifth vendor with a seed bank of 12seeds/L. Seed banks from various vendors selling balled andburlapped plants were more variable. Four vendors hadseed banks of # 6 seeds/L, three had seed banks of 14 to 38seeds/L, and one had a seed bank of 65 seeds/L. Thesevendor-related differences may have been caused by thepurchase of plants from suppliers with poor weedmanagement or could be due to poor weed controlpractices on the part of the vendor.

Import of container-grown ornamentals is a significantpathway for introduction of new alien species to Alaska.The pathway is larger for woody plants than vegetablestarts/herbs or herbaceous perennials and is especially largefor plants grown in mineral soil, which contained anaverage of 20 germinable seeds/L. In a relatively smallsample of the container-grown plants sold each year inAlaska, we found 51 alien plant species—including aprohibited species—and 9 others ranked as invasive inAlaska. The size of the container market in Alaska is large.The total retail sales of Alaska’s nurseries in 2002 was

Scientific nameCommon

nameField

surveyaVegetablestart/Herb

Herbaceousperennial

Smallwoody

Largewoody

Balled &burlapped

Taraxacum officinale G.H. Weberex Wiggers

Dandelion 1.3 1.3 6.6

Tragopogon dubius Scop. Western salsify XTrifolium pratense L. Red clover 3.6Vaccinium spp. Blueberry 3.6 5.8Verbascum thapsus L. Common mullein 2.9Viola sp. 1 Violet 1.3Viola sp. 2 Violet 1.3

a X 5 Plants were observed growing from pots during field survey of nurseries and stores.

Table 2. Number of alien plant species and number of seedlings found in soil from container-grown ornamentals of different types.

Container/Plant type n

Number of species Number of seedlings (#/L)

Broad leavesa Grasses Total Broad leaves Grasses Total

Vegetable start/herb 76 0.22 a 0.03 a 0.25 a 0.61 a 0.02 a 0.63 aHerbaceous perennial 88 0.97 b 0.10 a 1.07 b 1.43 a 0.07 a 1.51 abSmall woody 61 1.41 b 0.05 a 1.46 b 7.72 b 0.02 a 7.75 bLarge woody 26 1.54 b 0.23 b 1.77 b 2.85 ab 0.35 a 3.19 abB&B 34 1.18 b 0.26 b 1.44 b 15.56 c 4.61 b 20.17 cANOVA Pr . F 0.001 0.001 0.001 , 0.0001 , 0.0001 , 0.0001

a Values within a column followed by different letters are significantly different (P # 0.05) using Tukey-Kramer post hoc test.

Table 1. Continued.

Conn et al.: Invasion pathways N 335

$14.9 M (USDA-ERS 2003) and likely includes sales of. 10,000 woody perennials each year.

In addition to this pathway’s size, its risk for alienintroduction is further increased because the likelihood ofgermination and establishment of any associated introduc-tion via this pathway is enhanced. Ornamentals are plantedin ideal conditions and are watered and fertilized. Alienspecies that germinate in this environment are more likelyto survive than those that are introduced to otherenvironments.

The size of the containerized plant–soil pathway forintroduction of new alien plant species to Alaska could begreatly reduced if growers would use sterilized soil incontainers and weed control practices to eliminate seedrain. Vendors could help to reduce this pathway byimporting containers from growers that produce weed-freecontainers and by eliminating weeds growing nearcontainers at the retail site. Another solution would befor vendors to import bare-root ornamentals and plantthem in sterilized soil after arrival.

Acknowledgments

We thank U.S. Forest Service State and Private Forestry forhelping to fund this research and allowing Melinda Lamb tohelp with surveys and plant purchases in Juneau. MichaelShephard helped shape the study design and Julie Riley,University of Alaska Cooperative Extension Service, providedinsight into the nursery business and helped us contactnursery owners.

Literature Cited

Anonymous. 2007. Alaska Food and Farm Product Directory andResource Guide. State of Alaska Department of Natural Resources,Division of Agriculture. Online directory: http://www.dnr.state.ak.us/ag/ag_fooddirectory.htm.

Carlson, M. L., I. V. Lapina, M. Shephard, J. S. Conn, R. Densmore, P.Spencer, J. Heys, J. Riley, and J. Nielsen. 2008. Invasive Ranking

System for Non-native Plants of Alaska. Juneau, AK: U.S. ForestService Technical Publication 143. 218 p.

Carlson, M. L. and M. A. Shepherd. 2007. Is the spread of non-nativeplants in Alaska accelerating? Pages 111–127 in T. B. Harrington andS. H. Reichard, eds., Meeting the Challenge: Invasive Plants in PacificNorthwest Ecosystems. Portland, OR: U.S. Department of Agricul-ture, Forest Service, Pacific Northwest Research Station TechnicalReport PNW-GTR-694.

Case, L. T., H. M. Mathers, and A. F. Senesac. 2005. A review of weedcontrol practices in container nurseries. HortTechnol. 15:535–545.

Cross, G. B. and W. A. Skroch. 1992. Quantification of weed seedcontamination and weed development in container nurseries. J.Eviron. Hortic. 10:159–161.

Holt, J. S. 2004. Principles of weed management in agroecosystems andwildlands. Weed Technol. 18:1559–1562.

Hulten, E. 1968. Flora of Alaska and Neighboring Territories. PaloAlto, CA: Stanford University Press. 1008 p.

Klingman, G. C. and F. M. Ashton. 1975. Weed Science: Principles andPractices. New York: John Wiley. 431 p.

Mack, R. N. 2003. Global plant dispersal, naturalization and invasion:pathways, modes and circumstances. Pages 3–30 in G. M. Ruiz and J.T. Carlton, eds. Invasive Species Vectors and Management Strategies.Washington, DC: Island Press.

Muhlenbach, V. 1979. Contribution to the synanthropic (adventive)flora of the railroads in St. Louis, Missouri, U.S.A. Ann. MO. Bot.Gard. 66:1–108.

Proctor, V. W. 1968. Long distance dispersal of seeds by retention in thedigestive tract of birds. Science 160:321–322.

Radosevich, S. R., J. S. Holt, and C. M. Ghersa. 1997. Weed Ecology.Implications for Management. 2nd ed. New York: J. Wiley. Pp. 1–6,335–395.

Reichard, S. E. 1997. Prevention of invasive plant introductions onnational and local levels. Pages 215–227 in J. O. Luken and J. W.Thieret, eds. Assessment and Management of Plant Invasions. NewYork: Springer-Verlag.

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[USDA-ERS] U.S. Department of Agriculture, Economic ResearchService. 2003. Floriculture and Nursery Crops Situation andOutlook. http://www.ers.gov.

Received January 11, 2008, and approved June 30, 2008.

Table 3. The effect of container soil type on number of species and number of seedlings found. Means were separated using the Tukey-Kramer post hoc test (P # 0.05).

Soil type n

Number of species Number of seedlings (#/L)

Broad leavesa Grasses Total Broad leaves Grasses Total

Soil-less potting 146 0.60 a 0.06 a 0.66 a 1.12 a 0.05 a 1.17 aSoil-based mix 102 1.28 b 0.10 a 1.38 b 5.42 b 0.08 a 5.50 bMineral 37 1.32 b 1.32 b 1.59 b 14.33 c 4.33 b 18.66 cANOVA Pr . F , 0.0001 0.002 , 0.0001 , 0.0001 , 0.0001 , 0.0001

a Values within a column followed by different letters are significantly different (P # 0.05) using Tukey-Kramer post hoc test.

336 N Invasive Plant Science and Management 1, October–December 2008