Vol. 54—2000 - NorthEastern Weed Science Society

RESULTS AND DISCUSSION
Several newly released herbaceous perennialsoffered in the wholesale trade in 1998 and 1999
demonstrated the potential for significantinter-nursery spread and reclassificationas weeds. Plant
movement occurred in several different ways: from container to container, from container to surround
ing soil. from container to gaps in protective landscape fabric and from containers to soil under or on
sales benches. The reproductive morphologies were often different in each potentially weedy species;
their identity, methods and seasons of dispersal are given in Table 1.
It is interesting to note that neither nursery actuallybought any of the first nine plants listed in
Table 1; they anived into each nursery as weeds in other purchased plant material in 1998.
For comparativepurposes, Appendix1providesa compilationof herbaceousperennials
whose aggressive and/or invasivehabits have been documented and referenced intrade publications.
The distance each of these nine most-invasiveplants was able to move in the course of one
season varied with the plant in question and ranged from several feet to several acres, Table 2. For the
purposes of this paper. data relative to spread, aggressivenessandhost tolerance will be limited to
those nine previously identifiedas novel and potentiallyexplosive.
Several cw were able to infest more than one pot of nearly every other plant species in the
nursery. others infested fewer containers in a wide array species while others only infested
herbaceous perennials. Several cw set and dispersedseed sufficientlyearly to infest annualplants
located on the opposite side of the nursery, many of the cw were able to infest established
containerized or balled and burlapped trees and shrubs, Table 3.
The herbaceous perennial crop plants were grouped into categories according to the relative
amount of soil visible at crop maturity, August 1999. An estimated strength value was assignedto
each category as a method of inferringcrop resistance to these new weeds or weed strength against
crops that are occasionally weedy themselves, Table 4. Category I plants had 75% or more soil
visible and were considered weakly resistant to weed invasion while category 5 plants filledthe pots,
had less than 5% soil visibleand were considered strongly resistant to weed infestation.
The ability of novel weedy plants to infest crops by categories varied with the weed and the
category; many of these new cw were able to infest pots despite lack of visible soil, Table 5.
Herbicide use suppressed the development of cw principallyby delaying the infestation of host
plants until later in the season. Each of the herbicidescommonly used by these nurseries effectively
prevented cw in both sale and stock plants for 8-12 weeks depending on the compound. Little
carryoverpast their expecteddurationoccurred and rapid movementof cw followedthe declineof
herbicidal activity, Table 6.
CONCLUSIONS:
Of the many new cultivars and species availableto the nursery trade in recent years. nine
plants exhibit tendencies that would classifythem as weeds and potentiallyclassifY them as invasive
weeds. Often, these plants demonstrated more thanone reproductive morphology; these usually
occurred simultaneouslyduring the same andlor several season(s). These plants exhibited rapid spread
across the nursery and were capable of infestingplants in hundreds of square feet to several acres.
These novel weeds appearedin a wide variety of herbaceous perennials, containerized, B&B trees and
shrubs, annual flats and jumbo annual pots. The most invasive of these plants infested plants in pots
whose soil availabilitiesranged from little or none to nearly aUof the pot area and none of these cw
appeared inhibitedby or restricted from pots containing any particular species. The mechanisms of
crop stress and occasional crop loss incited by cw are undetermined; several possibilitiesinclude:
physical crowding. induction of nutrient deficiencies,induction of water stress, contaminating
pathologies or entomologies or alleleopathy.
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