The following 393 exotic species and infraspecies are currently categorized as waifs or very sporadic escapes from cultivation in Washington. Taxa may be included in this list for the following reasons:
Waifs known only from a relatively few old collections and not collected since (e.g., ballast plants).
Exotic taxa that have been documented as naturalized from only one or a few sites, especially if in urban areas.
Garden or crop plants that repeatedly occur only as sporadic individuals or small patches of plants temporarily appearing around dump sites, gardens, cultivated fields, recently seeded areas, and the likes, presumably without persisting or spreading to form naturalized populations.
Ornamentals and garden plants known only from a few adventive populations in urban settings (e.g., around arboreta, gardens, greenhouses, universities, landscaping beds, lawns, rockeries, sidewalk cracks), presumably not far from where originally planted.
Ornamental and garden plants represented only by seedlings or saplings near cultivated individuals.
Exotic taxa persisting at 1 or a few locations where planted, and slightly spreading to form populations at those sites, but not spreading to new sites.
Long-lived trees and shrubs that persist only where planted (e.g., old homesteads, vacant lots, highway plantings, or restoration/reclamation sites).
Species on noxious weed lists that are now eradicated.
Species on noxious weed lists that have not yet been documented from Washington with an herbarium specimen.