On the northern edge of the Great Plains in Western Canada, these seem
to be good indicators of past historic occupation, especially with farm
sites:
Caragana hedges (C. arborescens)
Lilacs
Box Elder (aka 'Manitoba Maple' in this region)
Conifers of any type, outside of native growing areas
The rather severe weather fluctuations in this region don't seem to
allow many other domesticated perennials to survive without human care,
although some examples such as rhubarb, hardier rose and
strawberry/raspberry varieties, and a variety of shrubs (e.g.
cotoneaster) may last for a considerable time on their own.
Larry Buhr
Univ. of Nevada, Reno