How do I know if a plant is a weed?
A weed is simply a plant out of place. By this definition, any plant could be a weed if growing in a location where it is not wanted.
There are some species of plants, such as thistle and poison ivy, that are rarely wanted in the lawn or garden setting and are often growing out of place. These plants are frequently classified as a weed. They are often tolerant of a wide range of environmental growing conditions making them more likely to establish themselves in the garden. These weedy species often propgate themselves easily by seed and/or by runners, stolons, or other vegetative means.
No one species of plant is always considered a weed in all settings and no species of plant is never considered a weed - although some species are more often growing where we don't want them than others!