Carolyn,
I vaguely remembered this quote but couldn't recall the source precisely. Looked it up and here it is.
"We can in fact only define a weed, mutatis mutandis, in terms of the well-known definition of dirt - as matter out of place. What we call a weed is in fact merely a plant growing where we do not want it. ~E.J. Salisbury, The Living Garden, 1935"