lathyrus latifolius (sp)
everlasting pea.....and it is everlasting :) very pretty but it seeds like crazy and the roots go very deep.

I've had this plant for probably 15 years. Some years I've pulled out every piece I could find and it still managed to come up the next year. As greylady said not only does it have those deep roots, but it seeds freely.

It is very lovely though if you can get it to grow where you want it to (and only where you want it to). I have some growing on a trellis in the lawn and it has no problem with the competition.