The flowers and even the leaves on this plant remind me of wisteria. What's different is that the flowers hang on upward spikes with Cow Vetch, Vicia cracca, rather than cascading downward.
From A Manual of Weeds:
"The most widely distributed of the Vetches, being very common in both Europe and Asia. Like nearly all of the Legume Family it has root tubercles which cause it to enrich the soil where it grows; it furnishes good forage and good hay, but its tough, creeping rootstocks make it so difficult of removal from places where it is not wanted that it must often be rated as a bad weed.
Stems tufted, slender, angled, branching, two to four feet long, climbing by means of tendrils at the tips of the pinnately compound leaves and forming dense mats, smothering grass or other plants that grow beneath, and entangling and pulling down the crop when growing in a grain field. Leaves sessile or nearly so, composed of eighteen to twenty- four thin, narrowly oblong, entire bristle-tipped leaflets. The whole plant is covered with fine, close- pressed hairs and is a soft olive green in color. Flowers numerous, on slender, one-sided axillary racemes about as long as the leaves, the standard and wings of the corollas being narrower than in the preceding species; each blossom is about a half-inch long, violet-blue in color, and hangs reflexed on its stalk. Pods smooth, about an inch in length, and contain five to eight small, dark brown, globular seeds. They are frequently an impurity of grass and clover seeds and are somewhat troublesome to remove."
Georgia, Ada E. A Manual of Weeds. New York, 1919. 248. Web. Google Book Search. 26 Jun, 2009.