Unwanted Seedlings Volunteers
Crossbow is a combination of the chemicals 2,4-d and triclopyr.
Are these volunteers suckering from roots of a tree that is still living or from roots from a tree that was cut down? If from a tree that is still living, then even though you spray the suckers, the plant has energy from the main plant to continue to send up suckers. If it is from a tree that was removed, it may take several years of treatment to exhaust the stored energy in the remaining root system.