I've read that fall is the best time to spray buckthorn. Heres from an article put out buy the WI DNR
Mark honeysuckle and buckthorn for fall control
Early spring is also a good time to be on the lookout for honeysuckle and buck-thorn, Kearns says. These invasive shrubs leaf out early. Mark them with your GPS unit or flagging tape and come back in the fall to control them. Controlling these shrubs involves cutting the stems and treating them with an herbicide. Spring treatments usually aren’t as effective be-cause the sap flow pushes the herbicide out.