The information was taken from a Montana State University Web Site. Good growers decades and centuries ago, as well as now, were very proficient at using these methods. Perhaps a short review of these methods will remind us of some the general principles involved. Deep-rooted perennial weeds can be killed through root starvation. This involves a deep tillage operation in the spring, followed by less severe tillage every 3 weeks through the season.
Nobody can afford that nowadays.
Work at OSU in the late 1930's found that only one season was required for Canada thistle. An important principle is involved here. When a perennial weed is disrupted by tillage, it starts sending up a new shoot. Even after shoots appear above ground, it takes a while before new leaves start producing enough CHO's to support newer leaves. Work back in the mid-1930's found that no replenishment of root reserves began for about 3 weeks after the last tillage.