Perennial Weeds

Perennial weeds typically rely more on vegetative means of replication than on seed. Rhizomatous species such as Canada thistle ( Cirsium arvense) in the photograph multiply quickly when the rhizomes are broken into fragments by tillage. In the profile, rhizomes spread laterally at a depth of almost 60 cm, thus they are are difficult to control by tillage alone.

One (non-chemical) way of controlling them is to fallow the fields for about 6-8 weeks once in four years. The thistle plants are allowed to grow up to their flowering stage, then they are mowed. Mowing is repeated 2 or 3 times at 3 week intervals. The early growth of thistle lowers sugar reserves in the rhizomes, thus by successive mowings, sugars are reduced to very low levels. Eventually there are not enough reserves to allow the plants to emerge from depth in the soil, and they die. This fallow is followed by planting a heavy feeding, deep rooted crop in the fall to further supress the thistle plants.