I live in south Burlington, ON. Likely as the result of a load of earth, for the past 2 or 3 years I have increasingly had a problem with perennial sow weed in one of my west facing flower beds. I dig them up but the runners are very difficult to pull completely out. Are there any solutions for this invasive weed?
Perennial weeds, particularly those that spread via their rhizomatic root system as well as by seeds, can be the toughest to eradicate. The perennial sow thistle, Sonchus arvensis, is one of these. It is an aggressive vegetative spreader: even the smallest piece of its root left in the ground can produce a new plant, and every plant can produce as many as 10,000 seeds.
Although there is no magic formula, there are some practices that work better than others. For this kind of weed, many experts recommend using a sharp knife to sever the plant’s stem from the root just under the surface of the soil, rather than attempting to dig it out, removing the plant’s food source and ultimately starving it of energy. Attempting to dig the weed out can result in inadvertently leaving pieces of the root system in the ground intact, as you have found, and this will encourage new growth from the root. Cultivation can also result in a surge of weed growth: turning the soil can bring dormant seeds to the sunlight, thus encouraging their germination.
Here is an article that outlines some steps which are relevant to the control of perennial weeds: https://www.finegardening.com/six-tips-effective-weed-control
You might also be interested in Omafra’s article on sow thistle: