This plant has powerful medicinal properties, with some toxicity, but at the same time it is also highly nutritious. It contains, per 100g, around 30mg of vitamin C, 1500 mg of Calcium and 45 mg of Iron. The dried leaves contain up to 28g of protein per 100g - a great nutritional supplement. Use only young leaves as edibles, raw, in salads or cooked, as spinach.
In the garden, they perform great function as a “sacrificial plants”, helping to control some fly, moths and aphids that attack some plants.
This strong plant grabs every little chance to grow.
Here is a video to help you to properly identify Sow Thistle:
Spiny-Leaved Sow Thistle, by the New Survivalist
More information on the medicinal properties of this plant:
Earth Medicine Institute: Sonchus oleraceus
Here you can get a thorough course on this plant:
Sonchus oleraceaus - Survival Plants Memory Course