Q.How to kill a weed that has overtaken my dad’s garden.
Years ago my dad’s friend suggested that he put some of his sheep manure on his garden. Ever since then there is a weed that has overtaken his garden. I even tilled my garden and it started growing here. Is there a treatment we could do in the spring to kill it? The first picture is it at full bloom. The second picture is of it taking over this row.

Certified GKH Gardening Expert
Use a pre-emergent in spring to eradicate quickweed. If any remain dig by hand or use a post-emergent herbicide.
https://www.gardeningknowhow.com/plant-problems/weeds/pre-emergent-herbicides.htm
https://www.gardeningknowhow.com/plant-problems/weeds/post-emergent-herbicides.htm