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Hairy Galinsoga, otherwise known as Quickweed, is considered a noxious weed throughout the US and Canada. As you have observed the most effective way to control it is by pulling expecially before it's seeds ripen and getg away from the mother plant. Putting down a good deep mulch (2 to 3 inches) will help deter it's advances. For chemical control a pre-emerent (trifluralin or Preen) applied after your garden seeds have sprouted will also help control the weed which usually spreads through it' seeds. Read the label carefully to see if a reapplication is necessary later in the season. Temperatures should be from 60 to the low 80's for it to be effective. Also a non-selective herbacide containing glyphosate (Round-up) will work. However, this cheical will killl everything that it touches so it's use must be carefully governed. Also for your reeading pleasure: www.ppws.vt.edu/scott/weed_id/galci.htm from Vermont
www.weedinfo.ca/en/weed-index/view/id/gasc from Canada