I weeded this garden a week ago, but you'd never know it. My arch enemy, oxalis, has popped up again.
I've been battling this weed for years. To pull it, I water the garden and grab the stems near the ground, hoping to get the whole plant as well as its roots. But the stems are delicate and tend to snap off. If it's gone to seed, the battle is already lost.
Part of the issue is that I haven't mulched the perennials the way I usually do, so weekly weeding is a must.
Here's another pest that I've had for the last few years. At least Virginia copperleaf is easy to pull up. It can reach three feet at maturity, but I haven't let it get that big!

And my old friend nightshade has sprung up again, growing remarkably fast. Again, it's easy to pull up if the soil is wet, but you have to grasp the stem at ground level to get the roots or the plant will snap off and regrow. The nightshades have some cool-looking flowers — they're related to tomatoes and potatoes — and some of the plants are poisonous. Again, if they seed they'll end up everywhere, so it's best to pull them before they flower. Here's some nightshade:

I dig up or pull all but the hardest-to-eliminate weeds, and resort to a weed killer only for plants like dandelions that are growing in a crack in the pavement or for really hard to kill weeds like thistles.
If you go the chemical route, make sure you pick the right chemical and read the label carefully. Any chemical with glyphosate in it is an indiscriminate plant killer, and will take out your grass and other plants as well as what you're trying to kill. It will kill the weeds in your lawn as well as the grass, as the owner of this lawn discovered:

Here's a guide to some of Minnesota's most common weeds: www.extension.umn.edu/gardeninfo/weedid/index.html
Do you have a weed that's driving you crazy?