Both: I go by numbers, soil and convenience.
If it's just a few in soft soil, I pull them when I happen to be right there.
If they're growing in hard clay, and there's a big enough patch or weeds, it's worth taking a walk to get a hoe or cultivator to scrape them off at the soil line en mass. (Or go deeper with a mattock.)
Maybe once or twice per year, if the whole yard is overgrown, and especially if wild blackerries are taking over, I'll mix a gallon of herbicide.
Corey
