Anyone who's ever grown peas, at least in my neck of the woods, can appreciate the parable of the wheat and the weeds. There's this plant—I believe it's called "pigweed"—that invariably comes up between the pea plants and even looks a bit like them at first. Once the shallow-rooted peas start sending out their tendrils, it's very difficult to pull up the pigweed without yanking out a few plants with it. The peas are what count, of course, so the wise gardener leaves well enough alone.
What's central to the parable of the wheat and the weeds is the preciousness of the wheat. The landowner refuses to lose any of it in order to get rid of the weeds. Readers of the Gospels will recognize a familiar theme, the same one that informs the parables of the lost sheep and the lost coin--and even the gathering up of those 12 baskets of leftover fragments after the feeding of the 5,000. In God's economy nothing is wasted; nothing good is counted as expendable.