To add to this, it's specifically wheat vs. tares. While it is growing, wheat and tares are virtually indistinguishable from one another. If you try to pull the tares out at this point, you will inevitably pull out the good wheat with it. So, the farmer waits until harvest time which is the time that you can finally tell them apart. The weeds, tares, are then thrown into the fire, while the wheat is gathered up for harvesting.