The weed described by the Greek term often translated 'tares' or 'weeds' is commonly understood as darnel, a grass that can resemble wheat during its early growth. Once the plants mature, the distinction becomes easier to see. Attempting to remove weeds whose roots are intertwined with the grain could also damage the crop.
Jesus later explains the harvest imagery in terms of final judgment. Harvest was already a natural biblical metaphor for divine judgment, and burning unwanted vegetation was an ordinary agricultural practice.