"We are told in the parable the farmer is the Son of Man and He planted good seed in His field (vv. 24, 37), then the enemy, Satan, planted weeds (tares or darnel). The enemy is no originating genius. He is an imitator. He produces counterfeits of the works of God. It is important for Christians to know this, so that they may be on their guard. If we study Scripture we shall not be ignorant of his devices (2 Corinthians 2:11). Note if you will, the enemy did not plant thorns or thistles. If he did this it would had been easily detected, and there would be no difficulty in distinguishing the false from the true. He planted "tares," or "darnel." It is impossible to distinguish the darnel from the wheat until the wheat begins to bear fruit. When the "servants" of the Farmer discovered what the enemy had done, they wanted to root out the darnel (v. 28). He told them not to root out the darnel. If they root out the darnel, they will uproot the wheat (v. 29) because the darnel wraps itself around the roots of the wheat. It is only when the wheat is harvested can the wheat and darnel be safely separated. As these imitation blades, green and flourishing, growing side by side with the real wheat, it appears there will be a good harvest. However, appearances are deceptive, and the harvest will be a disappointment and mockery to those who have spent so much time and labor on their cultivation."
(For full text see http://www.thewordofgodonline.net/wheatandtares.htm)