Charles Spurgeon Commentary Matthew 13:28-30

"And he said unto them, An enemy hath done this. And the servants say unto him, Wilt thou then that we go and gather them up? But he saith, Nay; lest haply while ye gather up the tares, ye root up the wheat with them. Let both grow together until the harvest: and in the time of the harvest I will say to the reapers, Gather up first the tares, and bind them in bundles to burn them; but gather the wheat into my barn." — Matthew 13:28-30 (ASV)

The servants said unto him, Wilt thou then that we go and gather them up? But he said, Nay; lest while ye gather up the tares, ye root up also the wheat with them. Let both grow together until the harvest: and in the time of harvest I will say to the reapers, Gather ye together first the tares, and bind them in bundles to burn them: but gather the wheat into my barn.

The separation will be more timely, more easily, and more accurately done when both have been fully developed—when the wheat has come to its fullness, and the counterfeit wheat has ripened.