24 Another parable He put forth to them, saying: “The kingdom of heaven is like a man who sowed good seed in his field;
25 but while men slept, his enemy came and sowed tares among the wheat and went his way.
26 But when the grain had sprouted and produced a crop, then the tares also appeared.
27 So the servants of the owner came and said to him, ‘Sir, did you not sow good seed in your field? How then does it have tares?’
28 He said to them, ‘An enemy has done this.’ The servants said to him, ‘Do you want us then to go and gather them up?’
29 But he said, ‘No, lest while you gather up the tares you also uproot the wheat with them.
30 Let both grow together until the harvest, and at the time of harvest I will say to the reapers, “First gather together the tares and bind them in bundles to burn them, but gather the wheat into my barn.”’”
The parable is explained in Matthew 13:36-43.
The explanation is given to the disciples alone (v.36), which makes it clear that the parable itself, along with the next two (the mustard seed and the leaven) were given to the crowd.
The method so far as the foe is concerned marks his wiliness, his cowardliness, his dastardly determination to harm. He was a trespasser, full of subtlety, animated by malice. There was no other motive in his action. He could gain nothing by sowing another’s field with darnel, for it is not a saleable produce, and no profit can be made out of its growth. It is as worthless to the man who sows it as to the owner of the field. This sowing, then, was the result of pure malice. — Morgan, page 151.
According to the common view, these tares represent what is botanically known as the “bearded darnel” (lolium temulentum), a poisonous rye-grass, very common in the East, entirely like wheat until the ear appears, or else (according to some), the “creeping wheat” or “couch-grass” (triticum repens), of which the roots creep underground and become intertwined with those of the wheat. But the parable gains in meaning if we bear in mind that, according to ancient Jewish (and indeed, modern Eastern) ideas, the tares were not of different seed, but only a degenerate kind of wheat. Whether in legend or symbol, Rabbinism has it that even the ground had been guilty of fornication before the judgment of the Flood, so that when wheat was sown tares sprang up. The Jewish hearers of Jesus would, therefore, think of these tares as degenerate kind of wheat, originally sprung at the time of the Flood, through the corruptness of the earth, but now, alas! so common in their fields; wholly undistinguishable from the wheat, till the fruit appeared: noxious, poisonous, and requiring to be separated from the wheat, if the latter was not to become useless. — Pentecost, page 216.