In the parable of the wheat and the tares (weeds), a landowner discovers that his wheat has been sown with tares, and decides to wait until harvest to separate the two. If you've ever weeded a field or a garden, you know that letting weeds grow just makes the work of weeding harder. This makes the landowner's decision look like one of the odd or unexpected twists that show up in Jesus' parables.
But it's not, because tares aren't just any old weed. Tares look like wheat when they're small. You can't tell you have weeds until the tares are full grown.
And by then the roots of the plants are so entangled that you can't pull out the tares without destroying the wheat. If tares are so much like wheat, why not keep them both? Because while wheat will make you bread, tares will make you sick.
So the landowner's decision actually makes sense. It also gives us a visual image for the parable, and a stark contrast pointing towards meaning - two things that start out very much alike, but that end up very differently. More on Sunday's Gospel tomorrow.