a weed that passes for wheat: darnel, which cannot be told apart from the crop until the grain forms
Darnel is a grass that grows wherever wheat grows around the Mediterranean and the Middle East, and while both are young almost nobody can tell the two apart. The difference shows only when the heads come out: wheat ripens into heavy golden ears that bend over, while this weed's seed stays small, thin, and dark. Its grain usually carries a fungus that makes anyone who eats it sick and dizzy, so the plant is not merely useless but harmful, and its roots knot together with the wheat's underground.
All eight uses of the word sit in one place, the story of the weeds in Matthew 13 and the explanation Jesus gives afterward, and the story leans on exactly what this plant does. The weeds are invisible while the crop is young, they give themselves away only once the wheat (σῖτος) heads out, and by then pulling them would tear up the wheat with them, so the owner tells his workers to leave both until harvest.
It helps to see how this weed differs from the other unwanted things a harvest turns up:
- thorns (ἄκανθα) are plainly not the crop and choke it from outside
- chaff (ἄχυρον) is the wheat's own husk, worthless but harmless
- this weed passes for wheat right up to the harvest, which is the whole point of the picture
In Jesus's explanation the weeds stand for people who belong to the evil one, but that is what the image represents inside the story, not what the word itself means. In all eight verses it names the plant.