Sow to yourselves in righteousness, Reap according to kindness. Break up your fallow ground; For it is time to seek Yahweh, Until he comes and rains righteousness on you.
So, because they had broken the covenant with God, they were swearing falsely when they made that covenant with God, thus God's judgment is gonna spring up upon them, just like weeds, the poison weed of hemlock just grows up in the fields.