A.T. Robertson Commentary Matthew 13:32

"which indeed is less than all seeds; but when it is grown, it is greater than the herbs, and becometh a tree, so that the birds of the heaven come and lodge in the branches thereof." — Matthew 13:32 (ASV)

A tree (δενδρον). "Not in nature, but in size" (Bruce). "An excusable exaggeration in popular discourse."