Charles Ellicott 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)

The least of all seeds — The description is, of course, popular and need not be pressed with microscopical exactness.

The greatest among herbs — More literally, greater than the herbs—that is, belonging to a higher order of vegetation.