When it is sown (οταν σπαρη). Second aorist passive subjunctive of σπειρω. Alone in Mark and repeated in verse Mark 4:32.

Less than all the seeds (μικροτερον παντων των σπερματων). Comparative adjective with the ablative case after it. Hyperbole, of course, but clearly meaning that from a very small seed a large plant grows, the gradual pervasive expansive power of the kingdom of God.