… but with what? Carl Fink submitted this Dark Side of the Horse, commenting: “So … planting seeds and clams pop out of the ground. And this is funny? A pun? What?“
…
P.S. To me they looked more like gigantic ravioli, but this strip also reminded me of those little joke packets of “Doughnut Seeds” (containing Cheerios or Froot Loops).
Comics I Don't Understand 
Could be large nuts in their shells? But why would they be that large!
I think the seeds grew.
I think they could be much larger versions of the seeds he is sowing… instead of the seeds becoming some sort of plant they just get vastly bigger.
I’m with narmitaj – he planted seeds which grew into big seeds. One wonders if he can then plant those seeds to grow even bigger seeds, and repeat ad nauseam.
He planted oyster crackers and grew oysters.
Instead of a flower bed, he got a clam bed?