[urgent help needed] How do I trigger a procedure immediately upon natural spawning of blocks?

How would I make plants with multiple blocks like this one repeat in clusters across large areas, like a plant? I have a procedure to generate one of these upon placement of a "seed" block, which I want to be fired immediately once it's naturally generated, or at least every few ticks so that when the player comes across it, it's full size by the time they come across it. However, when I try with the "On Placed" trigger, the "seed" just stays there and does nothing, even though I enabled "force plant ticking", however it does work when I place it by hand. Can someone help me out please?