As noted, the concerns are Serendipity, along with Fractal and maybe Mindgate.
Without those two/three cards, Fertilizer | Mulch may as well be free. (Using 3
for a mulligan is a bit too much. For the "Put-Pillars-Into-Your-Deck" Strategy, you're increasing the chance of drawing Pillars during your regular draw and Mulch is pretty useless after the first turn.)