I hadn't thought of leaving holes, ladybugs, butterflies and bees as possible ways to increase plant spawn. I get some spawning without leaving holes and with fairly diligent bug harvesting for coins. However, I don't get enough spawns for my tastes, so I should experiment with the idea. I know that at least one of those actions has positive effects, but it's not related to plant spawn. At least one of those actions has negative effects unrelated to plant spawn, too, so be careful out there!
Hmm. I'm going to have to try this, too! I haven't had many plants spawn recently, and I want more to appear. The other thing I will try to do is spread the flowers in my botanical garden out a little, so there's room for others to spawn, since I read that if your plants are just a little spread out, you will get more plant spawns.