I have the life house, and I planted a bunch of plants on the indoor balcony just above the first door as you enter. I panted two dandilions just below on the first floor, one on each side of the door.
I came in today and one of my first floor dandilions had a rank 1 fly swarm, so I zapped it. The fly swarm vanished, but I also got the message 'These pests are too strong!'. Sure enough my Helephant ears on the second floor had a rank 3 ghoulish wisp. For kicks and giggles I cast my Windstorm on the dandilion on floor one, and the ghoulish wisp was gone when I went back up to the balcony to check it. This seems to work for all the multi-plant spells (sunlight, rain, music, etc...), and it's saved me a redonkerous amoung of energy. I hope this helps you avid gardeners out!