Oh that makes perfect sense to me, I get it now. I thought perhaps they developed something sticky in the spring from being inside. I spray off my plants when they get dusty too, usually at the end of the dry season when the ones outside are getting their rain wash. I don't have any plants that are really 'inside' in that they are within 4 walls. I do have a lot of plants in pots on terraces and those are the ones I spray to get the dust off. I also spray the ones outside every week or so during the dry season so they can breathe. I think you are right, their pores would get clogged with dirt if they didn't get rinsed. Thanks for explaining.
