You plant a few trees around 5-10 then you wait till they need spraying with the pestacide stuff and when you've done that a few times you'll be able to click on your Sim and there will be an option to turn them into a plant Sim.
If you use too much on your dog (if you give a 40-pound dose to a 20 pound dog) you risk making him sick. But it will probably recover. If you give too little, then the flea protection may not be complete, or may not last as long as it should.