I use peppermint oil to keep squirrels out of my planters. Normally I spray a solution mixed with water to keep wasps etc away, but for digging squirrels, I use the straight oil. I'll use a cotton pad with a few drops of the oil on it, and then use some cheap bobby pins to hold it sitting above the soil. When they start digging around my tulips, I just put several drops directly on the soil above the tulip.
Another trick I've read and tried and also works is to place human hair in the planters. I often save the hairs pulled from my hairbrush just for this and hold them on top of the soil again with cheap bobby pins.
