After some use, plastic flowerpots can get a white, grainy look to them, especially if you use calcium and other minerals in your flower fertilizer or if you have hard water. This buildup of calcium is easily cleaned off to keep your plastic flowerpots looking as clean and new as when you first got them. Since the buildup is a basic alkaline substance, slightly acidic cleaners like vinegar work best to remove it.
If your flowerpots are occupied by flowers, transfer them to temporary pots to be able to clean more easily. This is easier if the plants are in pot liners you can move between pots.
Mix a cleaning solution of white vinegar and warm water. Add a half-cup of white vinegar to 2 cups of water, or equivalent proportions if you don't need that much.
Start out soft. Soak a cloth or rag in the solution and scrub down the flowerpot where buildup is showing. If this doesn't get it all off, move to a harder scrubbing method like a scrub sponge or abrasive pad. Use an old toothbrush if there are small details or lips to get clean.
Let dry and check for signs of calcium buildup. If it is still there after your initial cleaning, let the pots sit overnight in vinegar. Soak paper towels in vinegar-water solution, wrap the pots in them, and set them in plastic bags to keep the solution from evaporating. Set the bags on a tile or cement floor, or in a large plastic tub, so you don't have to worry about leaks.
Take off the vinegar-soaked towels and wipe pots down with a clean cloth. Much of the buildup should come off, but you may need to scrub on some hardened spots. Clean the pots thoroughly with hot water to remove any leftover vinegar, which is not healthy for plants.