As you will know, in London we are in an extremely hard water area which results in lime scale forming on virtually every surface that water comes into contact with. - If you have a water softening system installed, or are lucky enough to be in a soft water area, then you will not experience this problem.

Recently I experimented with a commercial toilet cleaner to remove the lime scale that builds up on plastic plant pots.  Unlike myself, you may not need to clean hundreds of pots each new growing season, but this tip is just as useful if you only need to clean just a few.

If you fill a plastic bucket (3 gall) with water to within two inches of the top, and then add 1/3rd of a bottle of toilet cleaner (normally it is less than 10% Hydrochloric acid).  Any commercial cleaner should be ok (such as Harpic or own brand labels).  Submerge the pots into this solution for a couple of days and every trace of lime scale will be removed, even very heavy concentrations.  The pots can be stacked and do not have to be placed in separately. It will also sterilise the pots at the same time.

After the second day just remove the pots and rinse under running water wiping them with a sponge.  If you do try this method please use rubber gloves and beware of splashes - you may not be like me with rhino skin!  This was tested purely on various sized plastic pots - I did not try this method to clean terracotta pots which may have different results.