Plant Doctor Archive

Water Gardens — River boulders

I AM told that river boulders are bad for a pond. Why is this?

IT is not so much that they are bad, it's just that river stones tend to make the water very alkaline and this has consequences. If you do use them, make sure you monitor and adjust the water pH regularly, especially for the first year.

I recommend Pond Balance for this purpose. Make sure you follow the instructions, as sudden large changes in pH are harmful to fish.

The worst consequence of a high pH is that it encourages blanket weed, a particularly nasty and difficult to control fibrous algae.

On a personal note, I do not like river stones in ponds as I have had to clean out many ponds with them. It is a very time consuming and tedious task I assure you!

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.