Plant Doctor Archive

Water Gardens — Bio filters

At what point in cold weather are bio filters on ponds no longer effective due to the low temperature?

THE nitrifying bacteria that purify pond water in a biological filter are active down to about 4°C. If you live in the North Island, then you will not need to worry about this at all. If you live in Invercargill then your filter probably ceased operating efficiently back in February.

However, I generally recommend leaving bio filters running all year. If your area experiences deep frosts though, you should definitely disconnect and take inside your UV filter as the cold can crack the quartz glass sleeve.

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