Question
Dear Jonathan,

I have a small pond in my garden with no filter and this year during the summer in England it has been completely covered by duckweed. There are still some plants growing in the pond such as lilies and iris. There are no fish but frogs were present during the Spring. When I did have some cold water fish I never had this problem.

I changed the water as much as possible but it is difficult to remove all this duckweed algae nd it soon covered the pond again. I tried a solution from a garden shop which is meant to remove duckweed but has had little success.

Please can advise be given how I can remove this duckweed and prevent it returning again next year. The pond has some shade from surrounding trees and plants.

Thank you

Brent

Answer
I'm sorry that I don't have any wonderful news for you.  The only successful way I have found to eradicate duckweed is to manually remove every last bit.  This is most easily done right before (or during) winter because the growth will be slower and it will be easier to keep up with.  If you still have no fish come this time, you can drain and clean the pond completely (or hire a professional) but with fish in the pond you have a biological equilibrium you don't want to tamper with.