Question
Hi Robyn.  I recently bought a new house which has an ornamental pond with waterfall -- maybe 8 x 15 feet.  The pond has in it two buckets with irises in them, and a few water-lillies, and I think there are frogs in it.  No fish (the neighbor said the prior owner tried fish but herons would show up and catch the fish and put holes in the liner besides).

The problem I write about is the duckweed ... I can skim it one day and it totally covers the pond within a few days later.  What should I use to control the duckweed that won't hurt the frogs and other plants?  Also, if we get a dog who drinks the pond water I don't want any chemicals to hurt the dog.

Thanks!

-David

Answer
If the pond is a few feet deep, it's probably about 1700 gallons or less if more shallow.  I've never had herons put holes in the liner but they certainly eat fish which is why my pond is netted.  Since the herons got all the fish, I'm guessing the pond is pretty shallow.  If you ever redo it, make it deeper with steep sides to deter herons.

Duckweed is eaten by goldfish, koi, ducks, and turtles.  Physically removal helps temporarily.  Duckweed also hates moving water so a fountain, waterfall, or other moving water (more than what you have) will reduce its numbers.  If you put a pump outlet coming out across the surface horizontally, that would move it a lot.  Other floating plants may also compete with it such as water hyacinth, water lettuce, and salvinia (all tropicals though).  I don't suggest herbicides because they would kill or harm most everything including the duckweed.  There is no safe water additive that would kill the duckweed and not the other plants.  Herons normally hunt by sight so they would have trouble seeing fish in that stuff so my guess is that when there were fish in there before, they ate most of the duckweed.  I can't keep duckweed alive in my pond with goldfish and koi but it's in my other ponds.  Good luck!

Robyn
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