Pond Boss Forums AQUATIC VEGETATION Controlling unwanted plants A Theory.....

Just recently drained my pond and it's filling back up slowly as I have a ditch shut off that feeds it. I drained this pond because of bullhead problems, mainly ,but also to try to discourage my duckweed and watermeal.
Below my pond I have another pond for raising fathead minnows, it's feed underground by water that leaks under the main pond's dyke, the water in the two ponds being about 50' apart.
So basically, the two ponds share the same water but the lower pond has it's water filtered by going underground. The main pond has a bad case of duckweed and watermeal but the lower pond is unaffected by the stuff.
With all this in mind, my thinking is to dig a hole in my ditch that feeds the main pond and to dump about 10 tons of #2 stone in to purify the water and see if this helps discourage the return of the unwanted plantlife. Oh, forgot to mention that the water feeding the main pond that's coming down the ditch is coming from a hemlock swamp and it's very acidic and whatever nutrients it has I blame for my weeds.
Comments and questions welcome.

I don't think the rock in the incoming ditch will make a difference in the duckweed. Leaving the main pond empty for the winter might make more of a difference. I drained a small shallow pond last year for the exact same reason (bullhead) and this year had no duckweed at all. It had been blanketed with the stuff for several years. The filamentous algae came back big time though. Also the bullhead somehow survived as I saw the schools of black fry this summer.

IIRC, the seeds from DW fall to the bottom and then float back up the next spring.

Like most plants, Duck Weed grows new plants from seeds. But it can also clone a new plant from a piece of leaf. Most plants can’t do this.
Duckweeds survive freezing as turions or seeds, which sink to the bottom after they are released from the mother fronds.
Dormant buds and seeds allow many species of duckweeds to survive periods that would otherwise be lethal. Some species can form dormant buds (turions), which are resistant to frost (but not to drought). Many species can flower and set seed. Duckweed seeds are resistant to cold and drought. Not all species of duckweed are known to flower. Those species may have lost the ability to flower, or perhaps we do not yet know the conditions which will cause them to flower.
Mud and silt can protect duckweeds: When their water dries up, duckweeds may become stranded on mud. Duckweed fronds are drought sensitive, but they will survive on a surface of wet mud. The small immature fronds and buds hidden inside the mother frond dry up last and have greater resistance to drought. These fronds and buds will grow out and re-establish duckweed plants if they are rewatered before it is too late. Winter-dormant seeds and turions resting on the bottom are insulated from the cold and drying conditions on the surface. Silt and sediments on the bottom may cover these dormant strucures and help protect them until good growing conditions return.

My most effective means of controlling duckweed has been surface agitation, either by aeration, or circulation. Ponds in my region that sit low and out of the wind have the worst problems. Ponds that are exposed to the wind, or that I've put a Kasco circulator on are always duckweed free.

As DJT hinted at, these nutrients, if unused by duckweed will be available for other plants.

BTW, your proposed rock filter may provide a myriad of benefits, but duckweed control won't be one of them.

A colleague of mine recently told me that goldfish with eat DW.
My only experience with this has been in a half wine barrel pond in my backyard which I stocked with goldfish and DW before I was given this info. The duckweed only lasted a couple of weeks but I can't say for sure whether that was the work of the goldfish or some other environmental factor. Does anyone know for sure?

One other thing about duckweed is that when it achieves significant coverage of the pond you will generally see less algae...not only from nutrient removal but also because the DW will block out virtually all the sunlight in the water column. No sunlight = no algae. This can be good or bad depending on your goals.

WW -
I can confirm that goldfish will indeed eat duckweed. In a pond though, they may find other stuff they prefer over the DW.

I know common carp will eat DW as well. But like Weissguy said, in an open pond environment, there are usually more preferred foods available.