COLDWATER - Clearing Oxygen Weed from a Neglected Pond

Clearing Oxygen Weed

from a Neglected Pond by Caryl Simpson article & photos

There are 4 plants commonly called oxygen weed in NZ and 3 of them are Class B noxious weeds. The only permitted one for aquaria and ponds is Elodea canadensis. You may possess Class B noxious weeds but you may not sell, or pass them on to others. This article refers to one of the noxious varieties and I have it in my pond because it is rampant throughout the local waterways and I collected it for free from a local park before I realised it was a noxious weed. My pond is approximately 3m x 8m and has been running for 15 years. A step-bystep article of the build, with photos, was in the May 2006 Aquarium World with an update added in the next issue. The main plants in it are lilies and oxygen weed with a large umbrella palm at one end and a lot of peppermint (or spearmint) at the other. Note how any area not covered by lilies or mint, is choked with oxygen weed. Last year the oxygen weed flowered profusely for the first time I can remember. Before that, I was unaware it flowered at all! It made a beautiful show across the water surface. In 2016, Kaikoura was devastated by a massive earthquake. Damage was also done up and down the east coast, including Marlborough, where I live. Our concrete pond started leaking after this event and we wondered what was best to do about it. Empty it and repair the leak? Empty it and line it? Empty it then fill it in? Decisions,

decisions. Another problem was the pump that powered the filtration and waterfall died and was not repairable. We finally decided we would like to raise the sides to make the pond more formal and to provide seating all the way around. To do this we needed to destroy the rocky waterfall, which explains the pile of rocks on the deck at the far end of the pond in one of the photos. While thinking about how to go about changing the pond, it was left to its own devices with no filtration. Years passed (5 to be exact) with the pond untouched, apart from the dismantled waterfall, as we couldn’t find anyone willing, or able, to do the work we wanted. Due to ill health, my husband and I are unable to do it ourselves. All that happened was the pond got topped up whenever the water level dropped too low. This week we have had to cut down a 42-year-old Japanese silk tree, Albizia julibrissin, as it would be shading the solar panels being installed on the roof. It also overhung the pond and made an awful mess each winter as it dropped leaves and branches into the water, forming a thick, matted, mass on the bottom. They say you shouldn’t site a pond under a tree but it did look beautiful and provided summer shade so green water was never a problem. You can see the trunk of this tree in the full pond photo near the end of this article. It is on the right-hand side at the far end. As the tree was being cut down, I looked at the pond for the first time in ages and realised the oxygen weed had got out of hand. It is called ‘invasive’ for a reason! The whole surface, that wasn’t covered in lilies, was a thick mat of oxygen weed. It was

wrapped around the lilies as well and there was very little surface area for the fish to come up and sun themselves. How to get rid of the weed? As it is Class B, you are not allowed to throw it back into the waterway from which it came. It can’t go to a recycling facility for garden waste either. We decided to haul it out, let it dry in the sun, then dig it into the garden. This involved wading into water that was just a little higher than the top of gumboots, so I wore jandals, and pulling the weed up by hand. For every small handful, about three times more came up from under the surface. It was entangled in the other plants and meshed with all the debris which had fallen from the silk tree. The photo shows just one of the piles of weed pulled from the pond – and that was only after clearing half of it! There were two more piles elsewhere and I still have to clear the other half of the pond but didn’t want to do too much at once in case the sudden clear water surface created an algae bloom. So remember - watch what sort of oxygen weed you add to your pond and make sure you don’t let it get out of hand, which it does quickly and easily. A reasonable amount is good for the water but left to multiply, it will choke other plants and form a thick mat over the water surface and form even more of a mat under the surface, where you can’t see, making it hard for the fish to swim freely. Within hours of clearing one end of the pond, I saw fish sunning themselves on the surface. I wasn’t even sure if there were still any fish in there, it had been so long since I had seen any. Hopefully we will find someone to fix the pond for us and make the required changes, or we might just fill it in.

Caryl Simpson