NEW Garden doctor
Nick Bailey examines this messy problem and offers his advice on the cure
How to tackle overgrown ponds
Ponds are among the most diverse ecosystems in our gardens. They teem with life, from the plants we introduce to fish, frogs and invertebrates. But left to their own devices, they can easily become overrun with triffid-like aquatic plants, algae, duckweed and a host of other ‘life’, which can turn a once beautiful feature into a silt-filled quagmire. I’ve rescued many ponds over the years and now is the ideal time to tackle yours if it has turned feral, is infested with duckweed, blanket weed or other algae, or is simply a smidgen wilder than first planned.
Balancing act
For a healthy pond, you need to decrease the amount of nitrogen in the water and increase the oxygen. The key to reducing nutrient levels is to remove any dead leaves, stems and plant debris from the pond – if you didn’t do this last autumn, do it now. Make sure you also have enough beneficial plants. Waterlilies are great for shading ponds and should ideally cover about 50 per cent of the surface area. This reduces the spread of oxygen-gobbling algae by hindering their ability to photosynthesise. Other plants, such as floating oxygenators, can also combat algae and balance the pondwater by consuming the nutrients that algae need to thrive. Unwanted floating plants, such as duckweed, and blanket weed, can be fished out now, but be careful not to remove frog spawn and other wildlife with it. The extracted weeds work well as a compost activator, because they’re damp and rich in nitrogen. There are several techniques to try if you have excess algae in a pond. UV water filters are very effective, but simpler ideas work well too. Putting a mesh bag containing barley straw in the pond can reduce algae, as its rotting process releases chemical compounds that inhibit algal growth. The effectiveness of this method is much debated, but it works for me. Another technique is to add a shading dye. It may sound an unnatural thing to do, but dark pondwater dyes made of food-based dyes won’t affect the plants you want or the wildlife, and work brilliantly at reducing blanket weed and other algae. If you have rampant plants taking over your pond, wait until September to tackle them, to avoid disrupting wildlife breeding cycles. Marginal plants can be lifted out, then divided using an old carving knife, in a similar way to terrestrial perennials. Replant them in baskets to keep them in check. With waterlilies, simply cut back their heavy rhizomes.
Drainage solutions
At the same time, you can also drain the pond if there is a build-up of silt and sludge. Store as much of the pondwater as possible in buckets to retain its valuable micro-organisms. The rest of the water can be removed using a submersible pump, hired from a builders’ merchant. Once empty of water and plants, the sludge can be removed in buckets. Refill the pond with your buckets of pondwater plus tap water and leave any extracted plants around the edge to allow trapped creatures to make their way back in. The water will be cloudy for a few days, but will return to clarity with the right balance of oxygenators, marginals and surface-shaders.