Keep your pond sparkling with pretty oxygenators
Water features are the garden design element with the single most transformative effect (after plants, of course). Yet despite their amazing wildlife value, ever-changing seasonal interest and the mesmerising beauty of their reflections, many newbies are put off by the fear of the crystal clear water in these features turning into a soupy swamp.
Fortunately, there is an underused group of pond plants that will decrease the risk of algal blooms by mopping up both the carbon dioxide and nutrients they need to live. Oxygenators, as their name suggests, in addition to out-competing pond slime, release oxygen into the water, which not only benefits resident fish and tadpoles, but prevents the bacteria that cause pond water to smell. That’s before we even start talking about how stunning many of these plants can be, providing vibrant colour and interest.
Above the waterline, the common water starwort (Callitriche stagnalis) has small, apple-green leaves that create ground-hugging mats that look very similar to Soleirolia soleirolii, also known as Mind-your-own-business. Underwater, the plants change their behaviour altogether. They stretch out to 20-30cm and form large, spoon-shaped leaves. The fresh, brightness of the green leaves is like a visual sorbet that, aside from being really effective water cleaners, are also excellent at hiding the liners or pipework of ponds, because of their ability to seamlessly clothe both the immersed banks and the deepest depths of the water in camouflaging colour.
If flowers are your thing, water crowfoot (Ranunculus aquatilis) is hard to beat for its delicate, pretty, white and yellow blossoms that float daintily on the water’s surface. Closely related to buttercups, their flowers and above-water foliage bear a strong resemblance to this much-loved wildflower, but below the water the leaves are entirely different. They become long, wispy, lace-like strands, like emerald-green feather boas suspended in the water. If you just can’t get enough of fluffy show-girl-costume foliage, there is also Ceratophyllum demersum, which will quickly colonise the floor of your pond with a mat of water-purifying leaves.
Finally, if you have a shallow area of your pond, where the water level is 30cm or less, the short, hair-like leaves of needle spikerush (Eleocharis acicularis) are an excellent candidate to create a lush underwater “shag pile” carpet lawn 15cm long. Want them even shorter? Track down the mega-dwarf form E acicularis ‘Mini’ from aquarium stores.
Mix and match from any combination of these oxygenators, and add a few water lilies or other surface dwellers to exclude sunlight to around half of the pond’s surface and you will slash your chances of algal invasions. Biological deterrents that are as effective as they are beautiful.