I’m going to tell you which I think is the most useful and all round best plant for growing in small garden ponds by far. It may sound a bit of an off beat choice, because the plant I’m thinking of has insignificant small white flowers and fairly boring green leaves. It has a habit of growing up out of the water, bunching and spreading invasively. In the margins, this plant will grow quite happily out of the submerged water completely, and may even be grown in flower borders. You can eat the leaves though, after a throrough washing of course, and so by growing the humble water cress you can incidentally turn small garden ponds into kitchen garden vegetable patches! But its not just as a useful herb or salad ingredient that I would recommend water cress, it’s also very beneficial for the small garden ponds themselves. Water cress helps to remove surplus nutrients, particularly nitrogen from out of the pond water and also helps to crowd out other less wanted plant species and algae such as blanketweed and even duckweed. The only problem with water cress is that it isn’t really terrible frost hardy, and it doesn’t seem to self seed very efficiently so what you need to do is to add a small bunch with some roots into your pond each year, early enough in the season to have the beneficial effect. You can just buy a bag of water cress from Tesco or Sainsbury or wherever, eat most of it and chose a couple of pieces that have the longest intact stems with some small white roots, or even just the beginnings of roots growing from the leaf axils on the lower part of the stem. Then just place it in the garden pond in the open water, where it will float, right itself turn upside down a few times over the next few days and then start to grow and spread. Green water will be a thing of the past as the water cress acts to restore the balance in the pond. Water gardeners have in fact known about this since a long time ago, but garden pond shops seem to have forgotten about it. Water cress beds have been used as filtration systems so it should come as little surprise really that the plants helps keep water clean and pure in the garden pond, even a small one.
Water cress belong to Brassicaceae (cabbage family)