Natural Swimming Ponds Part 2 – Plant Choices

Hello everybody

Natural Swimming Pond

In my previous blog you will have read about some of the challenges of building a natural swimming pond, and today I would like to talk a bit more about plant selection. The importance of having a large area of very shallow water available for marginal plants cannot be overstated; the best depth is 0-10cm, a number of plants will tolerate 15cm, but much beyond that and the plant choices will mostly be limited to tall, invasive plants with a limited colour palette of green and brown. If you are one of those people who wants the whole thing to have a wild and muted feel, there’s nothing wrong with that, provided that you understand the drawbacks of taller, tougher and more vigorous species.

Phragmites Australis

Here are the main ones: Tall and invasive plants like Typha (reed mace); Schoenoplectus (lake bulrush); Phragmites (Norfolk reed); Phalaris (Reed canary grass); Glyceria (Manna grass) and Carex (sedges) produce a lot of plant material very quickly. In the autumn this means quite a lot of cutting back to remove the dead leaves and stems to avoid all those nutrients being returned to the pond when it all decays in winter. Moreover, the fat poker heads of Typha break up into masses of fluff which floats on the surface and creates a mess. The new shoots of Typha and Phragmites, among others, grow horizontally at first and have sharp tips which can penetrate pond liner to cause leaks. These tall plants also cast shade well away from the perimeter, which can reduce the performance of any water lilies. The dense cover provided by such plants will create nesting habitat for water birds, which although desirable in other types of ponds, are not conducive to a hygienic swimming pond – water birds do make an awful mess with their droppings and will introduce undesirable protozoans into the water, these can cause disease and swimmer’s itch. It’s quite likely that after a while, one of the most invasive plants, usually Typha or Phragmites, will dominate and the others will be pushed out. If you do have a change of heart and want to thin or replace such plants, it is incredibly hard work and will almost certainly involve replacement of the liner, as it is next to impossible to dig out such plants without causing some damage.

If you’re still committed to a tall, wild look to the planting then that’s fine, as long as you understand the considerable drawbacks. In most cases my customers decide that they wish to have a compromise, with some coloured flowers for nectar and pollinators, and a mixture of taller and shorter plants with varying leaf shapes and textures, usually a good number of water lilies too. In this case a simple three level layout works best. The base of the swimming zone is usually 2 to 2.4 metres deep and this zone is usually left free of plants, few of which would grow at this depth anyway. This also allows the regular use of a pond vac to remove silt and debris from the bottom of the pond; this will help immensely with water clarity. Then you can provide a shelf for water lilies and/or other deep water plants such as water hawthorn and various oxygenators. The best depth for this will be somewhere between 40cm and 75cm but 60cm is a good compromise. This shelf doesn’t necessarily have to be on all sides of the pond and can be wider in some places and narrower in others to introduce a bit of variety and eccentricity to the look. Lastly, and most importantly, the marginal shelf should be as large as possible and at a finished depth of 0 to 10cm. Again, an eccentric or varied width pattern can help to introduce visual interest.

Once this has been achieved, the next step is to think about seasonality, as plants all have a season in which they look their best, usually when they are flowering; a period before and after in which they look reasonably good, and a considerable period when they are either dormant or of little visual interest. Given that you should be cutting back and removing as much a possible of the browning deciduous foliage in late autumn, the only plants which will look attractive in winter are evergreen ones. These are very few, limited to Acorus gramineus cultivars, Baumea, some Carex, Equisetum and some Irises. A further group including Juncus have a degree of green left in winter but aren’t truly evergreen and Aponogeton often has green leaves and occasional flowers in winter too. So, if you wish to have some green material in winter a few of these plants are a must.

In Spring everything will start growing rapidly; at first there will be few flowers but a lot of fresh foliage of all sorts of shades of green and many shapes and textures. Do think about foliage as much as flowers when planning your planting scheme, as the leaves will be there a lot longer than the flowers and can contribute greatly to a lush and interesting look. One of the first to flower will be the Caltha family, the marsh marigolds or kingcups. These vary from white to yellow and from small to large, all require quite shallow water to give their best performance. After that, flowers will come thick and fast, as many aquatic plants flower in late Spring or early summer. Irises will produce a month of glory between late May and early July, but after that they tend to look quite similar, so beware relying too heavily on plants that flower just once. A number of plants do produce a second or even third set of foliage and flowers if cut back before the last flowers fade, but not irises. A general rule is that the bigger and taller the plant, the later they flower, as they need time to build up the bulk and energy to put into the flowers. It’s not at all hard to have plenty of flowers in May and June, but then it becomes progressively harder to maintain a colourful display. Water lilies are the summer stars, but will flower in flushes, some only once but others more often.

As temperatures climb in summer (!) many marginal plants will start to brown off and look unattractive. Some plants have quite a short season of interest, and this should be borne in mind when choosing them and their neighbours. If possible, plan to have one group of plants nearby and ready to take over the display when the previous group is going over. Examples of short season plants are Alisma (water plantain), Butomus (flowering rush) and Sagittaria (Arrowhead).

Don’t forget to provide a significant number of oxygenators too. They are the Cinderellas of the pond plant world; nobody likes spending money on them as they are largely unseen, but are a pond essential. They are strongly seasonal, so a variety of types will be required, and these will need to be purchased in their season.

Thalia Dealbata (Alligator Flag)

In my next blog I will talk in detail about how to maintain the display at its best and avoid gaps, but there are a number of plants which really come into their own later in the year. These tend to be the taller and more architectural plants that take a long time to build up an impressive stand of foliage before producing their flowers. Examples are the Pontederias (Pickerel weed) and Thalia (Alligator flag) which will start to flower in August and go on flowering well into autumn, with masses of impressive tropical looking foliage. When these finally start to look sorry for themselves in October, it’s time to think about cutting back all the deciduous foliage and preparing for winter.

Next time: Maintaining the best possible display, the dreaded blanket weed and pond maintenance.

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