Pond Plant Sizes and the Concept of Invasiveness

How big do pond plants get?

On our website we state a 'leaf spread' for many of our plants. The leaf spread is based on the size we expect that plant to reach in around three years, based on the size we sell it at. (We have opted for three years because a three-year-old plant is usually fully mature, and capable of flowering). It is NOT the overall size the plant will reach, for the simple reason that almost no pond plants have a final size!

To understand this, it's useful to look at how plants spread in general. All plants have to spread to cover new ground, and they do this in three basic ways.

They can be a single individual plant, which gradually gets larger every year until it reaches a maximum size, and spreads solely by its seeds. Think of an oak tree, which sheds its acorns to make new trees.
They can be a single plant which makes new baby plants all around its edge, and thus very gradually spreads out from the centre, maintaining its shape but increasing in circumference. This is usually called 'clump-forming'. Think of bulbs or Hostas.
They can be a single original plant that puts out stems, which run just under or just on the soil and send up shoots along their length to make new plants. This is usually called 'creeping' if it is fairly slow, or 'running' if it is fairly fast. The plant can spread to new ground much faster than a clump-forming plant can, and will not usually maintain its shape - the shoots may come up some distance from the original plant and appear separate. Think of strawberries or bamboo.

For ecological reasons, most pond plants creep/run, while a few form clumps. (In fact, water hawthorn, Aponogeton distachyos, and the Pygmy Waterlily, Nymphaea tetragona, are the only pond plants which fit the first category of spreading by seed only). This includes waterlilies, which creep on rhizomes, and all marginal plants. A clump-forming plant does sort of have a final size - in the end it gets so wide that the centre begins to die off and it starts to fragment into separate plants. However, a creeping or running plant has no final size. If you placed a miniature waterlily in the middle of an empty lake and left it for decades, it would (if not impeded by other plants or damaged by herbivores) simply continue to creep until it had covered the entire lake. In practical terms, the size of pond plants depends mostly on their available space, which usually means the size of their pot.

So are they invasive?

Used in its correct scientific way, the word 'invasive' refers only to non-native plants that have been introduced to a new place, and are spreading and causing damage there. Most people have heard in the media about invasive pond plants, but this refers to foreign plants, such as Myriophyllum aquaticum (Parrot's Feather), which are spreading through Britain's waterways. These plants are generally illegal to sell anyway. (Although the picture is complicated in the UK - some plants are native to Great Britain but invasive on the island of Ireland).

But when customers ask us if a plant is "invasive", what they actually mean is, is it likely to spread in such a way as to cause me a problem? This is a much harder question to answer because, of course, the answer is "it depends". There are some plants which spread so fast and/or with such tough stems, or even shed so many seeds, that they are likely to be annoying if you have a natural pond or stream and are planting into the ground. However, by the same token these plants are usually robust, easy to grow, inexpensive, and give plenty of cover fast. If you have an artificial, lined pond, and are planting them in pots which will confine their growth, they are often a very good choice. Even if you are planting into the ground, if it is in a situation where there is a lot of competition from grasses and weeds, they may still be a good choice, as they can hold their own against the weeds without constant help from you.

To sum up:

If you are planting straight into open ground and don't want the plants to spread over the area, avoid any where we have commented on their page that they are best contained in a pot, that they seed everywhere, or that they spread fast and may need thinning.
If you are planting into pots in a lined pond with reasonable access, don't worry too much about 'invasiveness'. The pots will mostly limit the plants' spread and where they don't, you can reach in easily enough and pull out excess growth when you need to. Concentrate on picking things which are overall in proportion for the size of the pond, i.e. a small waterlily variety for a small pond.
Oxygenating plants rely on spreading fast enough to out-compete algae. As a result of this they will need to be thinned occasionally, to avoid their taking up too much of the pond. If you really don't want to have to thin them, then it is probably best to avoid them entirely. A plant that occupies the same part of the pond but does not spread quickly cannot really be called an oxygenator - it would be more a deep-water marginal.
For further information or ideas on planting a pond, have a look at our Pond Planting Plans or our Beginner's Guide.