How Big Do Pond Plants Get? And Can They Be Invasive?

How big do pond plants get?

On our website, we provide a 'leaf spread' for many plants. This represents the size we expect a plant to reach in about three years, based on the size we sell it at. We chose three years because a plant of this age is usually fully mature and capable of flowering.

However, the leaf spread is not the plant's maximum size. Most pond plants do not have a final size - they continue to grow and spread depending on the space available. To understand this, it's useful to look at how plants spread in general.

How plants spread

Plants cover new ground in three main ways:

- Seed-only spread. A single plant gradually grows larger each year and spreads solely by its seeds making new plants. Example: oak trees.

- Clump-forming. A single plant produces new baby plants around its edge, slowly expanding its circumference while maintaining a roughly circular shape. Example: Hostas or bulbs.

- Creeping or running. A single original plant sends out stems along or just under the soil. These stems 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 won't usually maintain its shape - the shoots may come up some distance from the original plant and appear separate. Examples: strawberries or bamboo. 

Most pond plants creep or run, while a few form clumps. This includes waterlilies, which creep on rhizomes, and all marginal plants. (Seed-only spread is very rare among pond plants. Water Hawthorn [Aponogeton distachyos], Golden Club [Orontium aquaticum], and Pygmy Waterlily [Nymphaea tetragona], are the only examples). A clump-forming plant does have a final size in a sense - eventually it gets so wide that the centre begins to die off and it starts to fragment into separate plants. But a creeping or running pond plant has no final size; it can theoretically cover a whole pond if left unchecked. In practice though, its size usually depends on the space it is given.

Are some pond plants invasive?

Scientifically, 'invasive' refers only to non-native plants which spread and cause ecological damage. In Britain, this includes species such as Myriophyllum aquaticum (Parrots Feather). 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 in Ireland).

When customers ask whether a plant is 'invasive', they are not usually referring to the scientific meaning, but are asking whether it is likely to spread in a way that could cause problems in their pond or garden. This is a hard question to answer because it depends on the individual situation. There are certainly some plants which spread fast enough 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 fast cover. If you have a 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.

Practical guidance:

- If you are planting into the ground and don't want the plants to spread, avoid any noted as "best kept in a pot," "seeds freely" or "may require thinning".

- If you are planting into pots in a lined pond with reasonable access, don't worry too much about 'invasiveness'. Focus on plants proportionate to the pond size i.e. a small waterlily variety for a small pond. The pots largely limit spread, and excess growth can be managed by hand.

- Oxygenating plants must spread fast enough to out-compete algae. Expect to thin them occasionally. Plants that do not spread quickly cannot effectively fill this niche. If you really don't want to have to thin them, then it is probably best to avoid them entirely.

For more planting advice, see our Pond Planting Plans or our Beginner's Guide.