The benefits of aquatic plant repotting and dividing....
Pond plants are just the same as any other plant, in that they have similar needs. Any plant that is growing in a container that is too small or overgrown is restricted in their growth. This reduces the potential of new growth and their ability to flower. Plants with yellow blotches on their leaves, small leaves or producing little or no flowers generally need to be fertilised or repotted. Overgrown pond plants can easily be divided to produce more plants. Old growth can be cut away and the healthiest plants may be kept to produce the best show.
Healthy vigorous plants are always more resilient to attack from pests. Dividing plants allows us to control their growth by reducing their size, if required, or to improve their growth in the following season by adding fresh soil and fertiliser when they are replanted/repotted.
When should water plant repotting and dividing be done?
Most aquatic plants can be repotted and fertilised during the growing season. The warmer water allows them to grow, settle in and re-establish themselves in the new soil.
More robust aquatic plants such as Hardy waterlilies can be repotted any time of the year, in Australia.
At Oz Watergardens we pot over 15,000 waterlilies each year. Our Winters (in Victoria) are not harsh enough to cause any problems or set backs to the plants. If however, your water lily plants have very small, weak rhizomes (the underground root), then we suggest that you repot when they are just starting to grow in Spring.
More tropical water plants should only be repotted when the weather conditions are warmer and the plants have begun to actively grow. Dividing and repotting these plants during Winter in the cooler regions, while they are still dormant, can lead to plant losses, as they do not have the strength or vigour to recover and grow away again.
The benefit of water plant repotting in Winter is that the plants are dormant, we can clean up and trim away dead foliage and the pond normally takes a good month to re-establish its natural ecological balance. If we do it later in Spring, the algae can take off in the warmer water before the plants have had enough time to settle and recover.
What soil should I use when repotting water plants?
In order to get the best from your pond plants, a good quality aquatic planting mix is essential. Unlike mixes for terrestrial plants, the mix or soil needs to hold the plant in place without floating, clouding the water or clogging pumps.
Most commercial potting mixes are made from pinebark or other composts. They are ideal for terrestrial plants but can cause problems in ponds. Many potting mixes are designed to be free draining and as a result can lose much of the fertiliser added to them. This can be a disastrous, in ponds, as the nutrients leach out feeding algal blooms or may become toxic to fish. Many composts release tannins causing the pond water to become a dark tea colour. A weak tea colour is normal in most ponds due to leaves and other debris falling into the pond water, however too much can become harmful.
Too much compost in the mix and you start to get strong organic pond smells, as they slowly decompose in the pond water. Not only can the smell be offensive, but they can also pollute the pond water, harming fish and other aquatic life.
In nature, most aquatic plants grow in high nutrient silt that has washed down into the waterways. This would obviously be the first choice, if we could choose the ideal growing media, as it would be full of minerals, nutrients and organic materials, perfect for aquatic plant growth.
However we can’t just go in and dig out wetland environments to collect the soil we need.
Garden soils can vary, depending on where you live, so our recommendations can only be done in general terms. Sandy soils do not hold onto nutrients very well and can increase algal problems. Clay soils can hold onto nutrients, but many cloud or muddy the pond water and can lead to major problems in water clarity.
The ideal soil would be a silt-like heavy topsoil.
A few Tips when using soils...
Even with soils, there are potential risks. You need to know and understand the soil. Are there any toxic residues from chemicals or sprays, weeds or undesirable pests in the soil? It may seem funny, but it could be a simple as someone dumping the left over cooking oil onto the garden.
As rule of thumb, we have found that soils that are suitable for growing good quality vegetables, are often suitable for aquatic plants.
A simple soil test would be to half fill a bucket with soil, top it up with water and stir it up. Let the bucket sit and if it clears up within a day, then it shouldn’t make your pond water look too muddy.
Choose wide shallow pots (preferably black). These will be hidden under the water so any appropriate recycled container can be used. Container size will dictate how big the plant can grow. If you need to control and restrict the growth, use smaller containers. Water lilies are heavy feeding plants and need a container that is at least 20cm in diameter. If you grow a waterlily in a smaller pot, then you limit the ability for the roots to support the leaf growth and potential flowering.
The how to's....of aquatic plant repotting....
Most aquatic plants are perennials and hardy waterlilies are often the easiest example to use. Perennial plant repotting and dividing is treated in much the same way as most aquatic plant repotting.
Hardy water lilies produce leaves and flowers that float gracefully on the pond surface. These leaves and flowers grow up to the surface from a rhizome that grows horizontally under the soil. When dividing and repotting hardy waterlilies, it is the rhizome that we are dividing and repotting.
For strong healthy plant growth, hardy waterlilies can be divided and repotted every year, however, we generally find that every 2 years is sufficient, provided that they are kept well fertilised.
Make sure that the soil is not too dry. Add some water and if you can scoop up a handful and roll it into a loose ball, then you have about the right moisture level.
Choose your container (at least 20cm wide or more) and fill ¾ with soil.
Add 2 x (10gram) slow release fertiliser tablets and push them into the soil.
Remove the soil from the waterlily, using a hose or dipping into a bucket of water to expose the rhizome.
Select the best growing shoots and cut them away about 8-10cm back from the growing tips (where the leaves are sprouting from). Discard the remainder of the plant.
Trim away the excess roots and damaged foliage from the selected piece/s. (If the water lily rhizome is to remain unpotted for any length of time, it can be wrapped in damp paper, or floated in clean water and stored in the shade).
Create a hollow on one side of the pot so that the soil is mounded up at a 45° angle towards the other side of the pot.
Place the rhizome at a 45° angle, with the cut end against the side of the pot and the growing point level with the top of the soil (aimed towards the centre of the pot). The reason for this is that the waterlily rhizome will continue to grow across towards the other side of the pot. Add some more soil around the rhizome and fill the pot to about 5cm from the top, taking care not to bury the growing tip. Firm the soil to hold the rhizome in place and add 2-3cm of clean pea-sized pebbles or gravel (again taking care not to bury the growing tip).
Gently add some water to the container, so that it slowly becomes saturated and most of the air bubbles have stopped. Then slowly lower the container into the pond.
You may wish to place the plant just a few centimetres under the water for the first couple of weeks. The upper surface pond water is warmer and you will get faster growth. Later you can lower the plant to the recommended pond depth. In the case of a waterlily, about 45cm deep. See the Oz Watergardens Pond Plant Zone Map for more info on recommended pond depths for all pond plants.