When potting your bare-root pond plant, you can use normal plastic garden pots (those with solid sides and just the holes in the bottom) like any other potted plant if you wish. However, if possible, opt for open-mesh aquatic baskets; these allow greater contact between the plant’s roots and the pond water, and plants generally grow better in them. If you use open-mesh baskets, you may need a hessian basket liner to hold the soil in. This depends on how large the basket mesh is and how fine the soil is that you are using - modern fine-mesh baskets will not usually need hessian liners, but if you are using a very fine sandy soil, and/or you have large fish in your pond which might pull or nibble on the pots, a liner may make it easier. An alternative to pots is to use aquatic planting bags – these are flexible, permeable, mesh bags which will not leach soil out and which can be fitted into awkward places.
Do ensure that whatever pot you choose gives your plants enough room. Many aquatic plants, and waterlilies in particular, need the space for a large root area. Do not place these in small or cramped pots if you want them to perform and flower well. Small marginal plants, oxygenators, and miniature waterlilies can be started in pots of around 1 litre capacity, but most pond plants are best started in pots of 2 to 5 litre capacity. We generally recommend that each variety of plant is potted individually, so that they are not competing for space in one pot.
Pot the plant in a heavy, loam-based soil. If you are potting waterlilies you can even use a clay-based soil. Special aquatic soil is available in most garden centres, or alternatively, normal garden soil from somewhere like a flowerbed, that has been raked or sieved to make it workable can be used. Do not use standard potting compost or any garden soil that has recently been fertilised, as this can cause excessive algae and/or green water.
When growing pond plants in pots, we would advise that they are re-potted or divided every two to three years, for best results. This is especially true for waterlilies and water irises. We would also advise that waterlilies are fertilised once a year, ideally in spring, if you are not re-potting them that year. It is best to use special aquatic plant fertiliser, as standard plant fertilisers can dissolve and leach out into pond water. You can fertilise a plant without re-potting it, if you don't mind getting dirty hands, by pushing a slow-release fertiliser tablet down into the soil until it is around the plant's roots. Pond plants other than waterlilies are usually fine without any added fertiliser in spring.