Between April and early August is the best period for stocking the pond, because the water is warm enough to get the plants growing away strongly. In winter the disturbed roots would probably rot, although there is nothing to stop you constructing the pool then, and stocking when the time is right.
Stocking the pool
The plants you are likely to encounter when beginning to plant up the pond can be divided into 6 categories.
1. Waterside plants require moist soil but should never be waterlogged and must always be planted above water level. They require a fertile loam and are best planted in spring, although with containerization the summer months are suitable as well. They are also suitable for a drier bog garden.
Astilbes – feathery flower spikes in pink, white and red.
Hostas — large-leaved herbaceous plants, some with variegations. Lily-like white or lilac flower spikes in summer.
Rheum — ornamental rhubarb. Huge leaves, suitable for large damp areas.
Rodgersia — cream or pink feathery spikes, spectacular bronze leaves.
2. Shallow marginal plants are equally at home in permanently wet soil or shallow water up to 6 in. (150 mm) deep. Suitable for wet bog gardens or marginal shelf planting.
Caltha palustris (marsh marigold). Yellow or white ‘kingcup’ flowers in spring.
Iris. Water loving varieties such as laevigata and pseudo-acorus.
Pontederia cordata (pickerel weed). Light blue flower spikes.
Veronica beccabunga (brooklime). Dainty marginal plant with blue flowers.
3. Deeper marginal plants will grow in water up to 12 in (300 mm) deep.
Alisma plantago (water-plaintain). Spikes of pink and white flowers.
Scirpus lacustris. Strong-growing bulrush for larger pools.
4. Oxygenating plants grow below the surface and are used for keeping up oxygen supplies in the water, which is essential for fish.
Elodea canadensis (Canadian pondweed). Strong growing.
Potamogeton crispum (pondweed). Seaweed-like foliage.
5. Surface flowering aquatics root into the bottom and require water at least 12 in. (300 mm) deep. They often have big leaves which provide useful shade for fish.
Aponogeton distachyum (water hawthorn). White flower spikes all summer and floating oval leaves.
Ranunculus aquatilis (water crowfoot). Dark green foliage and white flowers in high summer.
Water lilies. Many varieties and colours available. Some are more suitable for small areas than others so check with supplier before buying.
6. Free floating aquatics. The roots of these plants float in the water so do not require any soil. Some, like azolla (fairy floating moss) and lemna (duckweed) are too invasive for most ponds but Stratiotes abides (water soldier), which looks like the top of a pineapple with white flowers and sinks to the bottom for the winter, is interesting.