Feed large aquatic plants (if their growth or flowering last year indicated they need it) by inserting special aquatic slow-release fertiliser tablets well below soil level around the base of the plant. Waterlilies are also big feeders. Feed any plants not being divided.
Continue dividing or cutting back marginal and bog garden plants if overcrowded.
Divide and replant waterlilies once they show signs of growth.
Contain vigorous pond perennials by planting them in aquatic plant baskets and topping with a layer of gravel to prevent fish stirring up the compost.
Small plants in baskets can be raised up on bricks when first introduced then slowly lowered to the bottom of the pond as the plants increase in size.
Tidy up plants in the bog garden, and mulch with composted bark (or similar - avoiding manure if there is a chance that water may run off into a pond).
Troubleshooting
Don't worry if the water in your pond suddenly turns from clear to greenish, it's natural for a certain amount of algae to grow in the spring as life begins to stir after the winter. If the problem persists for more than a few weeks, consider adding more oxygenating plants to help clear the water.
