pruning, thinning out, pond plants

Aquatic plants can grow very quickly and can soon become overgrown, even if you have tried to restrict them by placing them in a small container. Therefore, pruning will soon become a necessity, particularly of the oxygenators. If it is not done regularly the soft, sappy growth will die down in the winter and pollute and deoxygenate the water. To prune them, simply nip the growth off at the base, leaving short stems. Young growth can be used as cuttings, if you want to increase your stock, but the stems will soon regrow. Mid to late summer is the best time to do this.
If you have newly-planted water lilies it will be some time before they require pruning. You can usually tell when they need it as they will tend to hold their leaves above the water and will cease to flower. You will need to lift the plants out of the water and cut off young growth about 8-10cm (3-4") long behind the growing tip. The older pieces and leaves can be discarded. The young tip is replanted as described in our planting section here. Waterlilies are constantly producing new leaves and any older yellowing ones should be removed regularly, so as they don't foul the pond.
As marginal plants begin to grow again in spring, the fresh new growth will appear as green tips in amongst the old brown foliage. The old leaves need to be removed and you can do this with a pair of shears as long as you are careful not to cut the tops off the new shoots. Secateurs can be used to remove any dry, fibrous old foliage.