Plant Problems

A well planted pond makes a striking centre-piece in any garden. Although in general water plants tend to be less bothered by pests and disease than their dry-land counterparts, there are still a variety of problems which can affect them.

Keeping your aquatic plants in good health calls for a little vigilance. If you know the tell-tale signs to look out for – and act quickly – you should be able to make sure that your pond is always looking its best and stays an attractive garden feature.

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Insect Pests

When it comes to pests, a very large proportion always seem to be insects – and the water garden is certainly no exception!

The 2.5cm (1 inch) long caterpillars of the China Mark Moth, for instance, emerge from eggs laid on the leaves of various kinds of water plants towards the end of the summer. These cream-coloured larvae, with their characteristic brown heads, bite oval-shaped holes in the foliage, fixing the pieces they remove together with silk to make a floating shelter where they feed on other aquatic plants, grow and ultimately pupate.

Although they are seldom a major problem, they can cause a fair bit of damage in small ponds – so, however fascinating you find their natural engineering skills, it’s best to remove any of their floating cases that you come across.

The Leaf-Miner Midge is another insect pest which, though not especially common, can be highly destructive to a wide range of water plants. The midge maggots themselves are small, thin and almost transparent, making them very difficult to see but, their presence is very easy to spot, especially if the attack is severe. Eating their way quickly through the soft tissues between the veins, the larvae rapidly produce skeletonised leaves in their wake.

There isn’t a treatment as such; all you can really do to improve things is pick off any affected leaves and destroy them.

Water Lilies

Of all pond plants, water lilies are probably the most commonly affected by pests and problems – some of which are entirely their own.

The Water Lily Beetle, for example, can cause major damage to their foliage, laying eggs that hatch into voracious black grubs with yellow undersides, which then eat their way through the leaves, causing big holes to appear. The damage they do eventually causes the leaf to shrivel and die, and if it is not removed, it will ultimately start to rot.

The only effective treatment is to ruthlessly remove and destroy the worst affected leaves and wash the remainder with a garden hose – and since this small brown beetle can lay three or even four clutches of eggs over the summer, you may have to do this more than once.

Fortunately this pest is only an occasional problem in small garden ponds, since it tends to prefer larger expanses of water, but if you are unlucky enough to find evidence of them, quick action will be necessary to save your plants.

Water Lily Aphids are another pest to be on the look-out for, especially during spells of hot, dry weather, when they may appear in huge numbers on the leaves, stems and buds. Although they are individually very small – much the same size as the more well-known vegetable black-fly – a large population feeding off your plants can cause weakened growth and distorted or absent flowers. Treatment again involves washing them off with a hose; they’ll make a tasty snack for any fish or wildlife in the pond.

Crown Rot

One of the most serious problems to affect water lilies, however, is not an insect, but a fungal disease related to potato blight – Water Lily Crown Rot – which can decimate a pond’s lilies if it is allowed to take hold. If you’re buying water lilies for your pond, it’s vital to inspect them carefully for any signs of infection, the easiest symptom to recognise being a black, soft spot on the lily’s rhizome.

The leaves of infected lilies will turn yellow and then break away from crown, which gradually blackens and eventually becomes foul-smelling; any affected plant needs to be removed and destroyed as soon as possible to contain the infection – which may spread to all of the lilies if left to run its course.

Fortunately, aquatic plants seem to be very resilient on the whole – which is a good job, given the obvious problems that trying to use any kind of pesticide in a pond would bring! Knowing what to look out for is half the battle and if you act swiftly to remove any affected foliage or plants as necessary, looking after your water garden shouldn’t give you too many problems.

Last Modified: May 24, 2022