While waterlilies are generally robust and trouble-free, they can become weakened when their cultural requirements are not met or when insects and diseases attack.
Nothing will dampen a water gardener’s spirits more than the sight of a once stunning waterlily’s leaves riddled with holes, or a promising flower bud covered with aphids. Luckily, these problems are often simple to prevent with proper care and maintenance, and most are easily treatable once present.
Aphids feed on waterlily leaves and flowers by sucking the nutrients from the plant material using specialized mouthparts.
Damage: Prematurely yellowing leaves and holes in the foliage.
Prevention/Control: Remove any weakened or dying leaves and flowers. Spray the plant with a stream of water from your garden hose. The insects will fall into the water, where your fish will devour them. For more severe infestations, sprinkle diatomaceous earth onto the leaves, which will puncture the aphids’ soft bodies, killing them.
Adult female beetles lay their eggs on waterlily leaves, which are used as a food source for the grub-like larvae.
Damage: Hatching larvae puncture the leaf surface to obtain oxygen, leaving a series of brown dots on the foliage. Once hatched, the larvae chew holes through the leaves, causing the injured vegetation to decompose and die.
Prevention/Control: Remove dead leaves and debris from around the pond¬—these provide shelter for overwintering adult beetles. Remove damaged waterlily leaves from the pond and discard. Inspect leaves for eggs and larvae and wash these off.
China Mark Moth
Also known as “sandwich men,” the larvae of this moth cut pieces of leaves to cover themselves as they “sail” from plant to plant, eating the vegetation.
Damage: Holes cut in leaves—foliage can be “shredded” in severe infestations.
Prevention/Control: Remove damaged leaves from the pond. Hand pick the larvae off of leaves. In severe infestations, Btk (Bacillus thuringiensis var. kurstaki) can be sprayed on affected plants. This organic control is safe for fish.
Midge (False Leaf Miner)
Adult midges lay their eggs in the pond, where the larvae then feed on waterlily leaves.
Damage: Larvae burrow into the top layer of a waterlily leaf and create a trail along the surface. The damaged tissue then rots, leaving the leaf in pieces.
Prevention/Control: Remove damaged plant material and dispose. Spray leaf surfaces with a hose to dislodge larvae. Mosquito Dunks can be used to kill the larvae before they reach the leaf surface.
This fungal disease, though luckily not common, can be transmitted from plant to plant, with devastating effects.
Damage: Leaves turn yellow prematurely while new growth is halted. The tuber or rhizome of the lily will begin to rot, turning soft and giving off a foul odor. The crown of the plant then rots away, killing the entire plant.
Control: Remove the affected plant immediately from the pond and destroy it to avoid infecting surrounding lilies.
The old saying “an ounce of prevention is worth a pound of cure” can be applied to the cultivation of waterlilies. Strong, healthy plants will be less susceptible to damage by insects and diseases.
Sunlight – A minimum of 6 to 8 hours of sunlight is required each day for optimal growth and blooming.
Nutrients – Waterlilies are heavy feeders and should be fertilized twice a month during the growing season to maintain vigor.
Grooming – Dead leaves and flowers should be removed regularly throughout the growing season.
Dividing/Repotting – Divide waterlilies every one to two years, depending on their growth rate.
Tamara Kilbane is a horticulturist and water plants specialist at Sarah P. Duke Gardens. She also curates the lilies entered in the International Waterlily & Water Gardening Society’s annual New Waterlily Competition, which are displayed each summer at Duke Gardens.