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Water Garden Plants: Problems and Solutions

By Judy Sedbrook, Colorado Master GardenerSM, Colorado State University Cooperative Extension, Denver

The reflection of the sun, dragonflies and butterflies dancing overhead, the tranquility of trickling water; the desire for just such an idyllic setting has made the water garden an increasingly popular idea.   Add in the affordability and availability of easily installed, prefabricated materials and the backyard pond becomes a practical option for many homeowners.

To go along with this, most garden centers and mail-order catalogues now stock a variety of plants specifically meant to be grown in or near water. Just as in any other part of the garden, though, these water plants are subject to a number of insect and disease problems.

By following a few simple guidelines, many of the problems you might encounter can be avoided.

  • When choosing plants for your water garden, be realistic. Do not choose plants with needs you cannot accommodate. Look for plants that are hardy in our climate. If you do opt for tropical plants, wait until temperatures are at least 70 during the day and do not drop below 50 at night before putting them out.
  • New plants should be carefully examined before adding them to your water garden. Check for the presence of insects or unhealthy appearing foliage. Make sure the root system is healthy. Roots should be firm and white.
  • Don't try to crowd too many plants into a small area. Make sure there is room between plants for good air circulation.
  • Monitor the pH of the water in your garden. Most pond plants will do well in a neutral range 6.2 to 7.4.
  • Know what sunlight is needed for the optimal growth of the plants you have chosen. Though most water plants require sunlight for 6-8 hours, there are some varieties that will tolerate more shade.
  • If you have fish, be sure to add a layer of gravel or small pebbles to the surface of each potted plant. This will discourage the fish from disturbing the roots of your water plants.

Mary Small, urban pest management specialist with Colorado State University Cooperative Extension Plant Diagnostic Clinic, has identified some of the more common water garden plant problems occurring locally. The following is a list of these insect and disease related problems, their causes, and what can be done to control them.

Petasites, Rotala and Water Hyacinth: Root Rot

Obedient Plant (Physostegia) and Water Forget-me-not (Myosotis): Impatiens Necrotic Spot Virus

Water Lily:

    Black Aphids

    Aquatic Leaf Beetle

Water Hibiscus: Scorch

Papyrus (Cyberus): Helminthosporium leaf spot (a fungus)

Joe Pye Weed (Eupatorium purpurea): an unknown virus

 

 

 

Petasites, Rotala and Water Hyacinth: Root Rot

Symptoms:

  • The plant is not thriving and there is dieback.
  • The leaves are wilting and yellowing.
  • The roots appear dark brown or black and few or no white roots or root tips can be found.
  • Roots are limp and not brittle and crisp as is found in healthy plants of all types.
  • The outer layer of cells easily strips off the roots leaving only the central strand of water conducting tissue.

Cause: These symptoms are most likely caused by root rot, a fungal disease. The fungus is usually introduced with new plants, but several conditions may be contributing to the problem.

  • There may not be enough oxygen in the water garden system.
  • The plants may not be well adapted to the climate or may have been put into the garden too early.
  • The water depth where the plant is growing may be inappropriate for the plant.

Control: Remove the plant from the water garden and monitor the remaining plants for signs of root rot. Correct any design deficiencies that may be adding to the problem.

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Obedient Plant (Physostegia) and Water Forget-me-not (Myosotis): Impatiens Necrotic Spot Virus

Symptoms:

  • The plant's leaves may be wilting, yellowing, mottled, or have 'chicken pox-like' sunken spots on them.
  • The plant may be distorted or stunted, with areas of necrosis.
  • There may be cankers on stems or stem death.
  • The plant may eventually die.

Spread: The cause of these symptoms is the Impatiens Necrotic Spot Virus (INSV). It is spread by thrips feeding on the plant. Thrips carry the virus in their bodies and introduce it to uninfected plants when they feed on them. The virus survives only if the host plant is alive. It can overwinter in the infected plant or other perennial host plants.

Control: Discard the plant. There is no control for the virus.

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Water Lily: Black Aphids

Symptoms:

  • The leaves of the plant are yellowing, browning and may be distorted.
  • They have small, black, pear-shaped insects on them. On close examination, these insects are seen to have "stovepipes" projecting from the rear end.

Spread: Damage is caused by the aphids sucking sap from the leaves of the plant. Black aphids can be introduced to the pond with infested plants. They overwinter on plum and cherry trees and these plants can be another source.

 Control:

  • Hose the aphids off with a strong stream of water. Once in the water your fish may eat them.
  • Keep yellowing and dead leaves removed from the water garden; aphids seem to be attracted to these.
  • Treat nearby plum and cherry trees with a dormant oil to kill overwintering eggs.
  • Remove the infested plants to a separate bucket or tub, use appropriate insecticides or soaps, wait the appropriate interval recommended on the insecticide label, rinse thoroughly and return to the garden. If fish are a part of your water garden, do not use products harmful to them such as pyrethroids, pyrethrum, etc.

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Water Lily: Aquatic Leaf Beetle

Symptoms:

  • There are holes chewed in the leaves of plants.
  • There may be stippling injury (speckled areas) on the plant.

Spread: Aquatic leaf beetles can be introduced with new plants, or they may just be attracted to the water garden.

Control:

  • Keep dead foliage cleaned up, especially at the end of the growing season. Adult insects overwinter in dead foliage at the water garden's edge.
  • Check the undersides of leaves and wipe off and eggs that may be found there.
  • Larvae, which look like small grubs, can be handpicked from the foliage.
  • In severe infestations, remove the infested plant to a separate bucket or tub. Apply insecticides, wait the recommended interval, rinse the plant thoroughly and return it to the water garden.

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Water Hibiscus: Scorch

Symptoms: The leaf edges are necrotic with no evidence of insects or disease.

Cause: Scorch is caused by the inability of roots to take up enough water to replace what is lost through transpiration. Scorch is worst during periods of hot dry weather, but can also occur when there is insufficient water present or when the roots are incapable of functioning well enough to absorb it.

Control:

  • Learn what conditions the plants requires for good growth and compare these to the conditions the plant is actually growing in.
  • If a deficiency is found in the amount of water the plant is receiving, if the plant is in an inappropriate location, or is a new transplant, take steps to improve its physical setting.

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Papyrus (Cyberus): Helminthosporium leaf spot (a fungus)

Symptoms:

  • There are reddish spots on leaves and stems.
  • There is yellowing of the leaves.
  • There may be some dieback of the plant.

Cause: This disease is stress related. Conditions that create stress include:

  • Overcrowding.
  • Poor air circulation.
  • Plants have been set outdoors too early.
  • The plant is in too small a container.
  • The plants have been placed in water that is too deep.

Control:

  • Improve the air circulation. This may just be a matter of increasing the space between plants.
  • Examine the roots. If the plant is potbound, replant in a larger size pot.
  • Remove and discard affected leaves and stems.
  • If the damage is moderate to severe, you may need to remove the infected plant to a separate bucket or tub and treat with Daconil. Follow the directions on the label and, after waiting the appropriate amount of time, rinse the plant and return it to the water garden.

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Joe Pye Weed (Eupatorium purpurea): an unknown virus

Symptoms: The plant's leaves are mottled and crinkled.

Spread: The virus is as yet unidentified and is awaiting further testing. The method of spread has not been determined.

Control: Discard the infected plant.

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Photo: Judy Sedbrook

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Date last revised: 01/05/2010