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For The Garden

Controlling Pond Plants

Author: Cathy Cromell
Issue: March, 2013, Page 144


Keeping a pond healthy is critical to its success, and to maintain a functioning ecosystem, ponds require occasional plant thinning or dividing.

Plants rooted in gravel and rocks on the bottom of a naturalistic pond spread more vigorously than those confined in containers set on concrete pond bottoms. Once established, foliage may cover the entire water surface within several years. Such abundant greenery, although lush to look at, inhibits water circulation. This, in turn, reduces the effectiveness of the pond’s biofiltration and skimmer system that maintains clear water.

When more than 50 to 70 percent of the water’s surface is covered by plant material—whether the plant roots have spread naturally or are restricted by pots—it is time to thin or divide.

Late March is prime time for do-it-yourselfers to perform this task, because plants are beginning their growth cycle and will recover quickly, says Paul Holdeman, owner of The Pond Gnome, a Phoenix company specializing in natural water features. Also, this is when pond water has warmed sufficiently to be comfortable to step into, but hasn’t become so warm that maintenance threatens fish health.

“Disturbing the pond stresses fish, and parasitic activity increases as water temperatures climb,” he explains. “The combination of the two equals danger to koi.” Finally, don’t thin plants during cool fall and winter months, when they are dormant, as this could cause them to rot.

Key maintenance procedures are explained below.

Many species can be thinned by pulling or digging out the excess, root and all. Holdeman suggests wearing a sturdy pair of neoprene gloves to protect your hands. Such plants include pennywort (Hydrocotyle verticillata), rush (Juncus sp.), water clover (Marsilea sp.), yerba mansa (Anemopsis californica) and taro, also known as elephant’s ear (Colocasia esculenta). However, taro’s named black varieties (‘Black Beauty’, ‘Black Magic’, ‘Black Ruffles’) and ‘Illustris’ should be divided as described next.

Other species of pond plants need to be lifted and divided, similar to perennials. To divide, carefully dig up the entire root ball (or lift it from its container, if applicable). Remove excess soil so that you can see the rhizomes—horizontally growing underground stems from which new shoots and roots will sprout.

Cut and divide the clump with bypass pruners so that each new section is left with at least 3 inches of healthy rhizome with growing tips. Healthy plant tissue will be firm and bright white. Trim and discard any mushy or brown material, which are signs of rot. In addition to the taro varieties listed under Thinning, plants that require division include canna, iris, pickerel (Pontederia cordata) and water lily.

Replant rhizome sections in the pond’s gravel and rock bottom, and then anchor with a handful of pea gravel, or replant in containers. For heavy feeders, such as iris, taro and water lily, you can tuck a slow-release fertilizer tablet next to the roots. Only use tablets formulated for aquatic plants and follow package instructions. Nutrient overload encourages algae bloom, so don’t be tempted to over-fertilize.
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