Tortuga Gazette 29(6) 6-7, June 1993

Pondscaping With Aquatic and Marginal Plants

by Mary Anderson Cohen

Outdoor artificial ponds are an excellent environment in which to keep aquatic turtles. Ponds have several advantages over aquaria including exposure of the turtles to beneficial natural sunlight, a reduced need for equipment (heaters, filters, heat lamps and broad-spectrum fluorescent lights) as well as water-conditioning chemicals, and the opportunity to introduce aquatic and marginal plants into the turtles' environment, creating a more naturalistic setting and providing natural, whole foods to supplement their diet.

There are basically two types of plants commonly used in and around ponds: aquatic and marginal plants. Aquatic plants are those which have leaves and roots but exist completely free-floating, either on the water surface (floating aquatics) or just below the water surface (submerged aquatics). Floating aquatics should never be placed in soil, but rather placed on the water surface and allowed to float freely. Submerged aquatics generally grow completely submerged (that is, under water). They are sometimes rooted in the pond bottom and sometimes float freely. Marginal plants are those plants which are rooted in soil but capable of growing with "wet feet", that is, at the margin of the pond or with their roots actually in the water. Some marginal plants are capable of adapting to conventional landscape situations, gardens with average irrigation; some require the bog-like conditions found in or near ponds and will not adapt.

As interest in water gardening has grown over the past several years, many unusual aquatic and marginal plants have become available through local retailers and mail order catalogues. In this article, the focus will be on less expensive, readily available plant materials. Because few people have a lake in which to keep their turtles, the plants discussed are all suitable for the small-scale pond. One bit of advice: many aquatic plants serve as food sources for aquatic animal life. Turtles can be voracious consumers of plants they relish. They are also very effective at flattening plants, breaking stems and generally rearranging their "furnishings". Before making a major investment in expensive plants intended for "permanent" pondscaping, it might be wise to introduce a single plant of the desired species into the turtle pond and see how long the plant lasts. For example, one-gallon named cultivars of water lilies run upwards of $30.00 each, making them rather pricey forage for hungry pond sliders.

Floating aquatic plants

Floating aquatic plants enhance the appearance of the pond. More importantly, they contribute to the health of the pond by cutting down on the amount of available light striking the water surface, thus checking the growth of algae. Coverage of approximately 60% to 70% of the water surface is generally recommended for algae control. Floating aquatics also act as "biological sinks", that is, they utilize mineral salts dissolved in the water, thus effectively competing with algae for available nutrients.

Fairy Moss (Azolla caroliniana). A tiny floating fern, with fronds less than 1 inch (2.5 cm) long, it may spread to carpet the surface of the water. It reproduces freely and may multiply explosively and block out too much light. It may create a nuisance in a filtered pond by clogging the filtration system. It survives but is reduced by frost. Turtles keep this plant well under control as they enjoy eating it.

Frog-bit (Hydrocharis morsus-ranae). Resembling a miniature water lily in appearance, its spongy, kidney-shaped leaves are about two inches across and it bears small, papery, white flowers in late summer. Plantlets borne on short runners enable it to form large groupings and to be easily propagated. It survives cold temperatures by dropping overwintering buds (called turions) to the pond bottom.

Water Hyacinth (Eichhornia crassipes). The most common and widely available floating aquatic plant, water hyacinth is native to tropical America. Its smooth, circular leaves are attached to inflated stems which are honeycombed in the center, trapping air and providing buoyancy. Water hyacinth bears clusters of lilac flowers intermittently during warm weather. Feathery roots float below the water surface and new plantlets are produced on runners growing from the parent plant. Its form is more compact in full sun, more elongated in partial shade. Roots, stems and leaves of water hyacinth are eaten by many species of aquatic turtle. Possession is prohibited in Florida and Texas because it has created a nuisance by clogging open waterways, but being frost-sensitive it is less of a threat in other areas.

Water Lettuce (Pistia stratiotes). With its vague resemblance to a loose-leaf lettuce, hence the common name, water lettuce forms rosettes of downy, pale-green, ruffled leaves; these leaves grow on swollen stems with a feathery root system under the water surface. Water lettuce grows best in partial sun. It is subject to occasional attacks by aphids and is sensitive to frost. Many turtles relish this plant. Possession of water lettuce is prohibited in Texas because it is a nuisance in open waterways.

Submerged aquatic plants

Submerged aquatic plants contribute to pond health in two important ways. These plants are "oxygenators", meaning they release oxygen into the water through photosynthesis, although at night time when there is no energy-giving light they consume oxygen. They can also act as "biological sinks" for nutrients dissolved in the water. Because algae need these nutrients to grow, they may compete and thus check the growth of algae in your pond. These qualities make submerged aquatic plants useful in maintaining pond water quality.

Many underwater aquatics are best adapted to cooler water temperatures and will look straggly when the temperature routinely exceeds 65° F, 18° C. The following plants are suitable for warmer water temperatures.

Anacharis, "Elodea", or Fishweed (Lagarosiphon major). The most common water plant sold for aquarium use. It sometimes roots in the pond bottom and sometimes floats freely. The long stems are covered with dark green leaves which curl back on themselves. It is an excellent oxygenator which grows very easily from cuttings, and will survive very low temperatures. Although Lagarosiphon resembles true Elodea in appearance, botanists have identified specific differences between the two. Nevertheless, most retailers in this area refer to Lagarosiphon as "Elodea" so if you wish to purchase it from an aquarium supplier, ask for "Elodea". Anacharis provides good grazing for pond turtles.

Cabomba (Cabomba caroliniana). An oxygenator whose bright green, feathery, fan-shaped leaves lie just under the water surface. It is easily propagated from cuttings taken any time during periods of active growth (warm weather). It may lose some or all of its foliage at low temperatures. Although decorative, turtles don't seem interested in eating Cabomba.

Tape Grass (Vallisneria species). The flattened, tape-like, bright green leaves grow unbranched and completely submerged. Many varieties are sold commercially. It can be easily propagated by removing and rooting the clustered plantlets which grow from the creeping root system.

Marginal plants

Marginal plants are moisture-loving species that are rooted in soil and grow in the mud at the water's edge or with their roots actually in shallow water (usually 6 inches or less). Many water gardeners choose to grow these plants in plastic containers. This technique has several advantages. In a preformed pond there are limitations to the amount of root space available. Vigorous-rooted species are more easily kept under control when confined to a pot. The pots make anchoring the plants easier and help keep them upright. During times when the plants are dormant or of poor appearance, they can be easily be removed and placed elsewhere. Ordinary potting soils are not suitable planting media for water-plants. They contain forest products (shredded bark, wood chips, etc.) which will rot with constant exposure to water, often causing the plants themselves to rot. These soils are also light in weight and may not be heavy or dense enough to stay submerged. It is better to use simple unamended garden soil with a high mineral component (such as adobe clay). Some nurseries sell specific potting mixtures for water plants such as "Water Lily Potting Soil."

Dwarf Cattail or Reedmace (Typha minima). This upright-growing marginal is the least invasive of the Cattail group, making it especially well-suited for smaller ponds. Dwarf Cattail has narrow, blue-green foliage and distinctive, rounded, brown seed heads and usually grows less than 24 inches (60 cm) tall. It is easily propagated by division of the root-ball in early spring as the plants begin to grow. It survives very low temperatures. Turtles bite on the stems of Dwarf Cattail but don't seem to eat it.

Dwarf Papyrus (Cyperus isocladus or C. haspan). Resembling the papyrus plant of papermaking history, this dwarf species is much more compact and better suited for smaller ponds. Slim, leafless, upright stems are topped by green and brown grassy tufts. The plant reaches a height of about 18 inches (45 cm).

Dwarf Rush (Eleocharis montevidensis). A member of the Scirpus group of grass-like marginal plants which includes Striped Bulrush (S. albescens) and Zebra Rush (S. zebrinus). Dwarf Rush features thread-like, emerald-green leaves topped by tiny brown seed heads. It forms 6 inch (15 cm) tall tufts. It survives low temperatures but is invariably flattened by turtles who find it a superior basking spot. It will push new upright growth if removed from the pond area for a short while.

Purple Pickerel (Pontederia cordata). This flowering marginal grows to about 30 inches (76 cm) tall. From underwater roots, heart-shaped leaves top long, spongy stems. Short spikes of purple flowers are borne in late spring and summer. It will go dormant in winter but will survive freezing winter temperatures provided the roots remain under water. Purple Pickerel may be propagated by division when the plant is actively growing. Do not divide when it is dormant or the plant will quickly rot. It may also be propagated from seed.