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Oxygenators or Submerged plants

Submerged plants are those that grow below the water surface. Also called oxygenators, these plants act as natural filters to help keep your pond in chemical and biological balance, making it more hospitable for fish and less inviting to algae.

Oxygenators grow entirely submersed except, in some cases, to bloom. They maintain foliage under the water so all of the oxygen that they produce goes directly into the water until it is unable to absorb it any further, whereupon the oxygen rises through the water as bubbles that emerge directly and observably from the leaves. All of the nutrients that these plants use to grow are derived directly from the water, which makes them useful as competition for some algae. They might root into the substrate or not, but they never leave the water except to bloom.

The juvenile forms of some ultimately emergent plants serve as efficient oxygenators while they are under water, and in fact, many aquarium plants are the immature forms of plants that really prefer to be above the water, where they can bloom and make seeds. They have simply evolved to withstand long periods of immersion, as opposed to short ones.

Every water garden will benefit from Oxygenators. These plants are an excellent way to remove nutrients from the water, helping keep the water clear.

Oxygenators tend to be vigorous growers. In a matter of a week or less particularly if the water is warm, they will take hold in their pots and start sending out new foliage.

Submerged plants play an important role in your water garden, as they produce oxygen by day. The oxygen helps keep your pond healthy. They are usually plain green and either grassy or made up of whorls of tiny leaves on long stems.

A healthy water garden is a living organism, and oxygenators are its lungs. Oxygenatorss also function as kidneys and biological filters by removing mineral salts and fish wastes that feed unsightly algae. Place a few weeks before you introduce any fish, to allow them time to get established and get your waters chemical balance on an even keel.

You should have one bundle of submerged plants for every square foot of the pond’s surface area.

Planting Oxygenators

Oxygenators are among the least demanding of garden residents. Because they obtain their nutrients from the water, you can plant them in sand or gravel and they will never need fertilizing.

Tip: Do not ever plant them in rich soil, manures, or composts, as nutrients will leach out of these and upset the ecology of your pond. In fact, oxygenators do not even need containers. You can band them in bundles of a half dozen plants and weight them down on the bottom of the pond with a rock.

If you are using containers, fill them with sand almost to the top. Then use your finger to poke each stem an inch or two into the sand. Plant one stem for roughly each inch of pot diameter. Top the sand with clean gravel, which will help hold sand and plants in place, and add some water to help displace air bubbles.

If you have Koi or other ravenous feeders, you may want to protect your plants with mesh, which you can buy from aquatic suppliers.

Lower the pots slowly into your pond. There should be 1 to 2 feet of water over the roots of the plants. If you have a deeper pond, set the containerized plants on blocks to ensure that they get enough sun. They will tolerate semishade, but they will die if sun is blocked by too many floating plants or by algae.

Keep the pots away from your pump; most water garden plants do not like much agitation.

Once the plants have grown about a foot long, you can cut off half their length and reroot the tops in new pots.


Oxygenators are generally easy to maintain. Snails cause them little harm, and while goldfish and Koi will eat them, oxygenators can grow fast enough to offset any damage. Indeed, by providing fiber and vitamins, they can be a useful addition to the diet of herbivorous fish.

The main chores in maintaining oxygenators are cropping back excess plants during spring and summer, and removing dead stems in autumn. Waste material needs to be disposed of very carefully. Almost all oxygenators have the potential to be invasive weeds once established in the wild. Several have even been banned in certain places; Myriophyllum spicatum, for example, is either restricted or banned from sale in the U.S. in Alabama, Colorado, Connecticut, Florida, Idaho, Maine, Massachusetts, Montana, Nevada, New Mexico, North Carolina, Oregon, South Carolina, South Dakota, Texas, Vermont and Washington. Similar legislation exists in many other countries, as well.

The main thing is to keep fragments from getting into ditches, creeks, storm drains or sewers. Instead, the material can be destroyed by incineration or composting, or else disposed in the household trash, ideally after being frozen or thoroughly dried out. The United States Department of Agriculture maintains a database of noxious weeds at plants.usda.gov or visit our Toxic Plants page, and your local fish and wildlife department will be able to tell you which oxygenators are restricted in your area and how to best dispose of unwanted cuttings or plants.

Oxygenators should be the first plants planted, these should be added before adding your fish. Oxygenators will help with NPS by removing nutrients from the water, they help prevent the eutrophic conditions common to new ponds, and in doing so, they starve off algae problems, including blanketweed and "green water". This is particularly the case if used in conjunction with floating plants like water lettuce and water hyacinth.

While oxygenators need control via pruning, especially the extraordinarily vigorous Canadian pondweed, oxygenators are otherwise among the easiest pond plants to grow. They are also inexpensive and easy to establish. Just take care not to add too many, and be careful how you dispose of whatever stems and plants you remove during the year.

Pondweeds and pH

Aquatic plants usually get the carbon dioxide they need for photosynthesis from the water around them, but many oxygenators are able to use a second source of carbon, bicarbonate ions, to supplement this source of carbon. This process is called biogenic decalcification, and in particular, species of Ceratophyllum, Egeria, Elodea, Lagarosiphon and Potamogeton species work this way at times, producing carbonate ions as a waste product and sometimes even secreting a kind of lime scale marl around their leaves as they do so.

The chemistry of biogenic decalcification is quite complicated, but the key thing for the pond keeper to know is that the uptake of bicarbonate ions and the resulting production of carbonate ions causes the pH to rise. At sunset, when photosynthesis stops, the pH drops back down again. As pH goes up and the availability of dissolved carbon dioxide goes down, this can cause problems for submerged plants that don't perform biogenic decalcification, including Cabomba, Fontinalis and Hottonia species. Extreme pH changes can also cause problems for fish and other aquatic animals. Clearly, it is important to regularly crop oxygenators to limit their overall biomass and thereby minimize day and night pH changes.

Recommended Oxygenators

Elodea canadensis or Canadian Pondweed

Description: The most popular oxygenator is Canadian pondweed. As its name suggest, it is a plant native to the cooler parts of North America, and it tolerates cold well. It overwinters beneath ice without any problems. On the other hand, it doesn't do as well in environments with mild winters and little change between seasons. For example, though it's native to Florida, it's rare there and ends up being much less of a nuisance than the invasive nonnative Brazilian pondweed (Egeria densa).

Canadian pondweed may be grown as a rooted plant in pots or as a floating plant at the surface. It needs bright light to do well, so unless the water is exceptionally clear, it shouldn't be planted any deeper than 18 inches. Water chemistry isn't of critical importance, though growth in soft water can be indifferent; a carbonate hardness level of 5 degrees KH or more is recommended. This is because this plant uses bicarbonate ions as a carbon source during photosynthesis.

Healthy plants are vivid green with short, slightly curly leaves arranged in groups of three (occasionally four) along the stems. The gap between each whorl of leaves is about half the length of the leaves themselves, and when the gaps between the leaves are greater than this, it can be a sign the plant isn't receiving enough light. Growth is fastest during spring and summer, and mature plants can have stems up to 4 feet long by the end of the growing season. During autumn, the plants tend to die back, but not before producing bushy buds that eventually fall onto the substrate. These buds don't start growing until the following spring.

Because it grows so quickly, Canadian pondweed has a bad reputation among some pondkeepers for taking over ponds and smothering all the other plants. Take care not to add too many at once, a bunch or two planted in spring will easily spread out across a couple of square yards by the end of the summer.

Elodea or Anacharis

Description: Ancacharis or Elodea is a vigorous grower with branchlets that easily break off from the parent plant to form new plants. It lives entirely underwater with the exception of small white flowers which bloom at the surface and are attached to the plant by delicate stalks. It produces winter buds from the stem tips that overwinter on the lake bottom. In the fall, leafy stalks will detach from the parent plant, float away, root, and start new plants.

Elodea is native to North America and is widely used as aquarium vegetation and known as the generic water weed. Anacharis can also be used in the class room demonstrating how plants use carbon dioxide with the usage of bromothymol blue.

Anacharis is very easy to grow and does not seem to have any special requirements except sufficient light. It grows better in potted soil or just left floating to form long roots that trail down in search of soil to anchor the plant. It needs to be brought inside from a pond at the end of the summer.

Anacharis is an important part of lake ecosystems. It provides good habitat for many aquatic invertebrates and cover for young fish and amphibians.

States that have declared Anacharis an invasive speices:

  • Oregon
  • South Carolina
  • Washington

Bacopa monnieri or Water Hyssop

Description: Native to coastal areas of southern and coastal areas of southern and central North America, water hyssop has a stiff, round stem 2 feet long that is covered with fine hairs. Egg-shaped leaves are about an inch long and relatively thick with hair underneath. Leaves are oblanceolate and are arranged oppositely on the stem. Light blue tubular flowers about ½ inch long appear above the water in the leaf axils (where leaves are attached to the stem).

Spreading is often achieved through cuttings.

Cabomba or Fanwort Carolina Water Shield

Description: This plant is native to ponds and streams from Michigan to Texas and south to Florida. Cabomba is frequently planted in aquaria, as an attractive-leaved water plant that is fast-growing (up to one inch per day). Underwater, its leaves are feathers, in a fan shape 1 ½ inches across. Leaves that grow above the water are linear, pointed, and ¾ inch long. The tiny flowers are usually white with yellow spots, although they are sometimes purple.

Fanwort require good light and warm water. When kept outdoors it is hardy to Zone 5 In the fall the stems break apart into sections and sink to the bottom in spring each one sprouts as an individual plant.


Description: Hornwort can be found worldwide, though they tend to grow only in places that are damp and humid. Hornwort develops stems 1 to 2 feet long, clothed in whorls of forked leaves, that remain completely underwater. Like some other aquatic plants, it reproduces from turions, buds that drop to the pond bottom in winter. You can pot up these turions when they start to grow in spring.

Healthy plants are dark green or slightly pink, if growing in very sunny spots. Hornwort flowers are tiny and so inconspicuous that they are easily overlooked, but hornworts frequently produce distinctive fruits about a quarter-inch long that are equipped with several spins. Pondkeepers frequently find this remarkable looking fruit and wonder what it is. The fruit overwinters on the bottom of the pond before germinating into new plants the following spring. Hornworts will sometimes persist through mild winters floating at the surface, but usually these plants die back after producing buds that sink into deeper water for the duration of winter. When the water warms up, they float back to the surface.


Description: Vallisneria is a submersed plant that spreads by runners and sometimes forms tall underwater meadows. Leaves arise in clusters from their roots. The leaves have rounded tips, and definite raised veins. Single white female flowers grow to the water surface on very long stalks.

Various strains of Vallisneria are commonly kept in tropcial and subtropical aquaria. These include dwarf forms such as Vallisneria tortifolia, a variety with leaves around 15 to 20 cm in length and characterised by having thin, tightly coiled leaves. A medium sized variety, Vallisneria spiralis is also very popular, typically having leaves 30 to 60 cm in lenght. The largest varieties are often called Vallisneria gigantea, these giant varieties are only really suitable for very large tanks or ponds, having leaves that frequently exceed 1 m in lenght, but they are quite hardy and will do well in tanks with big fish that might uproot more delicate aquarium plants.

Vallisneria are tolerant and adaptable but will do best in full sun, they will grow at a slower rate under moderate lighting. They multiply readily through the production of daughter plants at the end of runners. Once they have established their own roots, these daughter plants can be cut away and transplanted if necessary.

Vallisneria will accept neutral to alkaline water conditions (they do not live very acidic conditions) and do not require carbon dioxide fertilization.

Nasturtium Officinale or Water Cress

Description: Water Cress is a very good oxygenator. It is a hardy annual that grows naturally in the soil on the banks of cool to cold streams and spring-fed ponds. It can be found all winter long in unfrozen, sheltered areas.

The small floating leaves are rounded, dark green and waxy. The branching stems can spread out for 2"-3" over the surface. Slender roots hang down from the nodes of the stems. It develops many small white flowers over the growing season.

Plant in garden soil and put the pot by the edge of your pond with a couple of inches of water over the pot. It will do fine in an area of moving water.

Water Cress will quickly form a mass of foliage and can outgrow a small pot within two months. You may see the root mass growing over the surface of the soil them as they take in growth of the plant may slow and become scraggly. Either re-pot the plant in a larger pot or chop out a big chunk of the plant and the root mass then fill the hole in with soil. Water Cress tolerates sun, shade or partial shade equally well. It is easily grown frorn the tiny seeds found at any gardening center that has a decent seed selection. It is also a prolific re-seeder and will re-sow itself yearly. You can also start it from a freshly purchased bunch at the produce section of the grocery store. Make sure you can see the fine white roots on some of the stalks, push these into wet soil.

Potamogeton crispus or PondWeed

One of the less often seen oxygenators is the curly pondweed (Potamogeton crispus), one of at least 100 similar species in this genus. Compared to Elodea species (such as Canadian pondweed), Potamogeton crispus has a pink rather than green stem and leaves that are much larger, often 3 to 4 inches long. The leaves have distinctly wavy edges and obvious (usually pink) midribs and veins. Like the milfoils, P. crispus is a rooted plant and favors the same sorts of locations. Healthy plants form canopies of attractive leaves that float just below the surface of the water. It grows quickly but not as rampantly as Canadian pondweed, and it is therefore a better choice for smaller ponds.

Potamogeton crispus is hardy and tolerant of icy winters. Mature plants may survive the winter, but they also produce overwintering buds like those produced by the milfoils. Take care not to confuse P. crispus with Lagarosiphon major, both species are known to retailers and hobbyists as curly pondweeds.

Willow mosses are among the few fully aquatic mosses native to North America. In the wild, they live primarily in hill-stream habitats where the water is clear and cold: however, one species, Fontinalis antipyretica, does quite well in ponds where it can be grown on rockwork or in gravel beds in shallow water where water flow is brisk. Unlike most other oxygenators, F. antipyretica is also a favorite choice for wildlife ponds where its tiny fronds make good habitats for small aquatic invertebrates.

Shirley Bryne Waterlily

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