Floating Plants


(Unedited version)
Originally appeared in the June 2007 issue of
Tropical Fish Hobbyist Magazine

In the aquarium and natural waters, the top of the water and surface above it is the best place for light and gas exchange. Most plants, even when rooted in the substrate, try to send their leaves and stem close to the surface to take advantage of this prime aquatic real estate. Some plants have evolved to spend their entire lives floating in the water; several have even given up their roots finding them to be unnecessary in this environment.

Floating plants in the aquarium can be useful, particularly for breeding purposes and providing hiding places for fry. They can also be used to shade parts of the aquarium if needed. And they can be quite useful if you want plants in a bare bottom tank. Several floating plants are commonly found for sale for both aquariums and ponds with varying degrees of ease of growth and usefulness for the aquarium.

Azolla

Azolla is a floating fern. There are about 6 living species in the genus, though more than 25 species have been recognized in fossils. Azolla is mostly used in ponds, and can be difficult to grow in the aquarium. Low water movement, nutrient rich waters, and high lighting are recommended for their growth. Azolla caroliniana, filiculoides, and mexicana are native to some parts of North America. A pinnnata is considered an invasive plant on the federal noxious weeds list. Many Azolla have become noxious weeds in different countries across the world.

Unlike many plants that get in to the noxious weed list, Azolla are actually quite useful. Azolla have an interesting symbiotic relationship with a blue-green algae Anabaena azollae. Because of this relationship and the nitrogen fixing ability of the blue-green algae, Azolla are used as a green fertilizer. They are also supposed to be nutritious with a high level of protein and are used for livestock and even eaten by people.

Ceratophyllum

Hornwort is a very common feathery leaved aquarium plant that will grow well in many aquariums. Hornwort also stands up to many fish that may have tendencies to include salads in their diets.

Hornwort is unique in that it will never develop roots. You can put the plant in bunches in the back of your tank and it can look nice that way but the plant will roots just won’t ever form and if you try to place any part of hornwort under the substrate, that portion will die.

Hornwort can grow under lower and higher light levels. In lower light it will be brighter green and thinner. In higher light the plant gets bushier and can get red tints particularly in the stem. In fact hornwort is usually very easy to grow in the aquarium and pretty much anywhere else. Though a native of North America, this plant has been introduced in many parts of the world where it’s become a problematic weed.

The Ceratophyllaceae have their own family and its relationship to other plants is still undetermined. They seem to be confusing plants for botanists, the number of different species isn’t certain either. Earlier sources argued at 1 or 2, newer sources list 6 or 7; but over 30 have been described, though many of those are almost certainly the same highly variable plants.

Eichhornia crassipes

Water hyacinth is a beautiful flowering plant, originally from South America and used mainly in ponds. E. azurea is listed on the federal noxious weed list but E. crassipes is listed as a noxious weed in many states and countries. It’s one of the most problematic of the invasive weeds in the world. It can fully cover a lake or pond from shore to shore.

Water hyacinth isn’t a practical plant for most aquariums. The plant can get quite tall if it does well and it usually doesn’t inside anyway. Considering its invasiveness, possible illegality in many states, and the fact it isn’t a practical plant in most aquariums, I would not suggest using it.

Lemnaceae family

The duckweeds and their close relatives are some of the most well known of aquatic plants, seeming to occur just about everywhere there’s water worldwide. Often duckweeds can become a pest in the home aquarium, but they can also be useful and aquarists have used them pretty much since there have been aquariums.

The duckweed plants are all small but come in several sizes of very small. The smallest are the Wolffia. These tiny plants are the smallest known flowering plants in the world with tiny leaves 1 to 1.5 millimeters. Wolffia are sometimes called water meal because of their small size. Unlike the larger duckweeds, Wolffia don’t have roots. This is a neat plant to have in the aquarium but very hard to keep because most fish seem to think their on the menu.

The most common and middle sized duckweeds are the genus Lemna which have more than a dozen species. Duckweed is usually very easy to grow in the aquarium though I have had tanks where it just didn’t want to grow; some sources indicate this can mean there are nutritional deficiencies in the water.

Duckweed can look different in different aquariums, sometimes having very long roots hanging down in the tank, in other tanks fish may nibble on the roots keeping them very short. Sometimes duckweed may drop its roots making a mess of the bottom of the tank. Some aquarists use string or other barriers at the top of the tank to try to keep duckweed roped in to a specific area. The best way to control duckweed, if you don’t have a duckweed eating fish, is to remove the excess plants by hand.

Larger duckweeds include Spirodela and Landoltia. These are similar to the Lemna. Often in the wild and sometimes in the aquarium you will end up with several different types of duckweed growing in the same place.

Limnobium

Frogbit is another plant common for ponds that can grow in the aquarium. There are two species that may be available L. laevigatum, from Central and South America and L. spongia from North America. The plants are very similar in appearance. Both look like a very large duckweed but they aren’t closely related. In fact they are from the family Hydrocharitaceae and are related to such diverse looking plants as Blyxa, Najas, Elodea and Vallisneria.

Frogbit will need good lighting to grow in the aquarium. If it likes your tank it can grow quickly and will have to be regularly thinned. Daughter plants grow on stolons to the sides of the plants. Also remember these plants can have very long roots which in most aquariums will easily reach to the substrate where they will be happy to anchor. Remember that under these plants it will be shady. Be sure to plan where they frogbit will sit on the surface of the aquarium accordingly.

Pistia stratiotes

Water lettuce is another plant used often in ponds. As the common name implies this plant looks like a head of lettuce sitting on top of the water with roots hanging below. Like water hyacinth you have to consider the height of the plant out of the water when considering it for the aquarium. It would probably work better in an open tank than one with a cover. This is not an easy plant to grow in the aquarium and it needs a great deal of light.

There is some debate about whether water lettuce is a native plant in the US or not, but either way it has been here a long time, since the first recorded observation was in 1765. It has caused problems as an invasive weed in some places and is listed in some states.

Riccia

Riccia fluitans is a liverwort that’s been getting a lot of attention lately as a ground cover. Tied to rocks it does make an attractive lawn plant in the aquarium, but for years it’s been used as it grows naturally, floating. In the past this plant was commonly called crystalwort and is referenced in aquarium literature from at least as early as the late 1800’s. It’s a particularly useful plant in the breeding of some fish. The dense mats can hide tiny fry, much better than even other floating plants.

Salvinia

Salvinia is another floating fern most often used in ponds. It is interesting in that the “roots” seen floating below the plant is actually a modified leaf. There are considered to be 10 species of Salvinia. None of them are native to the United States though they are in the country now. 4 species are on the federal noxious weed list. They are also invasive in many other countries. Salvinia will sometimes grow in an aquarium under the right conditions but considering their status I would advise against growing them at this point.

Utricularia

Bladderworts are very interesting carnivorous plants, several of which you can keep in the aquarium. There are over 200 species of bladderworts world wide living both in the water and in moist environments. These can look like several other aquatic plants with their fine feathery leaves, but unlike the similar looking plants, bladderworts have tiny bladders. These small bladders capture tiny insects and crustaceans, sometimes even very tiny fish fry, depending on the type and size of the plant.

I’ve found bladderworts to be somewhat temperamental in the aquarium. Some are not very attractive and look more like hair algae with a few very tiny bladders thrown in. These types seem to grow about as well as hair algae in the aquarium and are generally a nuisance. Other more attractive bladderworts can be somewhat temperamental, growing well for some time then deciding to fade away, popping up again several months later. In tanks with added CO2 I’ve noticed they don’t play the fading away trick. I have noticed in the tanks I’ve put the bladderworts in with additional CO2 that the leaf growth isn’t as thick as the tanks without added CO2.

Bladderworts don’t usually flower in the aquarium but if they do the small and lovely flowers grow above the water. Depending on the species the flowers can range from yellow to violet.

Summery

There’s a large selection of floating plants that can be used in the aquarium for a variety of purposes. Sometimes these plants can be overlooked when deciding what to put in your tank but the floating plants have a lot to offer. Floating plants can be useful for utilitarian purposes like breeding your fish or using plants in a bare bottom tank. Some of them have very fast growth and can be used to help clean up your tank. Floating plants can bring the beauty and interest of long hanging roots to your pristinely aquascaped planted tank. And floating plants can add something uniquely unusual for the collector, like the carnivorous bladderwort. There are many more floating plant species and many more uses for them than I could possibly cover in one column, but perhaps this brief review will encourage you to try some of them in your aquarium.

Questions or Comments?

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Welcome to the Jungle | Into The Forest | The Creepy Crawlies | A Clearing in the Thicket | Algae Eaters for the Planted Aquarium
North American Natives | Why things go wrong Pt 1 | Why things go wrong Pt 2 (Algae) | Algae Eating Shrimp | Lo-Tech Tanks
Welcome to the Fish Room | The Stemmed Plants | Mosses | A Livebearer Biotope | Planted Tank Social | The Genus Hygrophila | Cyanobacteria
Easy Plants | What I Did Last Summer | Decorations in the Planted Tank | Botany-An Introduction to Plant Biology | Botany-Anatomy of a plant
Botany-How Plants Work | Easy Rosettes | Going High-Tech | Floating Plants | Dealing with Success | Bringing the Outside In | Vallisneria
Hair Algae | Flowering Aquarium Plants Part 1 | Flowering Aquarium Plants Part 2 | Liverworts in the Aquarium | Elements of Design
Planted Aquarium Maintenance | More Mosses | Invaders | Ferns in the Aquarium | Setting up a Planted Aquarium
Seaweeds of the Pacific Northwest | Proserpinaca | Hardware for the Planted Aquarium | Rotala | Neocaridina Shrimp | Lo-tech Tank Tips


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