Every so often, while I’m checking out aquatic plants at the aquarium shops, I run across terrestrial plants being sold as aquatic plants. I have never asked why such plants were being sold as aquatics though I have read somewhere that these plants may last longer than the real aquatic ones. I find this reasoning rather insulting. True, I’ve seen aquatic plants wither and die in my tanks but there is no way I’m about to submerge a Philodendron or a palm tree (no, I’m not kidding, I’ve seen it) in the hopes it will last longer than some more delicate aquatic plant.
So what’s wrong with submerging terrestrial plants? If you think killing plants and sometimes fish is OK then try it and you will be successful. These plants will hold their shape for a while but inevitably they will drown and rot. As they rot they pollute the aquarium, possibly killing fish in the process. Also, the plant may have been sprayed with pesticides while in a green house. Even pyrethrum-based insecticides, which are fairly harmless to us and pets, are deadly to fish.
So, how to spot a terrestrial plant in a tank full of aquatics? Here is where some basic plant knowledge goes a long way. I have a three-step test.
STEP 1: THE LEAF TEST
Submerged leaves of truly aquatic plants don’t have a coating that protects them from dehydration. These leaves are thinner and have a lighter, more delicate look than aerial leaves. They are often translucent and hairless. These leaves absorb nutrients from the water much as the roots of terrestrial plants. Aquatic leaves may also have air pockets to stay afloat. But thick, opaque leaves (usually with rotting edges) usually indicate a terrestrial plant.
STEP 2: THE RIGIDITY TEST
Fully aquatic plants don’t have to support their weight. Their stems are soft in order to bend with currents and may contain air pockets to help the plant float. Try picking up the plant and holding it out of the water. Plants that spend their whole lives submerged won’t hold their shape.
Bog plants and marginal plants will hold their shape quite well. That’s because they have to deal with fluctuating water levels. Many bog plants (e.g. Amazon swords, crypts, Java fern) will survive fully submerged though they will do better if allowed to send aerial leaves. Unfortunately these aerial leaves usually dry out in our climate or get burned by the aquarium lights. Keep in mind that aerial leaves usually fail step 1.
Marginal plants (e.g. cattails, rushes, irises) usually die if fully submerged. Their leaves are truly aerial, but the plant just adapted itself to live with its roots submerged.
STEP 3: THE GRANDMA TEST
Does the plant remind you of a plant your grandmother had? Have you seen something like it in a garden center? If the answer is yes, you can be 95% sure it is a terrestrial plant. At best it would be a marginal plant. Don’t put it in your aquarium.
All of the above tests rely on generalities. When one looks closely exceptions always come up. Some plants are truly amphibian and do well in either a warm and very humid environment, or fully submerged. Java moss, Java fern, and Anubias are good examples. Other members of the Java fern family (Polypodiaceae) are terrestrial ferns. Likewise the genus Anubias belongs to the family Araceae along with many familiar houseplants (Philodendron, arum, Anthurium, and Dieffenbachia) as well as aquarium plants (Cryptocoryne and water lettuce).
With time and experience one learns to identify the exceptions. When I friend of mine showed me his new Anubias barteri var. nana four years ago, I told him it would be dead in a couple of weeks because it was a terrestrial plant. After all it even failed the grandma test! Well, he was quite proud of himself when he showed me a large patch of it in his tank two years ago. He even game me a clipping. That clipping has grown fivefold fully submerged in my large tank and recently bloomed twice.
Another fellow club member has a less fortunate story to tell. He bought a variegated plant at a reputable store. I don’t know what it was but it was obvious to me it was a terrestrial plant. He suspected it too. The results were some dead and some blinded angelfish. Either the plant had pesticide residue on it, or it released its own toxins in the water.
The next time you see a rather unusual “aquatic” plant for sale put it through the three tests. If it fails, not all is lost. You can put it by itself in a quarantine tank and observe it. If it doesn’t begin to rot and starts growing you may have one of the exceptions. If it starts to rot, take it out of the water and see what happens. You may get a nice houseplant!
Bailey, Liberty Hyde and Bailey, Ethel Zoe (1976). Hortus Third. New York: MacMillan Publishing Company.
Scheurmann, Ines. (1985). The New Aquarium Handbook. Woodbury, NY: Barron’s Educational Series Inc. ?