Q. I have a well-stocked 55 gallon aquarium with a variety of aquatic plants, many of which are growing quite tall, and I want to know how I go about trimming these and how much I should trim at a time. Are there different guidelines for trimming smaller aquatic plants? Also, can I re-plant the trimmings, or should I discard them? Thanks for any insight.
A. All aquatic plants are not trimmed the same way, but it is less a matter of size and more a matter of the type of aquatic plant, liverwort (and moss), fern, stem or rosette.
Liverworts, such as Riccia fluitans or Monosolenium tenerum (erroneously called “Pellia”), are trimmed by merely separating a portion of the mass by pulling or snipping. Riccia fluitans needs to be tied down, or held to a rock or piece of driftwood with some hair net, or it will float. Monoselenium tenerum sinks and only needs to be tied down to keep it from wandering around in the water currents. Basically, parts that you remove are capable of generating new plant material, so any trimmings can be reused.
Ferns, like Bolbitis heudelotiior and Java fern (Microsorum pteropus), grow attached to rock or a piece of driftwood, and almost any piece can generate new growth. In fact, if you snip a large stand of Java and do not collect all the cuttings (even the small ones), they will probably move around in the water currents until they snag somewhere and start new growth. With ferns, you can find new plants in unexpected places, including between the stems of swords and Cryptocoryne. Somewhat slow growing, the larger the piece you trim, the better suited it will be for reuse. These aquatic plants develop a sturdy rhizome as they mature. You can always divide an aquatic plant by parting the rhizome.
Some aquatic plants grow leaves from a central base. Such rosette plants include the genus Cryptocoryne. The strong roots can grow into a dense mass, requiring a sharp knife to divide aquatic plants. Some rosette-type plants, including Cryptocoryne species, spread by sending out runners (like long, thick lateral roots) above or just below the substrate. Baby plants start growing at the end of the runner, and that plant then sends out another runner. Once a new aquatic plant is growing on a runner and has several leaves, you can snip the runner to divide the plants. You can leave these plants in place or replant them. If the aquatic plants are growing too closely together for you to be able to pick out individual rosettes, you might have to cut away at the roots that have meshed with neighbors. A portion of root base with a half dozen or so leaves usually is adequate for growing a “plant.” You can thin out a stand of Cryptocoryne by cutting away some of the stems at or near the substrate. If you can, try pulling away a few plants from the others – this will sometimes result in less damage to the roots.
A small amount of such thinning on a robust Cryptocoryne plant will not harm it. Unlike the ferns, these cut leaves will die and cannot be reused. Also, on a smaller aquatic plant, a Cryptocoryne might respond to leaf cutting by letting its remaining leaves rapidly deteriorate (Cryptocoryne meltdown). After storing up nutrients for a while, the plant base will begin growing new stems and leaves. A better method is to cut away a length of a root with a sharp blade. The cutting can be replanted. In my experience, meltdown is much less likely to occur with this method. Actually, I have never had meltdown from a root cutting but others have reported it. The Cryptocoryne that are more commonly available, such as C. wentii, tend to grow a lot of runners and rosettes rather than building a large rosette. So, separating aquatic plants does not usually require dividing a rosette.
Another group of rosette plants comprises the many Echinodorus species, many of which form large rosettes (E. bleheri), while others have small rosettes but a lot of runners (E. anugustofolius). The latter can be trimmed like the Cryptocorynes by cutting away runners and removing some of the rosettes. The larger rosettes, such as E. bleheri, can be trimmed in two ways. The first is by trimming away leaves as close to the base as possible. This is best done by grabbing an outer stem as close to the rosette as possible, and gently peeling and pulling the stem from the rosette; the new leaves grow from the center of the rosette, and the older leaves are at the periphery of the rosette. These leaves cannot be reused. If you cannot get your fingers near the base of the aquatic plant, you can cut the stem as close to the base as possible. Cutting a stem will leave a portion of it behind to rot, and it is possible that the rot will work its way down into the rosette, though I have not had a problem with this method on larger, healthy specimens.
Some Echinodorus, such as Amazon sword plant (E. bleheri), E. ozelot, and E. “Rose” will form a second rosette next to the first. It is as if the first grew so large that it divided itself, but the second rosette is actually a new one. Once the second rosette has a half dozen or so leaves growing, you can separate the rosettes by gently prying or pulling one from the other, then replant. A variation is to use a very sharp blade and cleanly divide a large rosette, and replant. I think there is more risk of one or both the rosettes developing rot if you try to divide a single rosette – it is best to wait for second a rosette to form.
Stems are the easiest aquatic plants to trim and replant. Ludwigia, Rotala, Hygrophila or one of the many other stem plants will grow planted or floating, can send out roots from any node along the stem where leaves form, and can be cut anywhere along the stem and reused. Almost no size is too small, so long as it includes a mature node. However, it is practical to have at least an inch or two if you want to aquatic plant the cutting. Stems are suitable for top cropping. When you crop a stem, many stems will develop two stems at the node below the cut, so judicious cropping is a way to help a stand of long stems develop a bushier appearance.
In taller planted aquariums, a stem plant will sometimes look nice closer to the light, but be pale, less leafy or even profuse with bare roots along the stem closer to, but above, the substrate. This is the result of stems shading their lower portions from the light. You can crop the nicer upper portion of a stem plant and use it to replace the older portion of the stem. Or you can place some shorter plants in front of the lower portions.
Stem plants can grow roots from the leaf nodes, and you can use this to your advantage when first starting a stand of stems. You can place a stem horizontally along the substrate, anchoring it in a few places or burying it in several places along the horizontal length. Roots will grow down into the substrate, and vertical stems will grow up from the horizontal piece.