For over a year now I have been growing most of my plants in conventional unglazed red clay flowerpots. In just two tanks I have Amazon Swords which are almost two years old, and they are rooted directly in the gravel. Their root systems are extensive and I have elected to leave them alone as long as they continue to do well. I keep Cryptocorynes and Anubias as well as Ceratopteris and Hygrophila in pots. The Microsorium, of course, is attached to the driftwood. I had read of potting water plants before, but never stopped to ponder such a proposition seriously until the idea occurred to me that doing so could solve some problems. What problems? And why do it? And how?
THE UNDERGRAVEL FILTER COMPLICATION
Whether to use UG or RUG goes to a basic overall philosophy of management design that is neither right not wrong, but only different from systems which do not employ them. I use reverse flow undergravel systems in all my tanks. My objective with the RUG is to maintain high aeration of the gravel bed so that its capacity as a biofiltration medium is maximized. The standard UG will do this also. Both types move water in a broadly distributed vertical pattern. Reviewing old hobbyist literature, one often finds statements to the effect that the UG filters are especially good for plant growth. Probably it was thought that the UG mechanical filtration which would draw organics into the gravel bed could be beneficial for enrichment of the rhizo-zone. In theory, there may be something to this, but those who use UGs and RUGs in this day and age are generally aware of the need to keep the gravel bed clear of detritus accumulation. The objective of maintaining the gravel bed at maximum oxidation potential is compromised by any other theory of using it for any other purpose! One experienced aquarist I know uses standard flow air-driven UG systems with "engineered" substrates and grows plants directly in the gravel bed. He has long-standing experience with such an arrangement and keeps plants with great success. I would view this as a management system of yet a different sort altogether and do not think of it as a compromise. But, by and large, I suggest that the basic idea of the undergravel filter as a high efficiency culture medium for aerophilic bacteria conflicts fundamentally with the general requirement for a reducing environment in the rhizo-zone which is optimal for plants.
The clay pot has a drainage weep, as should any planting container one might use for this purpose. Confining the root system to the pot isolates the rhizo-zone from the "oxidation enzyme reactor" which the gravel biofilter represents. Aerated water can circulate slowly through the pot, but its flow rate is greatly retarded relative to the flow rate through the gravel bed outside. I have never had a pot in which a plant was prospering go septic. The plant delivers oxygen through its roots to its microbial symbionts, and there is a very slow but positive flow of aerated water through the pot. I believe that the oxidation is controlled mostly by the plant so that a reducing environment is supported without anaerobism onset. This would be the objective in any successful planting substrate. With high-oxidation UGs and RUGs, it can be done by potting the plant!
CONVENIENCE AND ECONOMY
Whatever one is doing with the bottom of the tank, the portability of the plant once potted has its obvious benefit. It can be shifted about within the system, or even removed, without disturbing the roots. At such times that a thorough vacuuming of the gravel bed is in order, the entire plant along with its rhizo-zone can just be moved out of the way and repositioned anywhere in the tank, or even in another tank. If one wants to alter the location of a plant for any reason, the only restrictions which apply are the size of the pot and the plant's own space requirement. These advantages make aquatic plants in aquarium tanks no different in principle from potted houseplants moving about in the house or on the deck. Individually potting plants permits some breadth of scope for experimenting with substrates without committing entire tank bottoms to long-term trials. Potted plants can be set in bare-bottom tanks for easy maintenance without any gravel at all! This is how I grow out small specimens and cuttings in isolation tanks. I plant in straight SeaChem Flourite gravel with a few grains of Tetra Initial Sticks. In pots, a 15 pound bag of Flourite goes a long way, and when repotting, the substrate is reclaimed and reused with new Initial Sticks added. Or, read here any engineered substrate which might be undergoing experimentation or is in regular use. One need not spread the entire tank floor with optimized expensive substrate.
Consider also the ease of planting. One can set a new plant or cutting at a table using a small scoop, or otherwise carefully control the planting technique at normal gravity in a "dry" environment, then lower the pot into the tank with the plant intact and securely set. Certainly, there comes a time when repotting a growing specimen is desirable, as a thriving plant may outgrow its first pot. A true aquatic usually requires its own bouyancy to support itself. Sometimes repotting can be a four hand job if the plant is removed from the tank, but it is possible to do with help and due care.
One reason I prefer the customary clay pot is its weight. In the water, and especially with a large plant attached, the clay pot with its load of gravel tends to have a "heft" which I like. Any container with a gravel load will, of course, sink. But I just prefer the clay pot for its higher specific gravity. I would emphasize that this is a standard unglazed and undecorated clay pot which is non-bioreactive in any way. The sizes of such pots are relatively uniform along standards determined by the diameter at the rim. Three, four and five inch pots are employed, the smallest being, by far, the commonest in use. They are exceptionally inexpensive.
APPEARANCE CONSIDERATIONS
Ideally, one does not want to see the round rims of clay pots poking up out of the bottom of the tank. The three inch pot is not especially tall. If set directly on top of the UG plate, the surrounding gravel need not be at too high a depth to cover, or nearly cover, the rims. An aggregate of small stones piled up around the rims, or larger rocks and driftwood placed to mask the pots are conceivable strategies. A larger pot will almost always be taller than the surrounding gravel, and will present problems with hiding itself. Still, one wants a certain minimum depth of substrate as required by the plant, and the pot is a good way to get it whatever one chooses for a gravel depth overall. A high-velocity RUG is a good way to increase the bed depth without compromising the environment, and the oxidation enzyme reactor that results from this kind of gravel load makes the potting technique more attractive overall. The convenience benefit of potting plants obtains, however, for any type of system, with a deep gravel bed or no gravel bed at all.
HINTS AND TIPS
I use peat-paper pots as "liners" for the clay pots. This seems to make repotting and transplanting easier. Clay pots are porous, and tiny rhizomes tend to make roots stick to the pot. The peat used in these pots is not highly reactive, but there may be some benefit from whatever organic contribution may exist.
No attempt is made to re-fertilize by pressing material into the gravel. The potting technique facilitates resetting plants altogether. Also, once a plant gets fairly large, it is conceivable that it can become root-bound in a pot. It will, of course, be necessary to transfer it to a larger pot. These cases require removal from the tank, but give the opportunity to inspect the plants carefully and trim them under convenient circumstances. This can also occasion needed maintenance service while a pot is out of the way.
I use CO2 fertilization and moderate intensity fluorescent lighting. I use phosphate-removal resin to keep phosphate below .3 mg/l. I try not to allow the nitrate above 20 mg/l. I encourage high redox by means of DOC removal with activated carbon. These measures help to keep competing algal growth low.
I use Wiegandt Mira-Vit trace and Ferro-Vit fertilizer, as well as SeaChem Flourish. Soluble iron is tested in the water column frequently. 0.1 ppm is target minimum value. The plants seem always to do best with this much soluble iron (and other fortifications in the water column provided by supplements). Perhaps the plants use some of the stuff directly through leaves, but my feeling is that their main mode of uptake is by forced (however gradual) circulation through the rhizo-zone. The root system is mainly where the activity is, apart from the pre-eminently important photosynthesis process and CO2 uptake. In a pot with a weep positioned over a RUG, or a planting bed with a cable heater, there is a slow circulation of enriched water through the rhizo-zone. Eventually all molecules and ions present in the water column must pervade the substrate, where they are entrapped and used on command.
I cannot give a theoretically precise rationale for this method in the face of everything that is known about aquatic plants and how they prosper, apart from the outlines I have stated in terms balancing their requirements against those for other environmental considerations. There are issues concerning the correct proportions of macro and micro substance provision with which I have yet to learn to deal. Almost certainly, I have not hit upon any ideal. I can, however, vouch for the long-term vitality of the species of plants I mentioned at the beginning.
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