The use of aquatic plants for algae control follows two long-established principles among serious pond builders and water gardeners. They are that no pond will look its best without some plant material in it, and that those plants can be used to control algae. Still, in my experience, only a few builders actually provide for them, which leads to lost opportunities for plantings that are both aesthetically pleasing and ecologically useful.
The best way to think of plants in connection with algae control is to regard them as adjuncts to the filtration system. All aquatic plants that grow with their roots or leaves exposed to the water derive all of their nutrients directly from the water. Algae are no different from any other fast growing aquatic plant except that algae are more opportunistic than most other plants, and their ability to get a foothold in a pond while the other plants are in insufficient supply or are in their dormancy is legendary.
In any healthy, so-called “balanced” system, the ammonia that is produced by the metabolisms of the fish and other animals within the pond, as well as the proteins released into the water by uneaten food, decaying garden snails and other garden debris, smog, pine pollen, etc., is, in its turn, further metabolized by particular bacteria in a process called nitrification, in which ammoniacal nitrogen is oxidized into nitrate, which is plant food. Anybody who has kept an organic garden or who paid attention in school has heard of “nitrogen fixing” bacteria. Nitrogen “fixation” means that nitrification has occurred, and that the nitrogen that was formerly unavailable to most plants, now is.
I say “unavailable to most plants” because some plants can process nitrogen that hasn’t been completely “fixed,” and so can tolerate less healthy systems than species that depend upon well-fixed nitrogen. Many of these more tolerant species include some of the algae that plague pond owners everywhere. I’ve always said that there is an alga for everybody, and it’s true. Algae have been around long enough that they have figured out a way to live almost everywhere on the planet. The pond keeper must then acknowledge that there will almost always be some algae in his pond. He should keep the pond clean, but not spotless; well planted but not weedy or overgrown, or as I always say, “natural but not wild, maintained but not manicured.” All of that starts with the proper set-up. One in which there are provisions made for functional plants, and for efficient and adequate filtration and debris removal.
Before I go any further, I would like to say a few words in defense of algae. It is appropriate to bear in mind that a bit of algae in a pond is not the end of the world. Algae are noble organisms. In the natural state, they perform several useful functions. Among these are, obviously, the processing of nitrogen and carbon, and the production of oxygen. They also provide rooting media for higher plants, they provide food for many organisms including snails, fish, and birds, and they provide habitat in which other aquatic organisms can thrive. Anybody who has been in this business long enough has seen ponds in which the algae were the only things keeping the fish alive. Their capacity to consume nitrate and to produce oxygen makes them very effective filtering agents in their own right, and in some instances, they are quite attractive.
As before, there is an alga for everybody, and each pond will grow some combination of algal species according to the conditions in the pond. Factors such as water chemistry, sun exposure, fish population, feeding habits, how much debris gets into the pond from the garden, and how good the filtration is will all play a part in determining which species of algae will grow.
What is an Alga?
The term alga comes to us directly from Latin, alga, which means seaweed, or rubbish. Alga is singular, while algae is plural. They should both be pronounced with a hard g, as in good, but we in America have softened the g in the plural, while sensibly maintaining the hard g in the singular (on those rare occasions when we use it at all).
A host of organisms fall under the heading of “algae.” In the modern sense, the term “alga” refers to any simple, photosynthetic aquatic organism, and so applies to several groups. Some of the most common, such as “blue green algae” are actually bacteria, cyanobacteria in this case, that happen to produce chlorophyll and phycocyanin, which gives them their distinctive color. Blue green “algae” can be found growing in thick colonies in dark areas of many ponds, on piles of fine sludge or beneath other species of algae, and they can process nitrogen where other organisms cannot. In fact, they tend to fix the nitrogen for other plants. I have read that in some Asian rice paddies, Azolla, which is a symbiont of the cyanobacterium, Anabaena azollae, is used to good effect in conditioning the soil for each year’s crop, acting as a sort of green manure. Their presence is an indication that something might be amiss in the pond.
Other common species of algae include Spirogyra, or “string algae,” which forms dense colonies in water and at its surface. Unicellular algae, which stain water green or brown, are commonly seen in new ponds. Brown algae are often found in slimy layers inside of pipes or in blooms of suspended colonies comprising billions of unicellular organisms. There are more complex algal species, such as Chara, which forms dense, malodorous colonies throughout the water column, or Nitella, which forms dense, multi-branched, clumping colonies among higher plants. Both of these attach to the substrate with thickened stems that become buried in the silt.
In any pond it is usually the case that there is more than one species of alga present. One species might do well in a shady area of the pond while another might prefer the brightly lit weir of a waterfall. Their growth will come and go with the seasons, and while they might seem to have disappeared for periods of time, they will return when they can.
Whichever ones grow, it is certain that they are growing on a supply of nutrients that is not being utilized by anything else, either because other plants are not present, or because they are unable to gain access to the nutrient supply. This might be because the water hasn’t been properly conditioned in the filter, or because the water doesn’t flow enough, but more frequently it is because the plants have been put into areas where they never see the nutrients or where they might have grown to the point where the dissolved nutrients never penetrate to the active roots.
It follows, then, that where and how plants are placed, and which plants are used are three of the most critical factors in using aquatic plants to control algae.
Types of Plants
I think that a discussion of the types of plants that are available is warranted here. Aquatic plants can be divided into four loosely associated groups. The criterion for inclusion in any group is growth habit.
The first of the four groups is the so-called “oxygenators,” which are those plants that 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.
Entirely submergent Oxygenators – Species whose juvenile forms act as oxygenators. (All of the species listed are commonly available.)
Of course, oxygenators as efficient users of dissolved nutrients as they are, are not the only thing to use to combat algae. In fact, I can’t tell you how many times I’ve combed string algae from my Anacharis or Ceratophyllum as I cleaned a pond or collected the plants for orders. It is also possible to get too many of them in a pond. This can be because they just make it difficult to see the fish, or for the fish or other animals to move around in the water, or because plants reverse their respiration at night, consuming oxygen and producing carbon dioxide. I maintain a 230,000-gallon pond wherein the Ceratophyllum grew to such an extent that the fish were at the surface gasping for air one early morning. As soon as the sun touched the water, the fish were all right again. Lesson learned.
The second group includes those plants that grow with their feet in the substrate and their leaves afloat on the surface of the water. This includes the genera Nymphaea (Waterlilies), Nymphoides (Water Snowflakes), Hydrocleis (Water Poppy), Brasenia (Water Shield), Aponogeton distachyos (Water Hawthorn), and many of the Marselia (Water 4-Leaf Clover) species. Most of these do very little for water quality beyond the extent to which they can use nutrients from the substrate that would otherwise be available to algae. In one sense, these plants can be thought of as conduits for soil nutrients to get into the water because they use nutrients to create their leaves and flowers, which subsequently decay, releasing all of their constituents into the water. But they do shade the water, and none of us would willingly forego having waterlilies in our ponds just because they do virtually nothing against algae. I have used their pots to harbor Anacharis or other submergent plants. Also, all of the Nymphoides and the two species of Hydrocleis produce floating colonies of numerous, well-rooted plants, each of which uses nitrogen and other nutrients from the water.
The third group includes those plants that float directly on the water, extending their roots into the water and their leaves into the air. Within this group are the obvious candidates, Eichhornia azurea and E. crassipes, Pistia stratioides, Azolla spp., Lemna minor, Salvinia spp., and various creeping plants, such as Myriophyllum spp., Aeschenomene fluitans, Neptunia repens, and Ludwigia spp. These are particularly useful in stripping all types of nutrient from the water. Hyacinths are well known for their ability to cleanse water of everything from nitrogen to heavy metals. The group includes some of the more interesting and beautiful plants in the hobby today, as well as some that are wisely banned in many states.
The fourth group comprises those plants that have their feet in the water and their leaves well above it. It includes all of the familiar tall bog plants, including, Nelumbo, Sagittaria, Pontederia, Hibiscus, etc. As with Nymphaea, and some of its cohorts in Group 2, they do very little to filter the water while they are rooted in soil. I have used Iris pseudacorus and varieties of Pontederia (Pickerel Plant) as effective filters by planting them in pots and then allowing them to over-grow the pots. As soon as they begin to expose roots to the water, they begin to compete with algae for nutrients, but they won’t do it if they are rooted into a substrate.
Which are Best?
Which plants work best for filtering water? Plants that expose roots to the water are the best for algae control. Exposing submersed foliage is good, too, but nothing beats the power of water hyacinth or water cress to strip nutrients from water. Usually a combination of oxygenators and floating plants with roots in the water is sufficient to combat most algae.
Building the Pond for Plants
Any discussion of how to build a pond for successful use of aquatic plants must cover each aspect of the set-up, from digging the hole, to skimmer and filter placement, to the planting and maintenance of the pond. A poorly laid out pond will not perform as desired, regardless of how nice it might look on paper.
As a case in point
I was involved with a job many years ago wherein the pond was to be about 6 ½´ deep, about 100´ in diameter, with an island in the middle. The estimated volume was 238,000 gallons. The original plan called for the filter to be built into a bed on the bottom of the pond. In a meeting of the principals one day, I brought up the scenario of what would have to happen 3 or 4 years hence, when it would be time to clean the filter. They would have to dump 238,000 gallons of water in a city where they look you up if they see too much water running down the gutter, remove, stow, and maintain the many large Koi, clean the several cubic yards of gravel, going through all of the time and water that that would entail, then re-fill the pond and get the fish back in before they were all dead from shock and asphyxiation.
They opted to replace that idea with an upward-flowing separate pond in which they would place a grate and cover it with crushed cinders. This was of only limited value, however, because it was impossible to clean thoroughly, and eventually became the primary source of nutrients for algae. We had to scrap it, and rebuild it so that instead of 3´ of lava rock, the water passed through one foot of it. The results were astonishing, and when a pressurized unit was added some time later, the pond never looked better.
Nevertheless, when they later added swans and ducks to the pond, the filtration could not keep up with the load until we added Hydrocotyl ranunculoides and Pistia stratioides to the filter pond. These were the only two species that we found were not immediately destroyed by the birds. Adding those plants cleared the water within a month of the plants’ introduction. And you’ve never seen such water lettuce.
I won’t get into a discussion of filtration except to say that, as the episode mentioned above illustrates, no filter is any better than it is easy to maintain. In the old days, we were stuck with filters built into the bottoms of ponds, or with upward-flowing filter ponds, and/or with pressurized sand filters that were built for pools and spas. The pressurized filters could be modified by making new laterals for the outflow, and by using crushed cinders instead of sand, but, because of the weight of the lava rock, they were difficult to backwash efficiently, and the nutrient load could become enormous before anybody thought to really clean it out. Again, in these ponds the filter can become the major part of any algae problems in the pond.
Now, however, the industries manufacturers have caught up with the hobby and there are very efficient filters available. Even using the new media in the old filters is a good thing. Backwashing is far more effective, with the result that the filter is at peak performance more of the time than before. Backwashing takes only moments, and uses far less water than washing lava rock did.
I will also say that the first line of defense against the unwanted accumulation of debris in the pond is the skimmer, and how they are placed can make a huge difference. I believe that they should go downwind of the prevailing winds. If the wind blows in two directions, put one at each end of the pond so that leaves blown into the pond can be skimmed off immediately. A skimmer also makes a good hole into which aphids or unwanted small floating plants can be rinsed.
Knowing as we do that what comes out of the filter is nitrate, and that nitrate is plant food, it makes sense to get to that nitrate solution as soon after the filter as possible with plants that are capable of stripping it from the water as it passes by them. Putting plants in the small filter box at the top of the waterfall is dangerous because they can quickly become so dense that the water can’t flow out quickly enough to prevent the filter tank from overflowing.
Placing pools and streams between the filter and the pond ensures that some portion of the system can be planted in such a way that a percentage of the nitrate will have been removed from the water before it gets back to the main pond. It also drives CO2 out of the water as it splashes over waterfalls and rocks in the stream. In-stream pools also provide settling areas for the fine silt that gets through the filter. They should be built with shelves along the sides for plants to sit on, and with a deeper area left in the middle for silt to settle into. This can be removed with a net. Streams should meander for interest and should include broad areas and narrow ones. The broad areas can be planted with shallow water bog plants to lend a more naturalistic look, and to provide an area for trailing plants with roots in the water. The narrow sections can be used to create sound and water-play.
The most commonly missed opportunity for plantings is along the margin of the pond. Even if a pond system has no stream, it has a margin, at least, and either algae or deliberately planted species will grow there. I would much rather see a nicely planted pond edge than a row of rocks with algae stuck to them. The “shelves,” if there are any at all, are almost always inadequate. Sometimes I approach a pond to find that there has been absolutely no thought given to how the thing was to be planted. In many cases, all that is provided is a narrow, shallow rim that runs the circumference of the pond, with no change in its size or utility. Properly built shelves can support a very naturalistic planting, and if filtered water is pumped into these areas, the plants can be used to strip nutrients from the water.
Proper, filtering shelves should be deep enough for a 2-gallon pot to sit with its top about 4˝ below the water surface. Anything can be blocked up, but it is nearly impossible to create greater depth in most ponds once they are built, so it is advisable to make the shelves as large as possible and about 12˝ deep. It should be of random width, but it should always be wide enough to support any plantings that are anticipated. It should also be built with a retaining wall on the outside edge so that debris can collect there and not fall into the pond. The accumulated material can be removed with a net from time to time to eliminate it from the system.
As mentioned above, in order to maximize the filtering action of the plants, I have always recommended that people put filtered water back into the pond through the bog shelves. Even if it isn’t filtered, it will still be processed through the plants to good effect.
I prefer to establish anchor plantings in aesthetically pleasing ways, and to grow what I loosely refer to as “connectives,” or those plants, that start in one place and creep around (from Group 3), extending roots into the open water between the pots as they grow. What I call “anchor plantings” are specimens of tall, emergent plants (from Group 4) such as Iris, Pontederia, Hibiscus, etc, that are planted in pots if I intend to repot them each season, or want a tall mature specimen to develop, or it trays when I want colony of plants that need not be repotted more frequently than each two or three years. Trays allow rhizomatous plants such as iris to grow more naturally than a taller pot will.
If the plant population remains high relative to the nutrient supply, algae will be minimal. Seasonal changes demand that we plant a variety of species to ensure that something will be growing for as much of the season as possible. Here in southern California, that means all of the time.
Most of the time, when I approach a new pond for planting, I am faced with very little to work with. The ponds are usually full of loosely stacked rocks that want to fall into the pond when they are touched, and if any shelves are provided, they are generally hopelessly inadequate. Or the pond is just an empty box or bowl in the ground, with no features whatsoever. In some cases, it can be relatively easy to set trays at the appropriate level with concrete blocks. If the tray is filled with soil and the plants are properly planted in them, the planting will look good in very short order. Another option for featureless ponds whose owners do not want Goldfish or Koi is to plant the pond as one might plant an aquarium, with grass-like species (Sagittaria subulata, S. sub subulata, Lillaeopsis attenuata) growing in a sand medium, with Nymphaea, Echinodorus spp., etc. planted among them. It can be quite lovely, and should not be dismissed as a way to have a nice water garden in a small spot with little or no algae.
In the pond that has only rocks along the edge, however, planting can be a challenge. I get as much soil in there as I can by using pots, or by filling in between the rocks with gravel and sand until I can get something to stay in place long enough to gets some roots growing. I look for species that will grow almost anywhere and on almost nothing. Lippia nodiflora, Bacopa monnierri, Marselia spp., and Rotala rotundifolia, are all good for this. Once these are established and have formed a mat that is substantial enough to support them, I add other, taller species. This can take a couple of years, and can be avoided by a well-informed builder.
In some cases, objects that hang into the pond, such as the roots of Philodendron plants, or Ivy, or some other terrestrial plant that has reached into the pond, can be planted with a selection of plants that produce colonies of plants such as Hyacinth. By trapping a few plants behind the root, branch, or pipe that reaches into the pond, one can, after a year or so, establish a floating mat of plants such as Eichhornia (Hyacinth), Rorripa (Cress), Mimulus guttatus (Marsh Monkey Flower), and other plants can be established. If this colony can be established near a water return line, so much the better.
The cardinal sin committed by pond builders today, in my opinion, is the inclusion of rocks on the bottom of the pond. I have recently learned that I am not alone in my condemnation of this practice. Apparently, I am not the only person to have noticed that they make it virtually impossible to remove debris from the bottom of the pond, and that they, themselves, provide a good substrate for various algae species to grow upon. Not only that, they are dangerous. It is very difficult to walk around on a bunch of rounded, head-sized stones that are slippery with algae and frequently out of sight without twisting an ankle, smashing a toe, or otherwise endangering the flesh. Also, putting stones on the bottom of the pond ensures that the maintainer of the pond will not be able to easily remove the very sort of nutrient load that algae grow on. It is especially bad when there is more than one layer of them. I have removed hundreds of stones from ponds in disgust at the waste of time and resources they represent, and the ecological problems within the pond that they cause. Having stones on the sides of the pond is acceptable, but not on the bottom.
One last point about setting up – Go Slowly. Don’t immediately overwhelm the pond with fish before the plants have had time to establish themselves. If the pond can ultimately hold as many as 10 fish, stop at 5, then after the pond has become established to the point where there are sufficient plants to deal with the load imposed by the fish, build up to the larger number of fish slowly. You’ve got the rest of your life to enjoy the pond; don’t risk making it unenjoyable by overwhelming it from the outset.
Maintenance of Plantings
Maintenance of the plantings that you depend upon to remove the bulk of the nutrients from the water is not difficult, but it does need to be done regularly for best results. There are the obvious tasks of regular removal of dead leaves and flowers from bog plants and Nymphaea (water lilies), and seasonal repotting of Nymphaea, Nelumbo (lotus), and other potted bog plants. But then there are the less obvious ones of removal of accumulated root masses from floating/creeping plants such as Rorripa spp. (Cress) and Ludwigia spp. (Primrose Creeper), and controlling aggressive species to ensure that they don’t overwhelm the less robust plants growing in the bogs.
Allowing the root masses of water cress to remain in the pond is to encourage algae and fungi. The roots of cress are fine enough to make an excellent filter for the fine silt that gets through the filter, and when it all dies, which it will do, it falls apart directly into the water, releasing its load of debris and feeding algae all the while. All of that matter can be removed in one motion by carefully removing the root mass once it has died at the end of the season. It also allows the new roots of the cress to grow into open water again, instead of into the mass of dead roots, where it will not see new water. We do this in the spring when it has finished blooming, and in the fall when it is fully grown and needs to be thinned for next year’s growth. Removing only what is visible above the water leaves behind the real prize, which is all of that silt.
Regular removal of debris from the bottom of the pond is a must, as well. I like to leave some of it behind in the winter to allow the fish a warm place to go into when things really get cold, but I remove it as soon as I can in the spring. Sludge removal can be accomplished with a net, and does not need to be done with a siphon or other vacuuming device. These only waste water.
I know that it is popular to dump the water and to clean the ponds each season, but I disagree with that approach, and wouldn’t empty a pond unless there was no other option. This is an especially important issue in the southwestern states, where water comes at a premium. It is a different question in Wisconsin, but here, dumping water needlessly is rightly frowned upon by all. I have ponds that haven’t been torn down for 20 years and they have no problems with algae. I simply remove the debris with my net with each maintenance visit and that works fine. My ponds get partial water changes with the back-flushing of the filters.
If the pond has one of the upward-flowing “bio-falls” filters, I keep it clean, but I do not depend upon it to completely treat the water, especially in large ponds. I would prefer to immediately throw the bags of lava rock out and talk the customer into a pressurized unit that will really filter the water, instead of just providing a fat spot with useless inclusions into the plumbing system. If they don’t want the filter, I still feel as though I am better off without the bags of gravel. They are only as good as the bag is capable of passing water, and if it is easier for the water to flow around the bag, that is what it will do. Once the bottom of the bag plugs up, the water flows around the bag, leaving the ammonia essentially untouched. Also, these filters are often placed at the tops of hills, buried in landscaping, and so are difficult to service safely, so they are often neglected.
As part of the effort to limit the build-up of organic debris, I do not allow material from without the pond to get into the pond in the first place. I go to great lengths sometimes to anticipate the arrival of debris from the garden, going to points in surrounding rocks where debris piles up before going into the pond.
Feeding pond plants can be a matter of some concern among pond keepers because of the obvious issues associated with introducing fertilizer to the water. I feed my plants with a timed release fertilizer at the beginning of the season, and after that only if it is absolutely necessary. I feed established plantings of bog plants twice per year by gingerly sprinkling the same timed release fertilizer into the plants nearer the outside edge of the plantings than the water side. As it dissolves, the plants pick it up as needed.
While plants are an integral part of any aquatic system, and it is true that they can eliminate many problems related to algae, they cannot be depended upon to do the entire job all of the time. They might do it all of the time in a particular pond, but they will not be able to do it all in every pond. They need assistance, and that can come in the form of having proper places provided for them, and with proper maintenance. It also comes in the assiduous limitation and constant removal of organic debris from the pond by the pond owner, limiting fish populations to reasonable levels, and by the complete conversion of water-borne nutrients into forms useful to the plants.