A water garden retailer once asked me, “what are the best aquatics to use as natural filters for our display ponds and to sell for customer ponds?”
I have long been an advocate of using plants for pond filtration. It is a scientific fact that photosynthesis is the best means to purify water. It not only removes ammonium but also heavy or toxic metals from water. Water, without plants, will eventually degrade in overall quality and need to be changed. Thus, ponds with insufficient or no plants will need frequent water changes. Plants are nature’s filters and should always be included in ponds and aquariums.
The following is a list of benefits deriving from plants photosynthesis, taken from The Ecology of the Planted Aquarium by Diana Walstad:
1. Protect fish by removing ammonia. Aquatic plants have a preference for ammonia.
2. Protect fish by removing heavy metals from the water. Heavy metals weaken fish. Plants take up large quantities of heavy metals, such as: lead, copper and zinc. Also plant decomposition produces humic substances that bind and detoxify metals.
3. Control algae. Plants compete for nutrients and release allelopathic chemicals that are toxic to algae.
4. Stabilize pH. Photosynthesis is a major acid consuming reaction. This keeps water from becoming acidic over time. Nitrification caused by biological filters increases acidity of water.
5. Increase biological activity. Most bacteria live attached to surfaces. Roots and leaves that extend into the water column provide an ideal home for bacteria.
6. Oxygenate water. Plants release more oxygen then they consume. Photosynthesis releases oxygen. (Daytime)
7. Remove carbon monoxide from water. Excess carbon monoxide can cause respiratory distress in fish. Photosynthesis consumes carbon monoxide.
8. Prevent substrates from becoming toxic. Plant roots help keep substrates healthy.
Also a quote from the Dynamic Aquarium by Walter H. Adey and Karen Loveland is very much to the point: “the key to ecosystem management is stability achieved by locking nutrients up in biomass rather by using biological filtration.” Mr. Adey is a renowned expert on designing filtration systems for public aquariums and replicating natural eco-systems.
The best plants to use for filters are usually those that are aggressive spreaders and that make abundant roots. The faster they grow, the better they filter. Also the root mass acts as a mechanical filter harboring both anaerobic and aerobic bacteria. These bacteria are important to maintaining good water quality.
The length of the growing season is also important as plants with a long growing season perform the filtering function over a long period of time.
Another factor is salt tolerance of the plants for those of you that like to maintain a 0.3% salt level in your pond.
Looks are important too. Why not have beautiful plants for your natural filters?
Here are some of my favorite filter plants
Oenanthe javanica, Water Celery. Long growing season, edible, and attractive but not showy. Probably the best for filtering. Picture 1 shows Water Celery flourishing in my plants filter servicing my large pond. If you want a prettier celery, try the variegated version of this plant.
Cyperus alternafolia, Umbrella Palm. Stately and attractive but not a spreader. Very salt tolerant. Picture 2 shows Umbrella Palm growing in a bio-filter servicing one of my fish holding basins that has a .3% salt level.
Eichornia crassipes, Water Hyacinth. Short growing season except in the Deep South; attractive foliage and flowers; but very salt intolerant. A great filter plant. Picture 3 shows Water Hyacinth growing in the top of a bio-filter servicing a small pond.
Other salt tolerant plants are Hibiscus moschuetos, Swamp Hibiscus; Peltandra virginica, Arrow Arum; and Typha angustifolia, Narrow Leaf Cattail.
Ipomea, sweet potato plant. This beauty I discovered as a result of growing it in one of my island planters. Despite being terrestrial, it loves water and creates roots faster than almost any plant I’ve seen. In the picture on page 9, you can see a lime green variety shown in a wetland raft. There is also a variegated variety that is equally beautiful but not as robust. Ipomea is a short season plant, but would add showy color to your filter garden.
There are many plants that will do well as filter plants, and I recommend that you experiment with other plants and combinations of plants. Almost all pond plants and even some non-pond plants will do a good job.
If you are interested in building your own plant filter be sure to give it a surface area of 10-20% of the surface area of the pond you want it to service. Figure 1 is an illustration of a plants filter with bottom drain for draining off the sediment that will surely accumulate from your pond. You can build a plants filter from lumber, block or whatever materials you have on hand, but be sure to give it a smooth flat bottom for easy sweeping of debris and sediment. It must be higher than your pond and can flow into a return stream. The stream will enhance the benefits of the overall filter system.
Added Benefit – Plants are great for filtering your pond and attract additional wildlife to your garden.