The Grid Veggie Filter

Published on January 15, 2009


In this article I am introducing to PondBiz readers a unique technique for growing aquatic plants that enables them to be used as excellent phyto or veggie pond filters. At one time I was going to patent this technique, but I am now more focused on sharing the information with clients and other water gardeners.

Using plants as filters for water gardens and aquatic ecosystems is a very important concept, and is commonly used. There are always “new” techniques being developed for growing the plants in ways that improve their filtering capability.

Some enterprising water gardener installers and businesses are using bog filters planted with hardy plants. I really like these and can easily see an application for them in most water gardens.

>> Check out Nelson’s Water Gardens web site for ideas on bog filters,

There are many types of tropical plants that float with their roots dangling in the water and serve as excellent plant filters. However, overwintering large numbers of these plants to ensure having the same filtering capability in the Spring that we had from the Fall before, when the tropical floating plants were at their peak, is difficult in temperate climates. So, many of us are using hardy plants that are rooted in some kind of substrate in an attempt to have a head start on some type of veggie filter in the Spring.

Aqua Ultraviolet
 Most plants that are rooted in a substrate can extract the majority of their nutrients directly from that substrate and not from the water column, while the types of plants that float with their roots dangling in the water are dependent on getting most of their nutrients from the water itself. These floating types of plants are the best forms of “veggie” filters, because the huge mass of roots are not only extracting large amounts of nutrients from the water, but are also providing a huge surface area for a biofilm to exist. This biofilm is what I have termed “A Pond’s Patina,” or APP. In APP there are all kinds of bacteria, invertebrates and good algaes. This biofilm exists on nearly everything the water is in contact with. Roots have a type of surface that is difficult to simulate with any other type of material that we may want to use. So, it is unique, providing a different niche with more surface area, than the type of surface that rocks or liner provides. This means that more types and species of good critters can exist in an ecosystem. These bacteria and invertebrates provide a critical link in the web of life, or ecosystem.

There are water gardens that have a large bioload (overpopulated with fish and excess nutrients) that appears to be functioning well without an added biological filter. The fish seem to be doing great and the water is “gin” clear. In most situations, where these conditions exist, there is usually a huge proliferation of floating tropical aquatic plants. Most of us have seen this, but when the temperatures fall and the floating plants die, the conditions for the fish deteriorate quickly. Water quality starts to deteriorate, planktonic algae can proliferate and the whole ecosystem can suffer.

What is needed for those of us in more temperate areas is a type of plant that is hardy and can survive with its roots dangling in the water. All of the nutrients that the plant needs would have to come from the water and not from a “soil” or substrate. Ideally, the plant would stand upright and grow easily in these conditions. A water gardener would then have the ideal plant filter functioning 12 months a year. Yes, I did say 12 months – even during periods of heavy ice, this type of plant should be able to extract nutrients. Albeit, not as many, but it will still be filtering. I know that this type of plant can filter year round, but I am also certain that it transports oxygen to its roots beneath the ice, thereby adding O2 to the water column. This is a time of the year when more oxygen is always welcome.

It was by accident that I discovered such a plant. It is Iris pseudocorus, or yellow flag iris. I had a 14˝ diameter pot of it growing in topsoil and it had overgrown the pot. It literally kept growing outside the pot with its roots dangling in the water column. The yellow flag that was growing outside the pot was as vigorous as the plant that was growing in the soil inside the pot. It was easily 36˝ in diameter and only 14˝ of it was inside the pot! This plant is banned in a few states because it is so vigorous, but for those regions where it is not considered invasive it can be used to our advantage.

I have found that the yellow flag iris can be “planted” on a grid or grating that has openings of from 1˝ to 1 1/2˝. Plastic bread trays or plastic dog kennel grating works very well. I like to cut the size of the Grid to 1´ by 2´. The iris rhizome should have its roots pulled through the grid so they dangle below it. The rhizome is initially “tied” to the top of the grid with a black plastic tie. This provides the initial support for it to grow. I like to “plant” one rhizome per every 16 square inches on the grid. You could plant it heavier to jump-start to attain a fully mature grid faster. Once the iris rhizomes grow across the grid it then becomes self-supporting with a huge mass of roots dangling below.

The Grid needs to have stability, so a framework is built to support it. This can be done with?˝ Schedule 40 PVC pipe that is made into a table like framework. The joints will need to be glued together with PVC cement. The height of the framework can be adjusted for each pond’s depth so that the rhizomes are at least 4˝ below the surface of the water. Yellow flag iris can grow with its crown in 2´ of water, or even deeper, but does best with 4 to 8˝ of water over the crown. The important aspect is to provide as much space under the grid as possible so that the roots can dangle. If you plant it any shallower then fluctuating water levels can leave the rhizome out of the water. Any kind of desiccation of the rhizome can kill the plant or at least inhibit its growth.

It is very important to provide stability for the framework by including stabilizing legs that are perpendicular to the framework and on the bottom. When the iris grows on this grid the plant will become top heavy, and it does not take much wind to blow it over. You can add weight, such as a clay fired brick, to the bottom of the framework thereby giving it added stability.

The Grid Veggie Filter can also be used within a koi pond if the roots of the iris are protected from the foraging behavior of koi. This can be done by closing in all four sides of the Grid with additional grating. This allows free movement of nutrient rich water and will prevent fish from foraging on the roots.

Put it Anywhere

The Grid can be placed anywhere in the water garden, but the best locations are where there is some water continually flowing around the roots. This can be any place in a well circulated water garden. I believe the filter is most effective when placed in the faster water flow of a stream or biofalls.

Grids can be supported on the ledge of a biofalls without the PVC framework. They can grow very fast in these situations because of the upwelling of nutrient rich water continually bathing the roots. I recommend that the guts of the biofalls be taken out. Otherwise, the roots of the iris will intertwine in the bags of lava rock and the mats. This can be a nightmare to untangle. Most of the time these biofalls are not maintained the way they need to be anyway and the Grid Veggie Filter provides many times the filtration that the lava rock and mats did. Rock can be placed around the edge of the Grid to give it stability.

A good ratio of planted yellow flag iris Grids in a water garden or fish pond would be a 12˝ x 24˝ Grid per 2,000-gal of water.

Maintenance of the Grid Veggie Filter is Simple

Occasionally, parts of the plant will die for some reason and these sections will need to be removed. In the Fall, after the first couple of hard freezes, the iris on the Grid can be trimmed. I like to trim the iris fan to about 6 to 8˝ out of the water with hedge shears. I first take the Grid out of the water to do this so that all of the trimmings fall onto the ground and not into the water. If this is not done the top will bend over and decompose in the water over the winter. When the fans are cut back, you are removing a huge amount of nutrients from the ecosystem that were taken up by the plants. The plants remove the nutrients from the water and you remove the excess plant material.

Yellow flag iris is very hardy and can survive freezing in water even when it is simply floating. When it is anchored or “planted” to the Grid it becomes very robust even after a single growing season. I have attempted to grow many other kinds of hardy plants on The Grid but not with the success that I have with yellow flag iris.

The beauty of this filter is that it does not require a water pump for it to function. It can fit into any kind of water garden, even a container garden. It is a beautiful plant and more attractive than a black box that needs to hidden. Maintenance is relatively easy enough that anyone can do. It works 12 months a year, even during the winter, under the ice. Finally, I can honestly say that it is a very inexpensive filter that every water garden should have.

Kloubec Koi Farm

3 thoughts on “The Grid Veggie Filter”

    1. Iris pseudocorus is native to Western Asia and NW Africa. It is considered invasive in a lot of areas in the US. You will have to check if it is banned in your state. In most of the Midwest’s states it is not banned but is regulated, depending on the state. The plant is one of the best at growing in conditions without soil as long as there is nutrient (i.e.. fish waste) in the water. I am experimenting with other aquatic plants to find a substitute.

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