[from the June 2001 issue of Tropical Fish Hobbyist, pages 44-49]

Expanding the margins

by Tony di Fiore

Marginal plantings are an often-overlooked part of a complete water garden. A pond with beautiful water lilies in the middle of your garden in unarguably a visual delight, but add marginal plants and the pond integrates into the garden,forming a gestalt even more beautiful. Because these plants do not respect the boundary between water and soil, they seamlessly pull together your pond and the rest of your garden. In formal gardens, where the goal is not a natural appearance, the pond is often geometric in shape, and its margins are meant to be noticed. Even in such cases, however, marginal plants can add vertical interest and color, and they can be part of the synergic symmetry of even the most formal landscape.

What are Marginals?

Marginal plants are species that like to have at least their feet wet. They are bog or semiaquatic plants that grow in water less than 6 inches deep or in swampy soil. Called marginal because they hug the perimeter of the water, if planted in shallow water they may grow out onto the banks, and if planted on the banks they may grow down into the water.

The ideal way to grow marginals is on the ledges or shelves of your pond -- hopefully it has them! These are typically shallow ledges at the pond perimeters, but large ponds often have one or more marginal shelves out in the depths, producing an "island" of plantings to break up the open expanse of water. Although different species of marginal plants need different water depths but most ponds have a ledge at only one depth, it is an easy matter to place bricks under the pots to raise each one to its preferred location. This is only a little trickier in ponds without plant ledges; in such a case you must be careful to build stable platforms, not teetering towers that can easily tumble.

These plants range from tall uprights like rushes and cattails to groundcovers like pennywort (Hydrocotyle verticillata). Many of these plants are downright weeds, growing quickly and aggressively. For this reason they are usually grown in containers so that they can be kept from taking over the whole pond area.

Swamp Filters

Gardeners with swamp filters, on the other hand, do not use containers. Swamp or bog filters really allow you to have your cake and eat it, too, and to kill two birds with one stone. Do they really rate such cliches, and what are they?

Swamp filters are bog gardens that are plumbed into your pond. To make one, you excavate the area, put in a pond liner, and then refill it with soil. Water flows from the pond into the bog and from the bog back to the pond; obviously one of these flows will have to be pumped against gravity. The plants and the bacteria in the soil convert and take up massive amounts of fish wastes. The soil, of course, traps solid debris, providing mechanical filtration as well, and such a system typically serves as the only filter for the pond.

You can plant invasive marginals in the bog,since rampant and vigorous growth is what you're seeking, but the liner prevents them from taking over everything. You also get a beautiful display of marginal plants while providing enormous biofiltration, filtration that is often enough to completely eliminate algae problems by itself! If you'll forgive one more cliche, keeping your water clean and clear while enjoying a beautiful marginal garden is definitely a win-win situation for the aquatic garderner.


There is an enormous variety of species that can be used in marginal plantings, both hardy and tropical. Some of them will be familiar to aquarists, who may be surprised to find that an aquarium favorite is actually a bog plant that, iven the chance, will grow out of the water, including swordplants of the genus Echinodorus For the most part, tropical marginals are kept by specialist water gardeners or by those who have indoor ponds. The hardy species that will overwinter in place are much more common in outdoor gardens.

The only restrictions on the adaptability of marginal plantings are the imagination and sense of aesthetics of the gardener. They are used to blend the edges of a pond into a garden, to ornament waterfalls, to provide spawning areas for fish, to create a natural look, and to remove wastes from the water in swamp filters.

One of the most effective uses of marginals I have ever seen was one that emphasized the margins to the exclusion of the pond! It consisted of a meandering stream through the garden, which flowed around a course of large and small rocks. Between the rocks were planted a huge variety of marginal species, blooming in a profusion of colors. It was nothing but margins, and it was gorgeous!

Another place where marginal plants are extremely effective is in container water gardens -- barrels and tubs that are especially popular with gardeners whose only "yard" is a balcony. By adding emergent growth, marginal or two can add a vertical dimension to a container pond, and you can get an almost seamless integration effect by placing tall potted plants around the back of the container where the marginals are planted. Likewise, viny marginals that like to spread along the ground or up waterfalls will hang down the sides of the pond, further blurring the sharp image of a bucket sitting on the balcony.

Your nursery can guide you in selecting species suitable for your location and for what you have in mind for your water garden. Let's look at some of the more popular plants.

Reed-like Plants

To create a bed of reeds, you can use rushes of the genus Eleocharis, cattails of the genus Typha, sedges of the genus Carex, some hardy bamboos, or a plant clled dwarf bamboo (Dulichium arundinaceum), which is not a true bamboo but looks like one. A gentle sloping down into the pond can be achieved by planting tall (3 feet or more) varieties farthest from the water and graduating to dwarf types (one foot) next to the water.

Tall marginals are perfect for creating a focus in a pond located in a large garden, or for a background screen, or to create a sense of privacy and isolation. Most of these plants are extremely invasive; in natural ponds, they can quickly overtake not only the banks but also most or all of the pond as well. On the other hand, if rigorously controlled, they are a very effective planting when permitted to grow from the shallows to the shore. The same type of effect can be had with much less effort by using potted plants with a little bit of planning and ingenuity.

Other Tall Plants

Marsh lilies (Crinum spp.), flag irises (Iris spp.), taro (Colocasia spp.), and callas (Calla spp.) are some of the other tall marginals that can form a background planting but provide some contrast to the grass-like reeds and such. There are quite a few popular plants in this category that are tropical species. Potted, they can be enjoyed during the summer at the edges of your pond and brought in to be cared for as houseplants. Obviously you'll need to be vigilant to keep the soil from drying out.

Flowering Marginals

Many of te plants we've already mentioned have beautiful foliage, but some marginals are gron especially for their blooms. Prized for its bright red flowers on upright foliage is the cardinal plant (Lobelia cadinalis) and the similar Lobelia siphilitica with its blue flowers is, if anything, even hardier.

You'll find the marsh marigold, Caltha palustris, in almost every water garden. It is extremely hardy and has bright yellow flowers early in the season. It commonly takes a rest during the summer and blooms again in the fall. If you have a slightly shady pond, the marsh marigold can help bring some color to it.

The swamp daisy, Helianthus angustifolia, is a flowering background plant that is also very hardy. It can reach 7 feet in height and blooms a yellow flower late in the season.

Another tall one (4 to 6 feet), Hibiscus moscheutos, is hardy and blooms all summer with large red, white, or pink hibiscus flowers.

The very popular pickerel rush, Pontederia cordata, is also hardy and produces large, broad, pointed leaves and stalks of lavender flowers all summer.

A 12-inch marginal that does double duty with aroma and flowers is the mint Mentha aquatica, with mint fragrance and lavender blossoms. This plant also likes some shade.

These and many other species are available to fill just about any need in your water garden.


So how do you decide which plants are best for your aquatic garden? You should buy your plants from a reputable nursery, which can be a mail-order or on-line nursery as well as one you visit personally. The people there should be able to answer questions and make suggestions, and if you're working from a catalogue or online display, the pictures will tempt and delight you -- especially if there's snow outside your window the way there is mine as I write this!