Soggy Conditions? Some Plants Drink It Up.

Acorus gramineus, variety Ogon, also called grassy-leafed sweet flag.
Acorus gramineus, variety Ogon, also called grassy-leafed sweet flag. (Courtesy of the New York Botanical Garden)
By Adrian Higgins
Thursday, July 5, 2007

A neighbor once asked what could be done with his rear yard. He led me to a space about 100 feet deep and 150 feet wide. There were a few trees but mostly an ailing lawn yielding to moss and ground ivy.

The yard collected rainwater from neighboring properties, and there was no way to drain the water short of tying in to the city's storm sewer, which wasn't an option. I told him to build raised beds so that fresh plantings wouldn't rot in constant wetness. My advice today, 20 years on, would be quite different.

In areas that receive near constant moisture, the trick is to shift to a palette of plants that will thrive in such a place. Stick in the common fare of azaleas, yews, boxwood, rhododendrons, etc., and you will quickly see your investment shrivel and brown as their roots suffocate in wet, airless soil. But if you get to know some of the swampier garden plants, which have evolved for these conditions, the wretched problem can become a wonderful opportunity to have fun with captivating plants.

I have two soggy parts of the garden, in opposite corners, but both below a broad hill with seeps that inject moisture into the soil long after the rains have stopped. The first step was to add lots of organic matter to the areas so that the moisture would be even and persist in periods of drought.

In the smaller area, I have planted sweeps of two perennials, the marsh spurge ( Euphorbia palustris) and mountain fleece, variety Firetail ( Persicaria amplexicaulis Firetail). I am inflicting the botanic names on you in case you want to find them with confidence.

The spurge is big and shrublike, producing compact plants that in optimum conditions reach three feet high and at least as much in width. It produces bright yellow flower clusters in April before it fully forms for the season. The mountain fleece is quite different, upright with kinking stems and spearlike leaves, but topped with pinky red bottlebrush flowers. I can think of no other perennial that blooms for so long. The flowers emerge in July and are still effective in December. Together, the mountain fleece and spurge form a textural pas de deux that never tires.

In the other wet area, which has more elbow room, I planted moisture-loving trees, namely a black gum and dawn redwoods, which have grown vigorously. Around the black gum, I have planted the golden variety of creeping Jenny, and it has spread to form a glowing carpet beneath the black gum. Nearby, Siberian irises are lapping up the moisture.

Other irises would be just as happy (not the bearded types), namely the Louisiana and Japanese irises and, if conditions are truly wet, the flag irises. A similarly useful plant, an herb named Acorus, is also referred to as sweet flag.

The point is that there are so many plants that will work in wet soil that it is possible to go to town and make a real garden out of it, whether the site is in sunlight or shade. The shadier the site, the more you have to rely on leaf forms than flowers for ornament. The same holds true for normal gardens. Where the two differ, however, is that moisture-loving plants will sink fast in full sunlight if the soil dries.

You also have to pay attention to scale, as some of these swamp lovers may be herbaceous but get large with all that moisture. Given the right site, you could make wonderfully bold landscapes with coarse-leafed perennials such as joe-pye weed ( Eupatorium fistulosum: Gateway is a decorative variety) and ironweed ( Vernonia noveboracensis), which grows to seven feet by late summer, when it produces sprays of reddish-purple, thistlelike flowers.

The golden groundsel ( Ligularia dentata) is favored for its thick, large, round leaves, so decorative that when the plants produce their yellow, daisylike blooms, Jody Payne removes them as a distraction. She is the curator of the Rock Garden at the New York Botanical Garden, which has its share of wet areas and moisture-loving perennials.

Other larger-than-life swamp plants include the native turtlehead, Chelone glabra, the exotic butterbur, Petasites, and its native version, the umbrella plant, Darmera peltata, as well as the Rodgersia pinnata.

Payne likes those, and also commends the native marsh hibiscuses, the rose mallow ( Hibiscus moscheutos), with huge tropical-like flowers in August and September, and the swamp mallow ( H. coccineus). Horticultural professor and author Allan Armitage, in his book "Armitage's Native Plants for North American Gardens," also suggests the halberd-leafed mallow ( H. laevis).

Ferns are notoriously needy for moist areas. For out-and-out damp areas, Payne suggests the crested wood fern, Dryopteris cristata, and the toothed wood fern, Dryopteris carthusiana. "The cinnamon fern [ Osmunda cinnamomea] will tolerate a lot of moisture," she said, "and so will the sensitive fern and interrupted ferns" ( Onoclea sensibilis and Osmunda claytoniana, respectively). I would add the royal fern, Osmunda regalis, and the ostrich fern, Matteuccia orientalis.

Payne is happy to use plants that spread a bit too much in damp soil, as long as they are assigned their own area and are kept in bounds. This includes the horsetail, immortal if not universally beloved; houttuynia; and gooseneck loosestrife, Lysimachia clethroides.

Not all of these plants are big or bullying. Payne is a big fan of the candelabra primroses, notably Primula japonica and P. rosea. " Primula florindae is blooming right now, and it's gorgeous," she said. It's heavily scented, to boot.

Both the cardinal flower, Lobelia cardinalis, and the related great blue lobelia, L. siphilitica, are long-flowering perennials for the damp.

The globeflower, Trollius cultorum, blooms for weeks in late spring. Payne says the wet garden needs two delicate orchids. The grass pink, Calopogon tuberosus, is like the florist's moth orchid but upright and slender, with intense pink flowers in early summer. The orchid known as slender ladies'-tresses, Spiranthes lacera, produces white blossoms that spiral all the way up the stem.

Wet gardens and human feet don't mix. Mucky shoes are unpleasant for the gardener, but the gardener can also squeeze the life out of the soil. Payne wisely suggests that the swamp garden contain large steppingstones placed on gravel, to allow access and enjoyment. If and when the stones sink, you can lift them and top up the gravel base. It may be the only time you will curse the dampness.

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