Mary Meyer, Extension Horticulturist
My backyard is at the bottom of a slope that easily floods when we have a 'rain event' of a few inches. The water can even stand in this area for 12-24 hours - fatal for some plants. To make matters worse, the soil is sandy, so when rain is scarce, the soil is bone dry- again fatal again for some plants. I have three tough perennials that have lived well in this area, all of which are shade tolerant: ostrich fern, Canada anemone and gooseneck loosestrife. All three of these plants have aggressive rhizomes that have helped them to live in this difficult site. I believe all of these plants will grow well in USDA Plant Hardiness Zone 3. However, northern seaoats will only live as an annual, but it will self-sow enough to come back each year.
Ostrich fern (Matteuccia struthiopteris) is a tough native fern (photo 1). Some people may think the ostrich fern is too tough, with its rhizomes and substantial root system. I love the soft feathery fronds and the fact that it will tolerate standing water and survive droughts. In the driest years, the plants are shorter, the rhizomes do not spread, and the plants die back prematurely. In wet years, it begins very aggressively and grows into the adjacent lawn. I transplant it to other shady locations where I want it to grow. The fertile fronds are stiff, much shorter and still standing in the spring when I break them off and push them into the ground as a standing border for the sedge planting that is up the hill under the box elder. This acts as a border signaling my husband where to mow the lawn and not the sedges.
Canada anemone (Anemone canadensis) is another tough native that easily can be too aggressive due to creeping rhizomes. However, the tough conditions of this site keep it in bounds. It is a sea of 1-2 foot tall white flowers in the spring. The foliage is often confused with wild geraniums, but the single white flowers have a cluster of yellow stamens, typical of the buttercup family, and lack the beak-like style of geraniums. Canada anemone foliage makes a thick ground cover that competes well with other weeds, even buckthorn seedlings!
Gooseneck loosestrife (Lysimachia clethroides) is a 2-3 foot tall perennial native to China (photo 2). I am adding it anyway because it has lived in this tough situation as few other plants would. It also has aggressive rhizomes. In dry years the plants are short and do not spread at all, while In wet years it is tall and robust. I step on it to keep it in bounds as I go to the compost pile, or edge it with the lawn mower. The gooseneck flowers are fun to look at and a delight in floral arrangements. It is one plant that blooms regardless of weather conditions.
Other plants (grasses are my favorites!) that I could add in this site are:
Wood oats or Northern seaoats (Chasmanthium latiflium) (photo 3) are native to south central U.S. and marginally hardy in zone 4. This grass self-seeds and although one plant may die, another will likely come up on its own. This is a bunch grass, with no rhizomes. It has the best flowers for dried arrangements and will last for years if picked early before the seeds are fully developed. The pendulous flowers are flat and beautiful in the fall when they turn bronze and yellow. This grass is native in wooded areas along river banks and it prefers wet sites where it can grow to 4 feet. In drier sites it may only be 2 feet tall. A newer form 'River Mist' is yellow and white striped, much shorter e.g. 18 inches, and has only lived as an annual for me.
Prairie cordgrass or slough grass (Spartina pectinata) is a larger, 4-6 foot tall, long-leaved grass for wet sites that will tolerate standing water and lakeshores. Native to prairies and often found in roadside ditches, slough grass has creeping rhizomes ideal for binding lakeshores. This grass is large and coarse, good for larger sites. In areas containing only cattails or reed canarygrass, prairie cordgrass can be added to increase diversity. An ornamental form of prairie cordgrass ('Aureomarginata') has yellow stripes on the foliage and creeping rhizomes. Prairie cordgrass prefers full sun conditions, but can grow in light shade.
Native switchgrass (Panicum virgatum) is in almost every roadside ditch across Minnesota. Many ornamental forms are available; we are trialing 17 of these at the Minnesota Landscape Arboretum Grass Collection (Photo 4). Plan to come in September and pick your favorite. The 2014 Perennial Plant of the Year ('Northwind') was selected by the Perennial Plant Association for its 5 foot tall, stiff upright form and olive green foliage. 'Shenandoah' (4' tall) and 'Ruby Ribbons' (2' tall) both have red foliage and flowers. "Cheyenne Sky' is purple and red and grows to 4 feet tall. Most switchgrass plants will tolerate wet soils quite well. They are taller in wet sites and shorter under dry conditions. Switchgrass is a bunch grass and does not have creeping rhizomes, however, it can self-seed readily. Switchgrass prefers full sun conditions and will only tolerate very light shade.