Gardening With Natives in Wet Areas
PLANTING IN WET CONDITIONS
This article will address the challenges of planting in wet areas. We will begin by considering what constitutes a “wet” area, and how to define varying degrees of wetness. We will also focus on the factors that cause wetness, and whether a goal of decreasing moisture is a desirable, or even a viable, option. We will then consider native plants that are adaptable to wet areas. In addition to moisture needs, plant selection must also include an evaluation of other requirements and specific gardening objectives. These objectives might include attracting birds, contributing to the natural ecosystem for native insects, decreasing lawn areas, erosion control, bloom times, color of bloom, and many other factors. This article also illustrates how to use native plant databases to identify plants that meet specific criteria.
LACK OF OXYGEN
The basic components of soil are minerals, organic matter, water, and air. The typical soil consists of approximately 45% mineral, 5% organic matter, 20-30% water, and 20-30% air. Plants cannot tolerate extremely wet sites because soil that is water-logged has no room for oxygen. Where oxygen is lacking, water and nutrient uptake stops, plant processes and growth cease, and plants begin to decline or die.
Soil moisture should be viewed in two main ways: 1) drainage down through the soil and 2) runoff water across the site. Soil texture and structure will determine how fast the soil will drain. Texture is determined by the percentage of sand, silt, and clay. Sand particles are relatively large; clay particles are very tiny in comparison to sand; and silt particles are medium-sized. This means that sand drains the fastest and clay drains the slowest.
Soil structure is the arrangement of soil particles into aggregates. When you pick up a handful of soil, and it breaks apart into little pieces, you are looking at soil aggregates. Aggregate structures provide both large and small pores. Large soil pores allow water to quickly infiltrate the soil. Smaller soil pores can store available water in times of limited rainfall. To help you recognize your own soil properties, conduct a simple drainage test:
Dig a hole approximately one foot deep and fill it with water. Time the rate (on an hourly basis) of water drainage out of the hole by measuring how far the water line moves in inches. If the water drains away at about one inch per hour, you have a desirable, well-drained soil. If drainage is much faster, your soil is probably high in sand; if drainage is much slower, your soil is probably high in clay.
NATURALLY HIGH WATER TABLE
Perhaps your property is wet because it has a high-water table. Some sites, such as my local area, have natural springs and a high groundwater level. On August 23, 2011, a 5.8 magnitude earthquake occurred at a depth of about 3.7 miles beneath the town of Mineral, Virginia, located approximately 27 miles east of Charlottesville. The event is among the largest earthquakes recorded in the eastern United States and was known for damaging the Washington monument. It is of particular interest to this article because groundwater levels in our local area were affected. The most common groundwater-level response to an earthquake is a water-level oscillation. Changes can be large enough to make a well flow at land surface, or to cause a well to go dry. Typically, however, the water-level changes are several feet or less. Recovery to the pre-earthquake water level can be nearly instantaneous, or it may take as long as days, months, or never recover. Some properties in my neighborhood went drastically from dry to wet and negatively affected the plants that were thriving.
Before the earthquake, neighbors had beautiful fields of lavender of many different types. They extracted the oil from lavender, and dried the leaves. Lavender needs lots of sun and fast-draining soil. It will not survive long in shady, damp, or extremely cold conditions. After the earthquake, the neighbor’s property became too wet to grow lavender, and they had to let the fields go. They planted moisture-loving plants such as mountain mint (Pycnanthemum virginianum) and other wildflowers. A tulip poplar seedling (Liriodendron tulipifera) started on its own and has grown to a good size. In other wet areas, they planted sweet bay magnolia (Magnolia virginiana), arctic fire red twig dogwood (Cornus sericea ‘Farrow’ Arctic Fire), swamp milkweed (Asclepias incarnata), red maple (Acer rubrum), weeping willow (Salix babylonica), river birch (Betula nigra), swamp white oak (Quercus bicolor), and nuttall red oak (Quercus texana).
Man-made situations also can be the cause of wet sites. Water runoff from pavement, roofs, and other impervious structures can flood a site quickly. When slopes are cleared of natural vegetation that would normally absorb excess water, water runoff can become erosive. New construction, often accompanied by hard-surface driveways, disturbs soil and vegetation and can cause drainage issues. Water may be unable to drain through soil that has been compacted by heavy machinery, or even by heavy foot traffic. When possible, use permeable materials for driveways and paths.
You can test your water table to help you better understand your property: To test for a high-water table, dig several 2’ deep holes at the planting site and check them after 2-3 hours. If water has collected in the holes, the water table is high.
CAN A WET SPOT BE “FIXED”?
Soil amendments are only effective in relieving soil moisture problems if the entire root zone of the mature planting can be changed. Therefore, adding organic matter or sand to tree planting holes is ineffective and often destructive. The roots will be discouraged from extending beyond the amended area. Shrub beds may be amended with additions of large amounts of organic matter such as compost and composted sludge. Sand is ineffective in improving drainage unless more than 50 percent of the total soil volume is sand. Consequently, sand is usually a poor soil amendment. Trees on marginally well-drained sites may be planted with as much as one-third of the root ball above the existing soil line, and then mulched.
NOT ALL WET SITES ARE THE SAME
When you begin researching plants that tolerate wet sites, you will soon realize there are other factors that must be considered.
- Is your site sunny or shady?
- What kind of soil do you have?
- Is the excess water from natural or man-made causes, or both?
- Is erosion an issue?
- Is your site seasonally wet, and if so, are you willing to add supplemental watering in times of drought?
- Are you planting a wildflower type of garden or something more formal?
- Do you want plants that spread, or plants that grow in clumps?
- Do you have limited space?
- Do you have a significant deer problem?
- What is your definition of native? Is it North America? Is it more localized to state, county, or even zip code?
SELECT PLANTS THAT WILL THRIVE
It is easier, and more productive, to work with the conditions on your site than trying to adjust the site to fit the plant needs. Native plantings follow the simple rule of “right plant, right place.” There are native plants that will thrive in moist or wet conditions.
The amount of soil moisture a plant requires can be defined as:
- Dry – water does not remain after a rain; areas may be in full sun or in a windy location, on a steep slope, or have sandy soil.
- Moist – the soil is damp and may be occasionally saturated.
- Wet – the soil is saturated for much of the growing season, except in droughts. Some sites are wet for extended periods during spring and fall when rainfall is abundant. Although these sites may appear well-drained, they too are often not suitable for many trees and shrubs.
EVALUATING OTHER CRITERIA
Plant Virginia Natives recommends the following list of native shrubs for landscaping in a wet and shady area:
red chokeberry – Aronia arbutifolia
buttonbush – Cephalanthus occidentalis
sweet pepper bush – Clethra alnifolia
black huckleberry – Gaylussacia baccata
winterberry – Ilex verticillata
mountain laurel – Kalmia latifolia
ninebark – Physocarpus opulifolius
pink azalea, pinxterflower – Rhododendron periclymenoides
swamp azalea – Rhododendron viscosum
When considering shrubs, many gardeners need to consider deer-resistant plants. Of the shrubs listed above, red chokeberry, buttonbush, winterberry, black huckleberry, and pink azalea (pinxterflower) are all susceptible to deer browsing. In fact, deer have been eating the buds on our pinxterflower azalea this spring. On the other hand, sweet pepper bush (Clethra alnifolia) has not been browsed by deer. This shrub will tend to sucker, however, and so you will want to find a spot where spreading is a positive characteristic. If you prefer a tidier plant, you’ll need to spend some time pruning off suckers. Swamp azalea is listed as deer resistant, and although I haven’t grown this plant, I would guess that deer like to nibble off the buds since they seem to enjoy azaleas in general. Mountain laurel and ninebark are supposedly not eaten by deer; our ninebark has not been touched in two seasons and I don’t have first-hand experience with mountain laurel. So, for a gardener who is searching for deer resistant and moisture-loving shrubs for sun-to-partial shade, three of the shrubs listed above seem to fit the bill. Don’t forget to also consider if a suckering plant is suitable to your site.
NATIVE PLANT RECOMMENDATIONS FROM IOWA STATE UNIVERSITY EXTENSION
The Iowa State Extension has compiled the list, “Native Perennials for Moist to Wet Soils in Partial to Full Sun.” When searching online for recommended plants, you need to be aware of how “local” the native plants might be. You need to decide if you want plants native to North America, to a particular region or state, or to your zip code. I was surprised to find that all of these “midwestern” perennials recommended by the Iowa State Extension, with the exception of prairie blazing star, are also considered native to the local Charlottesville zip code 22901, or to Albemarle County. I found this information by using the Piedmont Native Plants Database and the Plant Finder by Zip Code database.
- swamp milkweed (Asclepias incarnata)
- white turtlehead (Chelone glabra)
- Joe Pye weed (Eutrochium maculatum)
- rose mallow (Hibiscus laevis)
- Southern blue flag (Iris virginica shrevei)
- prairie blazing star (Liatris pycnostachya)
- cardinal flower (Lobelia cardinalis)
- obedient plant (Physostegia virginiana)
White turtlehead, Joe Pye weed, rose mallow, and cardinal flower are native to zip code 22901. Swamp milkweed (Asclepias incarnata), Southern blue flag (Iris virginica), and obedient plant (Physostegia virginiana) are native to Albemarle County.
USING THE PIEDMONT NATIVE PLANT DATABASE
An online search will reveal many plant lists based on certain criteria, but now let’s use the Piedmont Native Plants Database to see how to make your own list using plants that are recommended for wet/shady areas in Albemarle County. I selected the following as my search criteria: shrubs, Albemarle County, moderate-to-wet moisture, partial shade to full shade, and deer resistant. Five shrubs are listed: buttonbush (Cephalanthus occidentalis), wintergreen (Gaultheria procumbens), fragrant sumac (Rhus aromatica), red elderberry (Sambucus racemosa), and southern arrowwood viburnum (Viburnum dentanum).
Note: I grow Viburnum dentanum and I know that deer eat it at least when the leaves are young. I have also read complaints on blogs about deer browsing on buttonbush.
If we expand the search, keeping deer resistance selected, to include all of Virginia, a sixth shrub, Yaupon holly (Ilex vomitoria) is added to the list. If we delete the requirement of deer resistance, but keep the statewide screen, 29 shrubs make the wet/shade list. As you can see, this database can be refined on any number of factors, including some we did not use, such as height, bloom color, bloom month, autumn foliage color, soil preference, and others.
Examples from the Piedmont Native Plant Database of native shrubs that like moderate/high moisture plus sun include:
- sweetshrub (Calycanthus floridus)
- buttonbush (Cephalanthus occidentalis)
- silky dogwood (Cornus amomum)
- American filbert (Corylus americana)
- deciduous holly (Ilex decidua)
- spicebush (Lindera benzoin)
- scentless mock orange (Philadelphus inodorous)
- swamp rose, (Rosa palustris)
- red chokeberry (Photina pyrifolia)
- winterberry (Ilex verticillata)
- red elderberry (Sambuca racemosa)
Another deer-resistant shrub that is native to the southeast U.S. and thrives in wet/sun conditions is Virginia sweetspire (Itea virginica). ‘Henry’s Garnet’ is the cultivar most often found at nurseries, and I have used the dwarf version, the ‘Little Henry’ cultivar, very successfully. See the August 2017 issue of The Garden Shed for information on this shrub.
Examples of trees that like moderate/high moisture plus sun include:
- red maple (Acer rubrum)
- downy serviceberry (Amelanchier arborea)
- paw paw (Asimina triloba)
- river birch (Betula nigra)
- black gum (Nyssa sylvatica)
- American hornbeam (Carpinus caroliniana)
- redbud (Cercis canadensis)
- American wild plum, Prunus americana
- umbrella magnolia (Umbrella tripetala)
- sweetbay magnolia (Magnolia virginiana)
Note: Many of these shrubs and trees tolerate a wide range of sunlight from full sun to part shade. They are often found on multiple lists, i.e., wet/shade and wet/sun. After finding a plant of interest from a list or from the Piedmont Native Plants database, I check the requirements (such as light and moisture), and characteristics (such as spreading tendencies and height) on another source. Useful sources include the Missouri Botanical Garden Plant Finder, or the Lady Bird Johnson Native Plants Database. I also use The DCR-Department of Conservation and Recreation for Virginia using the Piedmont region as the locale. This database has easy-to-use search tools using a combination of factors such as full sun and high moisture.
SOME GENERAL SOLUTIONS FOR WET SITES
In addition to moisture-loving plants, here are some steps that can help with excess water:
- Plant cleared slopes with ground covers.
- Use mulches to slow water and help it soak into the soil.
- Use permeable surfaces whenever possible.
- Divert runoff using perforated drainpipes laid on or under the soil.
- Dig shallow trenches or build retaining walls to channel away problem water.
- Plant tree balls high (several inches of root ball left above ground and covered with mulch), or construct berms.
What is the difference between a garden in a medium-wet to wet-site that uses plants that are tolerant of wet conditions, and a rain garden that uses similar wet-tolerant plants? A rain garden is a planted shallow depression that temporarily holds runoff from impervious areas (such as roads, driveways, and pathways) until the water evaporates, is absorbed by the plants, or infiltrates into the ground. The more area covered in impervious surfaces, the greater the volume of water runoff and the pollutants it carries. (See this article from VCE for instructions on how to calculate the minimum effective size of a rain garden.)
Rain gardens can be located anywhere along the natural drainage pathway of water through your yard, and at least 10’ away from the house foundation. You may feel that a location where water already ponds in your yard may be appropriate, but it is NOT. In this location, the soil does not allow adequate infiltration. Conduct the following drainage test to see if a potential site is feasible:
- Dig a hole that is about 1’ in diameter and 2’ deep (or as deep as your rain garden excavation will be).
- Allow the water to naturally soak into the soil and wait until the water drains completely.
- Within twelve hours, refill the hole with water and record the time it takes for water to drain. This should take no longer than 36 hours.
A rain garden’s finished depth should be 6-8” lower than the surrounding soil surface. The depression may need to be dug 12” deep if compost is being added to the garden soil, or if plants with large root balls are being planted, or if mulch is applied. The kind of soil in your rain garden is very important. The soil needs to be porous enough to soak up water within 48 hours to prevent plants from drowning, and to keep mosquitoes from breeding. Remember that clay soil is not ideal as it inhibits drainage. You may need to add compost to lighten the soil if you have heavy clay soil.
The effectiveness of a rain garden depends on the construction of several layers:
- Ponding area or depression that will provide the storage needed for the amount of runoff
- A berm that is at least six inches of soil or rocks that works like a dam to pond the runoff.
- Soil layer where the plant roots will collect moisture and nutrients
- Plants that will use the runoff for moisture and nutrient requirements
- Grass buffer strip around the garden to slow the velocity of the runoff
- A mulch layer to provide a medium for the biological activities to occur and to keep the soil moist
Sources of information on rain gardens:
“Stormwater Management for Homeowners Fact Sheet 5: Rain Gardens,” Virginia Cooperative Extension, https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/spes/spes-13/SPES-13.pdf
“Native Plants for Wildlife Habitat and Conservation Landscaping, Chesapeake Bay Watershed,” US Fish and Wildlife Service, https://www.fws.gov/Chesapeakebay/pdf/NativePlantsforWildlifeHabitatandConservationLandscaping.pdf
“Rain Gardens – The Basics,” PennState Extension, https://extension.psu.edu/rain-gardens-the-basics
“Rain Gardens for Rainscapes – Technical Design Manual, Montgomery County, Maryland, https://www.montgomerycountymd.gov/DEP/Resources/Files/downloads/rainscapes/fact-sheets/rain-gardens.pdf
“Rain Gardens: A Way to Improve Water Quality,” UMass Extension Landscape, Nursery and Urban Forestry Program, https://ag.umass.edu/landscape/fact-sheets/rain-gardens-way-to-improve-water-quality
When excessive moisture makes oxygen less available, water and nutrient uptake stop, plant processes and growth cease, and plants begin to decline or die. Wet areas in the landscape can be caused by natural conditions, such as underground springs, earthquakes, or runoff from elevations. There may also be man-made causes that impede drainage. A simple “hole test” can show if the soil is draining normally. Another related hole test shows whether the groundwater level is high. Once you understand your landscape’s moisture levels, and patterns of excessive water, you can start a search for native plants that thrive in moist-to-wet conditions. Other requirements should also be considered, such as light conditions, soil type, space limitations, deer resistance, and planting objectives. This article demonstrates how to use native plant databases to screen for plants native to Albemarle County, or to zip code, based on a combination of specific criteria. Rain gardens are both functional and attractive landscape additions that reduce storm water runoff and flooding in our yards, and help prevent erosion. Most importantly, rain gardens help preserve nearby streams and ponds by reducing the amount of polluted runoff, and by filtering pollutants.
“Introduction to Soils,” PennState Extension, https://extension.psu.edu/introduction-to-soils-soil-quality
“Landscaping in Wet Shade,” Plant Virginia Natives, https://www.plantvirginianatives.org/wet-shade
Piedmont Native Plant Database, http://webapps.albemarle.org/NativePlants/default.aspx
USDA NRCS, Plants Database, https://plants.sc.egov.usda.gov/java/
Plant Finder Database, Missouri Botanical Garden, https://www.missouribotanicalgarden.org/plantfinder/plantfindersearch.aspx
Lady Bird Johnson Native Plant Database, https://www.wildflower.org/plants/
Native Plant Finder by Zip Code, https://www.nwf.org/NativePlantFinder/Search
“Plants for Wet and Dry Sites,” Virginia Cooperative Extension, https://waynesboronurseries.com/plants-wet-dry-sites/
“Native Perennials for Moist to Wet Soils in Partial to Full Sun,” Horticulture and Home Pest News, Iowa State University Extension and Outreach, Richard Jauron, Dept. of Horticulture, https://hortnews.extension.iastate.edu/2018/02/native-perennials-moist-wet-soils-partial-full-sun
“Plant of the Week: Swamp Rose (Rosa palustris),” USDA, US Forest Service, https://www.fs.fed.us/wildflowers/plant-of-the-week/rosa_palustris.shtml
“Virginia Sweetspire,” The Garden Shed, September, 2017, https://piedmontmastergardeners.org/article/virginia-sweetspire/
“Organic Matter – The Gardener’s Silver Bullet,” The Garden Shed, October, 2017, https://piedmontmastergardeners.org/article/organic-matter-the-gardeners-silver-bullet/
“Groundwater-Level Response to Virginia Earthquake, August 23, 2011,” USGS (U.S. Geological Survey, Dept. of the Interior), https://www.usgs.gov/mission-areas/water-resources/science/groundwater-level-response-virginia-earthquake-august-23-2011?qt-science_center_objects=0#qt-science_center_objects
Feature Photo: Sweetshrub (Calycanthus floridus), Krzysztof Ziarnek, Wikimedia Commons (CC BY-SA 4.0)