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Surviving the Rain Garden

Surviving the Rain Garden

The rain garden concept for dealing with storm water is simple, but surviving the rain garden remains elusive.  Read about our failures, our few successes, and the insights that you can use to improve native plant survivability in rain gardens.  Learn about the 3 major rain garden problems and what you can do to mitigate them.

Rain gardens are now a preferred method of handling excess storm water on private and public lands.  City and Township regulations are increasingly recommending them and they are almost a necessity for new home construction, even on smaller lots.  The underlying principles are easy to understand: Adjust nearby grades and slopes, dig a depression downhill to catch rain water, fill it with a sandy soil, and plant your favorite flowers.  Viola!  An instant, attractive, self-watering garden that you don’t have to maintain.

The problem is it’s not that simple at all.  The wonderful native plants choices so often promoted for rain gardens look great for a year or two but quickly succumb to environmental pressures.  And the weeds!  Where do they all come from?

Take some solace in knowing that this experience is quite common.  But we have some ideas that can help to either design a rain garden successfully the first time or retrofit an existing rain garden without hiring an expensive landscape architect.

This article will talk about the 3 major causes of rain garden disappointment and how the homeowner, designer, or professional can overcome them.

Water in the Rain Garden

Water is the main antagonist in the rain garden drama.  Municipalities and engineers want excess water to stay on a property for as long as possible to protect water quality and reduce downstream flooding.  But homeowners and property owners want that water to disappear as quickly as possible lest the rain garden breed mosquitoes or cattails, all while minimizing the impact to our vision of the perfect lawn and landscape.  This is a very challenging set of requirements.  The now-standard rain garden design being installed is a reasonable compromise and some rain gardens do work and look great in lots of places around the country and even the world.  Australia is just as keen on managing storm water as is the United States.

However, rain gardens work better in some places and not so well in others.  In the Mid-Atlantic states, mainly Pennsylvania, Maryland, New Jersey, Virginia, and New York, rain gardens are fraught with difficulties and rarely look anything like what we see on the internet.

The culprit is the unique nature of the water itself.  Or more specifically, the seasonality of our water and of our soil temperature.  You see, from Washington DC to New York City to Boston, we have to deal with periods of intense cold and rain in winter and with periods of intense heat and drought in summer.  The sandy bottoms of our rain gardens are good at holding water for 3-4 days before leaking it away but they act like deserts during summer months.  The small pore size in a sandy rain garden clogs with ice easily and effectively acts like a frozen pond in winter.  This is a problematic combination for plants.

The charts to the right attempt a visual explanation.

The first chart shows the moisture availability profile in a typical rain garden.  Fully saturated, frozen conditions can occur in winter after it rains repeatedly in December and then freezes solid in January-February.  The same rain garden can then dry out excessively in the summer, particularly in July-August-September.  Wetland plant species might be able to survive the winter conditions, but succumb in the summer.  Prairie plant species that might rather enjoy the droughty rain garden in summer cannot survive the cold wet condition in winter.

The second chart shows the implications of the typical moisture profile on the survivability of a popular rain garden plant, butterfly milkweed (Asclepias tuberosa).  The moisture tolerance levels are depicted with the red lines.  The milkweed can survive soil moisture levels as low as 0.15 and as high as about 0.35, so it does great in the summer.  But the high moisture levels in the winter and spring will simply kill the plant.

The third chart overlays the moisture tolerance limits of cutleaf coneflower (Rudbeckia laciniata) on the rain garden soil moisture profile.  Its range is broader and more central, being from about 0.35 to 0.85.  However, the plant can still struggle in the heat of the summer or if a winter is particularly wet.

The fourth charts depicts how modifying certain elements of the rain garden design can improve the survivability of the cutleaf coneflower.  Improving moisture retention in the summer and reducing excess moisture in the winter to within the tolerance range of the species could help it survive our annual Mid-Atlantic cycles.

Soil in the Rain Garden

Sandy bioretention basin soils do a good job of draining away excess storm water but are subject to extremes that most native plants cannot tolerate.  Fortunately, there are several techniques that ArcheWild uses to improve native plant survivability in rain gardens.

One overarching technique is to vary the soil moisture holding capacity of the rain garden.  We do this by creating coarse pockets that drain better in the winter, moister pockets that don’t dry out as much in the summer, and different topographies that also influence soil moisture in the direction that we want.  It is important to remember that the average water holding capacity of the rain garden should not deviate from engineering standards, so you have to balance drier areas with moister areas in the modified design so that the whole system performs as expected.

Soil component quality, in our experience, dramatically influences the performance of rain gardens.  Reusing inexpensive ‘topsoil’ from an unmanaged or unknown stockpile usually imports too much clay and a tremendous number of weeds.  The rain garden quickly becomes overrun with a wide range of invasive species including crabgrass, stilt grass, thistle, and others.  Instead, use manufactured soil components for all aspects of rain garden construction that are clean and/or have been composted to eliminate weeds seeds and rhizomes.

Choosing a quality mulch for the top layer of the rain garden is also incredibly important.  Landscaper-grade hardwood mulch might be the least expensive but it, too, is often contaminated with weeds, particularly mugwort and Canadian thistle.  These weeds can rapidly overwhelm any rain garden.  ArcheWild uses only 3-year composted leaf mould for the top layer in our rain gardens.  It’s weed-free and provides an effective soil moisture retaining layer that better replicates the function of the ‘O’ and ‘A’ layers in a real soil system.  Leaf mould also moderates the dramatic swings in soil moisture in the typical rain garden much better than hardwood mulch.

ArcheWild can recommend suppliers of rain garden soil materials if you’re having trouble finding a local source.

Native Plants in the Rain Garden

Native plants are often promoted as being ‘more resilient’ because they’re native.  The harsh reality is that they’re very picky about their soils and their water.  Horticultural plants, on the other hand, have been selected and then bred to survive in as wide a range of soil and water conditions as possible, so that they will work in as many gardens as possible.  So in some sense, common-to-invasive landscaping shrubs should be more resilient in a rain garden, but we don’t recommend this practice due to their lower environmental and ecological value.

Successfully using native plants in a rain garden requires the precise application of ecological principles that combines what we know about how water and soil interact throughout the year and which plants can tolerate the resulting conditions.  As yet, there is no cookbook that we are aware of that can replace the role of an ecologist in forecasting the moisture profile of a newly built rain garden and choosing the right native plants.

But there are some things that a homeowner or a landscape architect can do to choose more appropriate plants for a Mid-Atlantic rain garden.

First, forget about using prairie plants.  This includes little bluestem, butterfly milkweed, sideoats grama, prairie dropseed, and myriad other species being promoted for the sandy rain garden.  Yes, they’ll grow during the summer but mostly all die off after a couple of wet winters.  Plus the excess moisture during spring and fall will simply promote taller plants that will shade out the shorter prairie species.

Second, try using coastal plain species.  In the Mid-Atlantic, we actually have a natural ecosystem that broadly resembles the rain garden.  Ecologists call the ecosystem the ‘Outer Coastal Plain’ and it occurs in easy-to-access locations on Staten Island, Long Island, southern New Jersey and all along the coastal regions down to North Carolina.  This ecosystem is unique in that it features sandy soils, can flood and even freeze in the winter, and become really dry in the summer.  Sound familiar?  Because of this unique set of conditions, very specialized plants have emerged that can tolerate these extremes.  Some excellent choices include shining sumac and groundsel bush, which look amazing when used in combination.  There are some excellent online resources describing the plants growing in the outer coastal plain.  If you need help navigating your options, write us.

Third, if there is a particular type of plant that you ‘must have’ in your rain garden, like a Hypericum, try what we call ‘book-ending.’  Book-ending is a concept wherein you plant different species of a genus in the rain garden that, together, can handle a wider range of conditions.  For example, rather than rolling-the-dice on just Hypericum prolificum, try planting Hypericum densiflorum (likes it a little wetter) and Hypericum kalmianum (likes it a little drier).  This technique improves the likelihood that at least one of the species will survive the conditions in the rain garden.  This technique can be easily applied to asters, viburnums, sunflowers, milkweeds, dogwoods, and many other attractive, desirable, and habitat-friendly plants.

Fourth, use plants grown by a reputable native plant nursery that uses local seed sources.  Much has been written about the need to stop using foreign genetics in our native plantings.  In the case of the rain garden, you’ll want to use native plants that were grown from seed collected in a region that as closely matches the conditions in your rain garden as possible.  What’s the point of planting common milkweed sourced from Florida if its genetics have never been exposed to the cold, wet winters in Philadelphia; it doesn’t make any sense.  To learn more about this important practice, read two articles about specifying native plants: A Case for Specifying Species and Ecoregion, and Native Species Procurement Specification.  Try to avoid plants grown by internet marketers with glossy catalogs.  Instead, find a small local grower that you can trust; there are many popping up in and around most major cities.  An excellent retail nursery is Good Host Plants in Philadelphia who openly declares the origin of their plants.  Of course, for large projects, you can contact ArcheWild Native Nurseries.

Native Rain Garden Examples

That’s about it for this article.  Hopefully some of these ideas can help you design a better rain garden.

Browse the pictures below to see several successful ArcheWild implementations of rain gardens to understand what’s possible when soil, water, and native plants are used in the proper proportions.

If you ever need any help with your rain garden, mail the folks at ArcheWild or give us a call.  We’re here to help.

rain garden engineering diagram

Typical rain garden design with sandy soil, berms, ponding water, geotextile fabric, and an underdrain.

Illustrative Moisture Availability Curve

Rain gardens in the Mid-Atlantic experience periods of low water availability in summer and excessive water availability in winter.

Butterfly Milkweed cannot survive in a typical rain garden

But plants can have narrow moisture tolerance ranges.  Butterfly milkweed roots start to rot when soil moisture exceeds 0.4

cutleaf coneflower curve

Cutleaf coneflower will die if it experiences either prolonged dry periods or overly saturated conditions

Cutleaf coneflower survives in modified rain garden

Modifying rain garden elements can moderate moisture swings, allowing cutleaf coneflower to survive in a rain garden

Hugel

Hugels are piles of organic material placed on a flat plane to create a planting zone with different characteristics than the base soil.  This permaculture concept can be applied in many ways for successful us in the rain garden.

Coastal Plain Shrubs - Natural Rain Garden

Coastal plain shrubs growing in a ‘natural’ rain garden.  This median depression on Ocean Parkway functions very similarly to built rain gardens.  Saturated and cold in winter, hot and dry in summer.

2016 ArcheWild – Plug 98 Sample – 20170215_123724

Try to use nurseries that declare the Ecoregion origin of their plants and/or that provide accession codes that provide complete traceability from seed location to planted plant.

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