Native Landscaping Manual

Chapter Two - Rain Gardening and Storm-water Management

A Landscaping Guide for Missouri

Introduction Elements of a Rain Garden How to Determine Your Goals Site Evaluation Planning and Design Location and Layout Soil Types Size and Shape Design Elements Typical Schedule Sample Designs Site Preparation Dealing with Slopes Installation Maintenance Species Selection Guide Native Seed and Plant Sources Related Links Bibliography

Order Chapter Two - Rain Gardening and Storm-water Management in booklet form

Introduction

The problem: Government studies have shown that up to 70 percent of pollution in aquifers, rivers, lakes and coastal areas is carried there by storm-water runoff coming from rooftops, roads, driveways, sidewalks and mowed lawns. These impervious surfaces have taken the place of soil that readily absorbs rainwater. The result is an increase in stormwater volume and velocity, creating an increase in water pollution, stream-bank erosion and flooding.

A natural solution: In nature, storm water flows down the hillsides into streams, riverbanks, and low-lying wetlands that form a watershed. In a healthy watershed, the roots of grasses, perennial plants, shrubs and trees capture rainwater, aerate soil and help water percolate into the ground, reducing erosion and flooding. Unfortunately, healthy watersheds and their associated plant communities have been damaged and/or destroyed by urbanization.

Rain gardens function like miniature natural watersheds. They slow water flow by using
elements similar to those in nature: plants, rocks, shallow swales and depressions that hold water temporarily rather than let it quickly escape. Rain gardens minimize flooding and loss of soil and improve water quality in lakes and rivers by reducing silt. Use of rain gardens also can save tax dollars by reducing the need for communities to build larger storm-water retention facilities.

Rain gardens provide beauty, natural diversity and wildlife habitat in areas that otherwise
would be a monoculture of lawns, pavement, concrete culverts and storm drains. This landscaping style is increasingly used by homeowners, commercial and residential developments, and by cities for park beautification.

(Above left, eroding creek bank.  Right, natural creek bank.  Below left, rain garden.)

Elements of a rain garden

(See illustrations below)

Retention Area A shallow depression (or series of depressions) that is planted with wetland plants and temporarily holds water. A rain garden may be an existing low area that holds water, or it may be dug and shaped to hold water. Its purpose is to reduce storm-water runoff and the potential pollution and erosion associated with runoff. Rain gardens range from small, conventional, homeowner gardens, to large, engineered wetlands. In either case, they are placed to intercept water runoff near its source and retain it long enough to allow percolation into the soil. In addition, natural"filtering" takes place as water moves through the root systems of plants.

Berm Soil excavated from the uphill side of a rain garden is moved to the downhill side to create a dam. Water is retained for a longer period of time, allowing for better percolation into the ground as well as uptake by plants. If the rain garden is on flat ground, the berm will surround the area.

Settling Basin or Ponding Area An area that is deeper than the rest of the rain garden (six to ten inches deep). The majority of the water is held in this basin, especially when soil is saturated. Most sedimentation and evaporation take place in the settling basin. If the soil contains more clay, infiltration will be slow. The settling basin may need to be larger to allow for more water storage capacity.

Transition Zone The area that surrounds the settling basin will dry out first. Take this into consideration by selecting plants that tolerate dry conditions at times. Many beautiful native plants have evolved with alternating wet and dry periods and are good choices for this region.
(See Species Selection Guide).

Bioswale (input) A shallow channel (swale) is constructed to direct storm-water runoff from its source (rooftop, pavement, or lawn) to the rain garden. It is planted or seeded with moisture-loving plants. The plants reduce the flow rate of runoff and encourage soil absorption of water even before it enters the rain garden retention area. Bioswales can be constructed independently of a rain garden if space is limited. In this case, they guide runoff to existing storm-water systems.

Overflow Spillway (output) A small area in the berm should be somewhat lower, creating an outlet for water. In the event of a major downpour and a full rain garden, a bioswale below the overflow is used to direct water to the storm-water system.

Please Note: Overflow spillways may not be necessary in some situations. If you have not altered the flow patterns of the yard, the rain garden should not create any further runoff
than before and the existing drainage path should be adequate.

Determine Goals

Before beginning rain garden construction, evaluate the site. Ask yourself why you are building a rain garden and what objectives you have. The answers will help with the decisions on site location, layout, size and plant choices.

Consider these five reasons to build a rain garden:

Erosion Control: Soil loss can be reduced or eliminated by installing water-retaining rain gardens. They help prevent water from quickly running off a site, reducing water volume and rate of flow.

Maintenance: Poorly drained sites are difficult to mow and care for in rainy seasons because of standing water. Mowing can be eliminated by planting moisture-loving native plants in low-lying areas.

Beautification: Rain gardens planted with wildflowers and sedges bloom throughout the season, attracting a variety of colorful birds, butterflies and insects.

Education: Over 85 percent of Missouri's wetlands are gone. A rain garden provides opportunities to teach the importance of water cycles, stormwater containment, and biological diversity.

Wildlife Habitat: A rain garden with a diversity of native plants attracts many insects, birds, amphibians and mammals, providing opportunities to observe nature up close.

Site Evaluation

Map the area.
Note the size and shape of the site. Add structures, location of utility lines and traffic use. Next, note the north-south aspect, soil types, vegetation, patterns of shade and sunlight. Slope, soil moisture, drainage and the potential for erosion also are important. A map, drawn to scale, will help decide where to locate the rain garden and what size and shape it will be. A scale drawing will help determine the number and placement of plants. This map will be useful in explaining the project to neighbors, city officials or maintenance crews.

Survey vegetation.
A plant survey of the area may reveal remnants of the plant community that previously existed on the site. For example, if native wetland grasses and forbs grow nearby or on the site, include those in the plant list. Seeds of these plants could be collected and used in the project. The site may be covered with shrubs, vines or weedy vegetation. If so, determine what vegetation should be removed.

Research land use history.
(Call 1-800-DIG-RITE or local utility companies to locate underground pipes and power lines.)
Are there existing low depressions, swales, or ponds? Sometimes damaged ponds are difficult to see since they no longer hold water, but they have the potential to make excellent rain gardens or overflow areas. Look for evidence that a wetland existed on the site. Do neighbors talk about wet areas, flooding, or standing water? Do wetland plants exist in the area?

Planning and Design

Location and Layout

Observe topography and water flow during periods of heavy rainfall. Ideal sites have a gentle slope and a naturally occurring low area or an area where downspouts and other runoff can be directed. You may need to direct runoff from its source to the rain garden by reshaping existing soil contours. A good rule of thumb is to locate your rain garden at least 10 feet away from buildings. Direct the overflow spillway into existing drainage ditches, storm-water sewers, ponds or creeks and away from neighboring houses, driveways, or sidewalks.

Soil Types

Determine if your soil type is sand, loam, or clay by digging a test hole a foot deep. Fill it with water and observe how quickly the water disappears.

• If water drains in less than an hour, soil is sandy or rocky
• If water drains in less than 6 hours, soil is loamy
• If water remains in the hole after 24 hours, soil is clay.

Sandy or loamy soils are more porous and therefore drain better than less porous clay soils. The goal of a rain garden is to encourage rain infiltration into the ground. Sandy, rocky or loamy soils are ideal because they drain quickly. If you have poorly drained clay soils you will need to compensate by digging a larger rain garden. (See "How to Calculate Size" ahead).

Size and Shape

Rain garden size is related to soil type and the amount of impervious surfaces (rooftops, driveways, sidewalks or mowed lawns). The larger the surface, the larger the rain garden.
Also, clay soils require larger rain gardens.

How to calculate size:

According to many years of rainfall data collected in Missouri, the typical rain event produces one half-inch of water (there are thirty of these each year). Because of this, the following recommendations are based on a half-inch rain event.

Let's begin with a house that has 1,100 square feet (50 feet x 22 feet) of impervious roof
surface area.

It receives 342 gallons of water per half-inch rain event (one cubic foot holds 7.5 gallons
of water). That is about 80 gallons per downspout (assuming 4).

So a rain garden receiving water from one downspout that has sand or loam soil needs to be about 4 by 5 feet wide and six inches deep to capture one half-inch rainfall on your house. If you run two downspouts into a single rain garden, make sure it is 8 by 10 feet and six inches deep.

A rain garden with clay soil should be slightly larger or about 6 by 8 feet and six inches deep per downspout.

Design Elements

Determine the design style for the rain garden.

Natural style: This style emulates a natural wetland. Plants are randomly placed, approximately one plant per square foot. The design should include elements such as groundcover or mowed turf edges, split-rail fencing, boulders, birdhouses or feeders to give the garden an intended appearance. Natural gardens often have equal portions of forbs (showy flowering plants) to grasses and sedges. These gardens are often seeded or planted with small plugs.

Traditional style: This style is considered a conventional approach to garden design. It may involve massing single species, repetition or planting in regular lines or curves. Also, a larger portion of flowers to grasses results in a more conventional appearance. This style of garden is planted with three inch to one gallon size plants. For a dramatic effect, choose plants whose leaf textures and forms have good contrast. In other words, combine plants with large coarse leaves next to those with narrow fine leaves. Also include plants that flower during spring, summer, and fall for color all season.

Typical Schedule

Before going further, create a step-by-step schedule so each step is done in sequence.

Sample Designs for Rain Gardens

Photos of a sample rain garden:

Case Study: Shooting Star Nursery, Frankfurt, Kentucky.

Downspouts on utility building (left) flow into drainage tile under parking lot to rain garden in forground (right).

Left: Rain garden with berm used as path.  Right: Rain garden overflow constructed with 4 inch rock.

Rain garden on right drains into overflow and then into lower pond on left. Photos taken at Shooting Star Nursery, Frankfort Ky.

Case Study: Missouri United Methodist Church, Columbia, Missouri. Left:Water flows off roof and into bioswale.  Middle: Water runoff from parking lot enters bioswale.  Right: Rain garden basin with wetland species and river gravel.

Site Preparation

Call 1-800-Dig-Rite: At least three days before any digging, excavation or bed prep, call 1-800-Dig-Rite to have underground pipes and cables located and marked. Stay two feet away from utility lines

Layout: Use rope or garden hose to define the basic shape. Mark the final layout with stakes before digging.

Piping: Replace, repair or relocate gutters and downspouts where necessary. If you choose to run water underground to a bioswale or rain garden, connect downspouts to 4-inch, corrugated drainage tile (6-inch or larger for commercial applications).

Soil Shaping: Soil dug from the bioswale, rain garden, and settling basin typically is used to onstruct the berm. Step back from time to time to observe elevation and slope. Use a line-level
when you are close to the final grade to ensure your rain garden base and berm are level. The overflow spillway should be a few inches lower than the top of the berm.

Rock Placement: Place large gravel, rocks or boulders in the bioswale to slow waterflow and create visual interest. Place rocks or a flat stone beneath downspouts or at point where drainage tile enters the rain garden to prevent erosion. Rocks also may be placed in the rain garden and within the overflow spillway.

Soil compaction: Water infiltration rates can be increased in clay soils by loosening compacted soil with hand-digging, tilling, plowing or with the addition of humus, gypsum or sand.

Dealing with Slopes

Steep slopes pose problems. Slopes with a grade above 10 percent may need to be stabilized. There are several materials available to keep seed, plugs, and soil in place while plants become established.

Installation

Planting: Small rain gardens (up to 300 square feet) should be planted with three-inch to one-gallon size plants in the spring. While more expensive than seeding, plants mature and flower more quickly. If you choose a more traditional garden style, plant larger plants two to three feet apart and mass single species for greater flowering impact. If you desire a natural style, plant plugs on 1-1.5 foot centers. See the "Notes" column of the species selection guide for spacing
recommendations. Water every 2-3 days until plants become established and begin new growth (about 3-4 weeks). Mulch can float away during rain events; therefore, mulching is not recommended until spring rains end.

During the first growing season, vigorous species will take over if not kept pruned.

Seeding: Seeding is recommended for large areas (from 300 square feet to several acres) because planting plugs on this scale can be cost-prohibitive. Seeding should be done in early winter. Seeding requires three years to mature and flower, so patience is needed. For detailed instruction on seed collection, cleaning, storage, mixing, sowing, maintenance, and seedling identification, see Chapter One-Prairie Reconstruction

Sample seed mix for a one-acre wetland.

How to calculate an acre of land

An acre contains 4,840 square yards or 43,560 square feet. If your plot is about 200 feet by 200 feet then you have 40,000 square feet or just under one acre.

Maintenance

Maintenance of a mature rain garden is considerably less than a comparable area of lawn.

Pruning and dead-heading: Leave stems and seed heads standing in fall and winter to add visual interest to the landscape and to provide food and cover for birds. Remove dead vegetation in spring with a string trimmer or pruner.

Fertilizing: Don't fertilize a rain garden. It is not necessary and will stimulate weed growth. Light annual application of compost improves soil fertility and is beneficial.

Mulching: Annual one to two-inch applications of compost in late fall or early winter are beneficial in first growing season. An annual application of mulch is not necessary once plants are established. However, mulch does add a manicured look
and provides a "garden" appearance.

Weeding: Most weed seedlings cannot survive periods of flooding, a definite advantage in weed control. Weeds that persist after flooding should be pulled manually. By the third year, plants should be mature enough to compete and crowd out most weed species so weeding will be minimal.

Maintaining edges: Borders are important. A border defines the edge of the garden just as a frame defines a painting. A strip of mowed turf, buffalo grass,
or a walking path at the edge of a rain garden helps set the area apart. Borders may include split-rail fences, low walls, shrub masses or a simple trellis.

(Left, stepping stone path at edge of rain garden. Middle, gravel path. Right, split-rail fence.)

Mosquitoes: Rain gardens typically don't have standing water for more than a few days, which is not long enough for mosquitoes to complete a life
cycle. Gardens that have a settling basin planted with a diversity of native plants will attract mosquito predators such as aquatic insects, dragonflies, tadpoles, frogs, toads, some bird species and bats.

If your rain garden develops a population of mosquitos in the early phases of installation, a good commercial product to use is Mosquito Dunks. Top minnows are very effective control. They are available at bait shops, pet stores and garden centers who specialize in aquatic plants.

Species Selection Guide

Rain gardens are full of water during storms and dry out during dry weather. The plants recommended in this manual generally tolerate both extremes.

There are three descriptions in the 'Light and Moisture Requirement' column that merit attention. They are defined below to aid in your decisions on placement of species within the zones of a rain garden.

Wet to mesic: These species are well suited to the alternating wet and dry zones of a rain garden, specifically the bioswale, the transition zone and the overflow spillway.

Wet to submerged: These species grow at the consistently moist margins of ponds and rivers. They require an area of constant moisture and are suitable for the settling basin of a rain garden.

Submerged: These species grow in the water zone of ponds and rivers. Their roots consistently are under water. The settling basin can be made to permanently hold water by constructing the bottom out of compacted clay or a pond liner.