by Sue Ellingson, Madison, Wisconsin.
A Rain Garden Isnt a Pond
A rain garden is simply an ordinary garden thats planted in a shallow depression and filled with native plants.
Rainwater from a downspout or driveway is directed to the garden. There, the water soaks into the earth, instead of running into the street and straight into the nearest lake or river.
A rain garden isnt a pond. The rain soaks into the ground, usually in a matter of hours. The garden is dry most of the time.
Digging one is easy. Best management practice manuals advise about special soils and perforated pipes. Professionals need to know that stuff for commercial gardens. But home gardens dont have to cope with such enormous volumes of water. Unless your lot is quite unusual, your rain garden wont need any engineering.
Heres how to dig a home rain garden:
Click here to learn about
Watershed and Why Protection is Vital
A Little Design Goes a Long Way
Your rain garden doesnt have to be a saucer parked in the middle of the yard. In fact, it will frame the yard and house better if its a border off to one side. It can have square edges or curvy. It can have bushes or stepping stones or a bird feeder.
Its easy to get caught up in the engineering and forget to apply the art.
Until recently most native gardeners have been diehard prairie enthusiasts. They planted mini-prairies in a suburban yards. Typically, design was never a consideration. The gardens often looked unkempt and abandoned.
But native plants can be used in ordinary gardens. In fact, they have many advantages over traditional plants. And good design principles can be applied to plantings that are all native, to stunning effect.
This section will cover landscape design for your whole yard, not just for rain gardens. Its important because natures needs will survive you and the garden should, too. If it fits as part of an overall design, it will be accepted by succeeding owners and an advertisement for native gardening. With luck, even your neighbors will want one.
The goal is a landscape that not only works for nature, but is beautiful and easy to maintain, too.
The purpose of design is to give a feeling of order to the landscape around us. Orderly environments are appealing. We feel safe and relaxed there. On the other hand, people like nature and natural areas, too
Native gardens can be wild and natural and can look orderly at the same time by following a few simple rules.
A crisp edge is the most important factor in giving a native garden the impression of planning and order.
The very best ones have a border of mowed grass, as well as a path or two. The paths make the prairie look accessible. And the sharp contrast between the mowed area and the natives makes the whole look like a garden, instead of a vacant lot.
A second important point about edges has to do with the design of the overall garden.
In The Secret Garden a lonely little girl finds the key to a walled garden. Imagining this safe, secret place a garden, no less is magical.
Its a garden room in the most literal sense. Its also an evocative vision for the often repeated advice to create garden rooms when designing your yard. People like the feeling of coziness, of security, that walls or well defined edges give.
Putting gardens at the edge of the yard, rather than in the middle, is what makes the walls of the room.
Besides, foundation plantings mostly benefit the neighbors. An edge garden you can see from inside the house. Its good reward for your work.
Avoid plants that are too tall!
Most discussions of garden design dont talk much about the size of the plants. Thats because plant cultivars the garden plants were used to have been bred to mind their manners. Native plants, on the other hand, grew up in the rough-and-tumble. They had to get more sun than the plant next door or die. So they strive to be tall.
If you have a big yard and a deft touch at design, you ll find a spectacular spot for seven foot tall big bluestem. But probably not right by the front door.
More about this in the section about plants.
The outline of your gardens can be straight or curvy or even circular. These days, most folks make curvilinear gardens. But old Italian villas use rectangular layouts that look even more formal, more orderly.
To develop a theme and add order use the same pattern throughout your yard. Curvy gardens, a curvy walk up to the house, a sinuous deck in back.
Sketch a layout
The first job is to make a map of your yard. You should do a 15 second sketch. Plan view is easiest to work with that is, looking at the yard as if you were a swift taking a nose dive straight for the ground. Estimate relative dimensions, rather than measuring. The back yard might be, oh, about half again as wide as it is deep, for example. Add in the house, existing gardens, trees, and bushes.
Of course, you can measure carefully and lay it out on graph paper. But, hey, we arent building the Eiffel Tower here. In your own yard it probably doesnt matter whether the garden is off a few feet.
A quick sketch has the important benefit of being disposable. You can loosen your creativity by trying out half a dozen ideas in just minutes. When you dont like one, toss it and try again.
On one of the sketched maps of your yard, think about what you use the space for. The whole drawing should be filled with functional areas, no blanks anywhere. The back yard might have areas for eating, socializing, screening, vegetable garden, or play. Examples for the front yard are entry, sitting area, and framing for the house.
Heres a bit of advice that turns landscape planning on its head. Plan for the amount of lawn that you need; let the rest of the yard go native.
Rain Garden How To
A rain garden should be 10 from the foundation of your house.
Generally, water traveling through the soil will spread out 1 for every 2 of drop. So, in theory, if the garden were 5 from the foundation, the water would miss a foundation wall 10 deep. Soils are different everywhere, though. And water behind the foundation can potentially cause serious damage. A garden thats 10 from the house should be enough.
A rain garden should not be dug where the water table is very close to the surface. Pollutants, especially soluable ones, can contaminate the groundwater. Salt is soluable enough to be a concern. Atrazine, a herbicide, and nitrate, a fertilizer, have contaminated groundwater in many farm areas.
A rain garden should not be dug just above a retaining wall thats more than about a foot high. A retaining wall can fail catastrophically if the soil behind it gets too wet. That is, one day the wall falls down without warning. If your lot is steeply sloped, you should consider planting natives, but leave the ground as is, without the saucer-shaped depression of a rain garden.
A rain garden should be 150 to 300 square feet.
Generally, for heavy soil, the garden should be 30% of the roof area that drains into it. For sandy soil, 20%.
However, the size of a home rain garden is best driven by aesthetics, how much work you want to put into it, and maintenance requirements. Even a too-small garden will reduce runoff substantially.
The essential questions are: What size garden works in your yard? And how much weeding can you handle? Once its established, it wont need much maintenance, but the first two or three years will require work.
Most states offer a soil test for a modest fee. Its $15 in Wisconsin. The test will tell you about the nutrients in your soil. But it wont tell you about the texture or infiltration capacity.
Odds are, if youre a gardener, you have a pretty good idea of your soil texture. Just dig up a spadeful of earth when its a bit moist and feel it. Is it gritty and sandy? Crumbly? Sticky and clay-like? Thats a good enough guide for the home rain garden. If its sandy, a small rain garden will suffice. If its clay, put in a larger and shallower garden.
To learn how to judge your soil by feel, Click here for a PDF of Identifying Soil For Rain Gardens from UW Arboretum.
Or here is an at home soil infiltration test:
Dig a hole 6" deep and 6" in diameter. Fill it with water. Let it stand 1 hour.
Refill the hole with water. Measure the depth of the water with a ruler and make a note of it.
Let it stand another hour. Then measure the depth of the water a second time.
If the water has dropped more than 5/8", the soil has enough loam or sand for good infiltration. If the water has dropped less than 5/8", the soil has poor infiltration. Make the garden shallow and choose plants for clay.
But in my opinion, unless soil type is extreme, it isnt very important. When I compiled the list of rain garden plants, I found that virtually all of them would grow in a broad range of soil types. There were so few that didnt, that I dropped the Soil Type category altogether.
Call the power company
Before you start any digging, call the power company. Theyll come to your house and mark the underground services, usually for free. In Wisconsin call Diggers Hotline, 800-242-8511.
Most of the time the buried services are more than deep enough to accomodate a rain garden. But not always. A five minute phone call is cheap insurance.
Get rid of the grass
For a single garden, even one thats 300 square feet, its possible to cut the sod by hand. The sod makes great compost.
For larger projects, the choices are to rent a sod cutter or a rototiller, to smother the grass, or to use herbicide. Or, of course, to hire a landscaper to remove the sod. One worker with a Bobcat can clear an area in an hour.
For my garden, I had hired a landscaper for several projects in the yard. He tilled the garden area, too. Tilling often encourages previously dormant seeds to germinate. So a passel of weeds can sprout that require repeated tilling. This wasnt a problem in my garden. A good tilling, followed by a dressing of mulch, worked well.
Sod-cutters are available at rental stores. Running one isnt complicated, but it is hard work. The cut sod is heavy, too, and you need a place to put it or compost it. If youre doing a large area, it can take a weekend.
To smother grass, cover it with black plastic in the spring or fall when the grass is growing. Leave it for six weeks or until the grass is dead. The disadvantage, needless to say, is having to look at that hideous plastic for a month and a half.
Its also possible to put down ten layers of black-and-white newspaper (color is toxic) and cover it with a thick layer of mulch. The newspaper is never removed. It composts in place and you plant right through it.
I dont like herbicides. Ive come to realize that there is no such thing as washing away. Using water to remove dirt or some other gunk really only gets rid of the gunk if theres a process for degrading where the water ends up. Mechanical methods work well to remove grass. Why add to the bouquet of poison?
Forming the garden
Gardens planted on mounds were popular a few years ago. A rain garden is just the reverse a garden in a shallow depression, rather than on a mound. Its best if the bottom of the rain garden is level, though shaped more like a saucer, rather than a bowl. That way water will infiltrate equally well everywhere in the garden.
The garden only needs to be about 3 deep. Put the soil thats dug from the center of the garden around the edges and make a little berm.
If you do end up with extra dirt, you can put it along your foundation. Water leaking into the basement can usually be cured by making sure that the soil along the foundation is sloped away from the house.
Leveling the bottom
Laying an ordinary level on the bottom of the garden is usually good enough. Or laying a longer board down and setting the level on it. Even judging by eye works reasonably well. If one end of the garden ends up lower than the rest, you can put more wet-tolerant plants in that spot.
If you put in a garden as a group project, its likely that somebody will have access to a rod and transit, also known as a surveyors level. Thats the easiest way to level the bottom. The rod is nothing more than an oversized yardstick, usually 6 long, with the inches and feet clearly marked. The transit is a little telescope on a tripod.
Its difficult to make the transit perfectly level, which it must be. It may be practical only for people who are experienced with the tool.
Cut and fill
Most yards are sloped at least a little. Builders have always graded the land around new houses so rainwater will run away from the foundation. To make a level rain garden in a sloped lawn without bringing in extra dirt or taking any away you have to use a technique called cut and fill.
Put a stake at the top of the slope, another tall stake at the bottom, and as many in between as you need to keep a string tight. Tie a string at the base of the top stake and along from stake to stake to the one at the bottom.
Hang a line leveler from each section. (A line leveler is a little level designed to hang from a string. Theyre available in hardware stores for just a few bucks.) Make the string tight between the stakes. Scooch it up and down until its level.
Once the string is in place, measure the distance from the string to the the ground at the bottom stake. Half of that distance, plus a couple inches, is how high the down-slope berm or wall has to be. (The simplifying assumption behind this advice is that the slope is perfect. No mounds or swales. No cross slope. Still, it should be a good enough approximation for a home rain garden.)
The wall shouldnt be much more than 12 to 15 high. When it rains and rains, soil can get so saturated that it turns to liquid mud. Nothing will hold it back, certainly not a rock wall. A rain garden on a shallow slope works well. But theres a chance that a high retaining wall on a steep slope will fail.
Its important to think about where the water will go if the garden overflows. It will certainly happen sometimes, especially before the plants get established. If the water heads back toward your foundation, thats trouble.
Typically, this isnt a problem because, as I mentioned before, the land around most houses has been graded so rainwater will run away from the house. When the garden overflows, the water will simply follow the existing slope of the yard.
The easiest way to get water into a rain garden is to put the garden in a low place in the yard. The water will flow as it always has. Rather than rotting the turf at the low point, though, the garden will infiltrate it. And you wont have to think at all about how to direct water. Most people dont have a spot like this.
The second easiest way to get water into the rain garden is to dig a swale from your downspout to the garden. Start at the end of a downspout extension and dig a little ditch to the garden. If you have a rod and transit, you can check to make sure that it slopes away from the house and toward the garden. Plant the swale with turf or natives, or even fill it with rocks, depending on your situation.
In my yard, a previous owner had buried plastic pipe that led from the downspouts all the way out to the street. Its light-weight, semi-flexible, ridged, black plastic pipe about 4 in diameter. Its available at any home center or hardware store. I simply cut off the pipe where it entered the garden and laid a few rocks at the open end to prevent erosion.
Compost absorbs water and is recommended for rain gardens. In fact, engineered rain gardens on commercial or public sites will typically replace the existing soil with a mixture of equal parts sand, loam, and compost.
Many cities give away compost made from leaves and other yard waste for free. Click here for Dane County compost.
If compost isnt easily available, just leave it out. The rain garden will do fine without it. Most native plants are well suited to poor soil. It might even help keep them from growing too tall.
Plant selection is the toughest problem in designing a garden.
Trying one engineering solution or another digging a garden this way, or a swale that way can be done, or re-done, in a few days. What works and what doesnt is clear right away.
Plants, on the other hand, do their own thing. And they take their time about it. They wont fully mature for several years. Then some are stunted; others as tall as Shaquille ONeil. The one that should have been perfectly suited to the site dies; an obnoxious few multiply faster than ants at a picnic.
Its impossible to get a lot of experience in a little time. Gather information, talk to people. Then dive in this is a garden, not a skyscraper. But to feel comfortable with the plants means living with them awhile and that takes time.
My list of of rain garden plants is a compilation from seven sources, mostly native nurseries. I also added plants that met the criteria of being bi-modal that is, plants that will tolerate both wet and dry conditions. And I excluded a few plants that were not available for sale at any nursery.
But dont choose plants just from the list. Add ones that you particulary like or that you just want to try. I planted false indigo and butterfly weed, despite the fact that they prefer drier and sunnier locations than my garden. Im happy to report that they arent dead yet.
Click here for plant lists sorted by scientific name, common name, height, and plant community.
Height is the most important characteristic
Avoid plants that are too tall! Big bluestem can grow to 7 or 8. It takes a good bit of skill to find a place on a city lot where an 8 plant wont look overwhelming.
The exception to this rule is seasonal, especially spring, plants. Some small plants will come up in the spring and disappear entirely by the time the summer season plants grow up. You dont want to miss out on violets, Virginia bluebells, Jacobs ladder, and others.
Academic landscape architects have recommended choosing a palette of plants based on their natural communities. These plants are naturally well suited to one another. I include information about Prairie, Savanna, Upland Forest, and Lowland Forest communities in my plant lists. The descriptions that I used of Wisconsin's plant communities are no longer available. Here is another source to try: dnr.wi.gov/topic/endangeredresources/communities.asp
Avoid cultivars and hybrids. These plants typically have a name in quotation marks. New England aster Anna Potschke, for example. Native plants have co-evolved with native insects and animals. A surprisingly small change can make the plant inaccessible to its partner. A change in flower color, for example. Ive watched a hawk moth dip its remarkable proboscis into all the purple flowers in a flowerbed and ignore the red ones. When the plant ceases to do its job, like mowed turf, it becomes an ornament.
Dont skimp on the bones
A third to half of the plants should be sedge or grass. A sedge looks like grass, but it isnt; its sedge. Most grass doesnt like wet feet and often rots in a rain garden. Prairie dropseed is an outstanding exception. But most sedges are perfectly suited to wetter soil. In fact, sedge meadows are wetland communities that are among the first to be damaged or destroyed when urban development reduces groundwater and increases runoff. They can use a second home in our gardens.
Sedges serve two purposes. They compete with the forbs for space and nutrients. That keeps the size of all the plants in check. Theyre less likely to reach Jack-in-the-beanstalk scale. And they make the planting dense, physically supporting other plants so they wont fall down.
Put in one plant per square foot.
Wild gardens work better
Ultimately, the planting should be completely filled with plants. Natural landscapes are thick with plants. Like your lawn, not like a flower garden. Its interesting, actually, that we expect a garden to be individual plants framed by bare soil. Living things battle for space. Its folly to engage all of nature, ultimate defeat a surety, just because, well, a garden is supposed to look that way. When desirable plants fill the space to overflowing, weeds dont have much chance to take hold.
If you let the garden grow wild, the plants will reseed and spread on their own, mixing together freely. It doesnt matter too much where the plants are initially.
If you long for a garden in the old fashioned sense of the word the plants arranged in a living bouquet then of course you should do it. Youll have to weed; defying nature doesnt come free. But theres no reason this style wouldnt work perfectly well as a rain garden.
Not too early, not too late
In southern Wisconsin, a rain garden can be planted from early May to mid-August. Earlier than that and the plants might be damaged by a hard frost. Later, and they might not root sufficiently to survive the winter.
Add woodchip mulch
Add a thick layer of woodchip mulch. Wood chips reduce nitrogen in the soil, but, again, thats good for a prairie garden. In Madison, get free woodchips at Warner, Sycamore, Elver, and Garner parks.
Water at first
When the garden is newly planted, it needs water. Water it just like any other garden, when the weather gets too dry. The second year the plants are much tougher. By the third year, it wont ever need water again. Im not kidding. As I write this, weve gone a whole month without rain and my plants look almost as fresh as springtime. Its astonishing.
Weed until the plants get established
A little weeding is necessary even in a native garden. Invaders include crab grass, creeping bellflower, and woody plants. After the plants are well established, the garden will need some weeding, but not much.
Cut it down in spring
Beyond that, the only necessary job is to cut down the dead stalks in late spring. Break them up and add them to the compost pile. Its best to cut it down spring rather than in the fall so the birds can eat the seeds and overwintering insects attached to the plants. Besides, the stalks are delightful in winter, iced in delicate layers of snow.
If you know how to burn prairies, or if you want to learn, a spring burn is good for rain gardens, too. Typically a permit is required before burning inside a city. Contact your fire department. The Wisconsin Prescribed Fire Council provides links to burn training in Wisconsin.
The cost varies a lot, of course, depending on how you do the work.
If you propagate plants from seed and dig the garden yourself, you can probably do the whole project for next to nothing.
In 2001, I spent about $300 for my garden. That included approximately $100 for the tilling and $200 for plants. The garden was dug and planted by Wild Ones as a group project. I picked up free wood chip mulch from the city and free compost from the county. I reused stones already in my yard to make a small retaining wall.
Having the garden dug professionally from start to finish would probably cost in the neighborhood of $10 per square foot, or $3000 for a 300 square foot garden.
Wisconsin Dept of Natural Resources
City of Madison
Wild Ones Natural Landscapers
Friends of Lake Wingra
Copyright © 2005, 2008 by
Sues Rules for Raingardens