Saturday, June 29, 2013

How to build a large rain garden at a school or park

This is a summary of what we learned during the construction of a large rain garden at Thoreau School.  It was an Eagle Scout project for Jack Nolan, age 14.  It was funded by a $1,500 grant from the Friends of Lake Winga, and supervised by David Thompson.

Planning and timing

A large garden is a complex effort.  It takes time to get the permissions, select the location, make the design, and order the plants.  Teachers and classes can't be included unless there's plenty of lead time.

Thoreau garden #2 planning was started in October of 2012, and built in June of 2013.   But this was a stretch!  I'd suggest a two-year process.  In the first year, secure your funding, select a tentative location, begin the permission process, and identify the group to be involved (such as scouts or a class).  No later than early fall of the year before construction, begin your detailed planning.

Plan construction and planting for May.  Having two weekends, one for construction, and the second for planting, worked well for us.  Sunday of each weekend can be a rain date.  The week between work days can be used for mop-up from the first work day, and preparations for the second work day.

The best overall manual is the one by Roger Bannerman, available from Amazon or the web.


is extremely important.   I suggest starting a project with several alternative sites, narrowing down to one as information is gathered.

Swales--at schools or in parks--can be an ideal location.

They are usually away from trees, and have a gentle grade.  All you need to do--to form a basin--is remove some sod in neat rectangles, and use several layers of sod to make shallow berms, at intervals along the swale.  The place where you removed the sod becomes the basin, and it's now free of turf.  Start with one basin, and add more basins along the swale in subsequent years.  Use the first garden as a nursery, to produce plants for the next year.

A major advantage of swales is that building and planting each basin is a manageable task.  Different classes (or citizen groups) can "adopt" each segment (basin) of a swale.  More basins can be added each year along the swale.  After several years, a large area can be transformed, with a reasonable amount of work each year.

Terraces (between the sidewalk and the street) are also ideal locations.  Because the turf around sidewalks is  usually raised, the sidewalks become creeks during rain, sending water down the sidewalk and into the street at the first driveway.  Any terrace, with the sidewalk sloping towards the terrace, is a good spot (except very close to trees).   But the best terrace locations are along the borders of parks or schools, where the sidewalk can collect water from a considerable distance.

At these locations, you don't have to persuade a homeowner, but permission from the Parks Dept or MMSD is required.

Risk factors

The Thoreau garden #2 had a challenging location due to slope, shade, and volume of water.  The location should match the capabilities of the people building the garden.  Serving a single downspout is the simplest garden design.

Besides runoff volume, be sure to evaluate other risks such as snowplows, lawn mowers, salt damage, children at play, and pedestrian traffic.

Dealing with overflow 

Volume of water is the biggest risk factor.  We want gardens where they will infiltrate the most water. But large volumes also put gardens at risk.

For this reason, potential sites should be observed during storms, or at least the size (in square feet) of the collection area should be estimated.  If the garden is at risk of being overwhelmed, design needs to include bypass and/or overflow channels.  It's possible to design a garden so that, once it is full, the excess water bypasses.

If the garden has risk of damage from large volumes, it should be carefully monitored after construction.  Corrective action should be taken after storms.

Arrangement of plants

Creating clumps--putting multiple individual plants in the same area--is the best plan.  It looks better, and the planting process is easier to organize.  Clumps will simplify weeding, because anything that's not the expected plant for each area must be a weed.  Obviously, clumps of taller plants should go near the center of the garden.

Once placed in the ground, the plants begin to compete with one another.  Some species--usually the larger ones--are going to overwhelm others.  For that reason, planting in clumps is best.

Should you have many species or a few?  The simplest design is best, for ease of ordering, planting and weeding.  However, which plants will thrive in your location is unpredictable.  So you don't want to put all your eggs in one basket--one species--unless you know for sure it will thrive in your garden.

How many plants per square foot?  A rule of thumb I heard was one plant per square foot.  (In other words, planting them about one foot apart.)  However, I believe you need to take the mature size of plants into account.  Large species can go two feet or more apart.  Small species, like Pennsylvania sedge, might go closer than 1 foot apart (especially if you want the garden to look good, early).

Order plants from Plant Dane! or The Prairie Nursery, or the UW Arboretum native plant sale.  If you can justify wholesale, try Agrecol.  Be aware of order deadlines.

Avoiding trees

It's said that you should place your garden outside the drip line of large trees.  This avoids harming tree roots, and dealing with dense shade.  But if you can't avoid trees, it's still possible to build a garden if you don't dig a basin.  You might use silt socks, or a berm, to retain water.

Soil improvement

"Professionally built" gardens may start with a deep basin, which is then back-filled with a mixture of sand, topsoil, and mulch.  This improves the infiltration, but is obviously expensive and labor-intensive.  If you are excavating a basin of any depth, this takes you down to subsoil.  Poor soil requires enrichment.  One way to do this would be to use a rototiller to work in compost, after the basin is constructed.

We didn't use back-filling or a rototiller.  Instead, we used about 30 gallons of compost (my home brew).  When planting, we dug a larger hole than needed for each plant, then added compost with the plant.


Most types of mulch will float away the first time your basin fills.  It's more than wasted effort--the fugitive mulch can bury your baby plants.  Grass clippings may work better, because (if applied soon after cutting) they can form a "thatch" that resists moving.

The best bet may be an erosion control blanket, held down with a few garden staples or wooden stakes.   You place it before planting, then cut holes with scissors for the plants.  An EC blanket performs some of the jobs of mulch (moderating temperatures, retaining moisture, stopping erosion), but it won't stop the weeds.

We used an EC blanket on our berm (light green).

Maintenance and followup

Essential!  Includes jobs like repair of damage from storms, watering, weeding, checking for rabbit damage, digging buried plants out of shifting mulch, and cleaning mud off leaves of flooded plants.  You have to mark the garden's boundaries for lawn mowers and snowplows.  Plan for signs to explain your garden.  Place permanent metal signs identifying your species.  Make a manual, with photos of plants and a planting diagram, for people who will weed the garden in later years.

Selecting your species

This was one of the hardest jobs for the Thoreau garden.  Unless you are a Master Gardener, it seems like selecting a pig in a poke.

There are lists of recommended plants, and tables showing the characteristics and requirements of plants.  But to our consternation, we found different sources contradict one another.  If you aren't familiar with plants, have an expert select your plant list.

We ended up making a spreadsheet showing the numbers of plants we had ordered, along with their characteristics and requirements.  The spreadsheet was very helpful later.

Obviously, getting the right sun/shade tolerance is the most important job.

Paradoxically, drought tolerance is important--unless you are sure you can water regularly.

Soil preference would be important, except I suspect that most soils in Madison are going to be pretty much the same--a sort of average, lawn-type soil.

I'm of the opinion that any native species that survives, and thrives, and out-competes weeds--is a success.  However, other people may want showy flowers (butterfly weed), or attractive foliage (wild ginger, ferns, Pennsylvania sedge), or a continuous succession of flowers.

One thing no one warned us about...  Some of our baby plants HATE being periodically submerged in muddy water.  This is especially true of fragile, spindly plants like wild geranium or golden alexander (photo).

They may become very robust when mature--but fresh from the nursery, they are delicate primadonnas.  In contrast, big-leafed aster is wiry and tough as a little bulldog.  So if you have a deep basin, you need to avoid the delicate plants (or avoid flooding).

Educational component

Education about stormwater, biodiversity, and watershed improvement, are all important reasons for building a garden.  Involving school classes is labor-intensive, and takes a lot of lead time and planning.  There are alternative ways to provide education.  A prominent location, with signage, is one way.  And, the group that provides the labor will learn a lot.

Recruiting your workforce

We had about 17 people constructing, and 12+ planting, working from 9:00-1:00 pm.  Sub sandwiches, drinks, and cookies at noon were provided.   However, only about 2/3 of our 500 plants got in the ground by the time volunteers left.

For your primary workforce, target a club, class, garden group, or Scout troop.  Recruiting by email isn't effective.  You really need to speak to people individually, and get RSVPs.

About a week before the work day, you need to take stock to see if you have enough people.  If you don't, then send out an emergency request to people you know you can count on.  It's important to allow flexibility.  People don't have to work for the whole four hours.  Tell people they can come for less time, provided it's at least an hour--but get a definite commitment.  With planting, because space is tight, you want to make sure not everyone comes at the same time.

Organizing construction

During construction, the boundaries of the garden and of the berm need to be marked with yellow rope.  You may need to show some novices how to dig effectively.  You'll need to arrange for a sufficient number of shovels, root cutting tools, rakes, and level (for the berm).  Two or more wheelbarrows are essential if you are creating a sizable berm or basin.

Select a reliable person to shape, level, and compact the berm.

You may end up with a large amount of excess soil.  Who will haul that away, and how will it be protected from rain and runoff?

Organizing planting

For planting, the zones for each species need to be marked in advance with chalk, fluorescent tape, or rope.   Locations for individual plants also need to be marked with a little flag, plant label, or Popsicle stick.  At this stage, you may discover you've ordered too many of one species, and too few of another.  So you may have to adjust the sizes of your zones, or even enlarge or contract the whole garden.

In our case, the berm took up more room than we expected, and the gardens (because of snowplow setbacks) were smaller than originally planned.  So just before planting day, we added a third basin, so we could use all the plants we had ordered.

Several organizers are needed to hand out planting assignments, plants, tools, and compost.  The organizers may also have to instruct people in planting, since many have no experience.  We divided people into teams of two--one inside the garden to plant, and the other person outside, to hand in plants and compost, and take away soil.

Each day, a few organizers are needed at 8:00 am to set things up.  A few need to stay for an hour or two after 1:00 pm to clean up.

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