Monday, May 23, 2011

How to build a large rain garden on your terrace


Building this garden doesn't have to cost anything.  Once established, little maintenance is required.  The basic idea is to dig out the terrace, to below the sidewalk level.

The garden shown above was built by botany professor Bob Kowal at 537 Gately Terrace.  I described it in a previous post.

Before you begin, there are two important issues to think about:

Find a source of native plants

There are sales about Madison in the spring.  Or, you can beg plants from neighbors.  If they have a garden of native plants, there's a good chance they have extras.

Native plants require less maintenance, and are very attractive.  Pick plants appropriate for your conditions--sun or shade.  Link to plant lists.

Unless your sidewalk is on a hill, you probably won't get a lot of water in the garden--so you don't have to worry about finding wetland plants.

Some native woodland plants like sweet woodruff spread rapidly.  You can plant these first, and let them fill out the terrace.  Later, as you get other plants free from your friends, you can add them.

Find a place for the soil you remove.

It's going to be a lot of soil!  For my first garden, I separated the soil from the roots, which was a lot of work.  Then I was able to spread the loose soil on my front lawn, rake it smooth, and seed it.

For my second terrace garden, I advertised on Craig's List--"Clean fill offered."  Within an hour, someone responded, then parked a trailer next to my future rain garden.  Not having to move the soil, or separate it from the turf saved much time and work.

I've also used soil removed from rain gardens to make little dams about my yard, or to fill in gullies in the neighborhood.

Why dig out the soil?

With the garden below the sidewalk, rain flows from the sidewalk to the garden.  Your garden gets extra water, so it's more luxuriant.

But another important reason is to allow plenty of room for leaves to accumulate.  Never rake your terrace again!  The leaves smother most weeds.  Gradually, the soil will become very rich as the terrace fills with composting leaves.

Digging all the soil from sidewalk to curb makes an attractive garden.  However, you may want to bevel the edge on the sidewalk side, for added safety.  (But no one has sued Bob during the last 15 years!)

To be on the safe side, call diggers hotline.  My garden is over the gas line, but I didn't get deep enough to hit the pipe.

When you finish digging, enrich the soil

The "Catch 22" of this job is that you've removed the top foot of good soil.  There's a good chance that what's left is gravel or clay.  So, you have to enrich the soil enough to get your plants started.

Add compost if you have it.  I added coffee grounds from the neighborhood coffee shop (now in short supply).  Bob Kowal added bales of oak leaves he bought at Forest Lawn Cemetery.  Or, you can dig deeper, then add back some of the best soil you removed.


Put the plants in clumps of the same kind, with plenty of room between each plant (about 18", because they will grow).   Put the taller plants towards the center of the terrace.

Finish by mulching, to stop the weeds and keep in the moisture.  Grass clippings make good mulch.  Bark isn't a good idea of your garden will overflow, because bark will float away.  Water until your plants become established.


For the first few years, keep enriching the soil.  If some plants don't thrive, replace them with others.  Be sure you remove maple seedlings, before they become too large to pull easily.  Watering is needed only during drought times.

Future site of terrace rain garden.  Pour water to make sure sidewalk tilts toward garden.
Spreading the removed soil on the lawn nearby.

Rain garden in winter, ready to absorb spring meltwater.

Next spring--rain garden on May 12.  Not bad for the first year.

Medium-sized terrace garden.  Doesn't look as good, but there's less soil to move.  Bevel the edges.

Slide show of flowers in the garden here.

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