Design

 Location

It's not recommended building rain gardens under a tree canopy, because you could damage the tree's roots, while the roots interfere with digging.  Likewise, rain gardens on a slope invite erosion and make it harder to excavate a basin.

So why did we build this garden on a shady slope?  Floods of runoff collect here from the paved playground, sidewalks, and the school's roof.  Flowing down towards the playground below, the floods were eroding a deep, unsightly gully (photo on right).  The resulting sediment was harming Lake Wingra.  This patch of land under the tree was the only bare ground along the water's path large enough to build a rain garden.

Esthetics of planting

We place plants of the same species in clumps, which is how nature tends to do it.  This clumping creates areas in the garden with the same texture or color, giving it a visual structure that is more pleasing than a chaotic mixture of all species together.

We tend to place taller plants in the rear, so they don't obscure the shorter ones.  This also adds to the pleasing structure.

Layering to reduce weeds

We create multiple layers of plants to increase shade at the surface, which helps control weeds.  There simply isn't enough light left at ground level for most weeds to grow.  Layers and shade also preserve soil moisture during droughts.

Top layer  The trees. 

Tallest plants  White snakeroot, golden alexander,  large sedge, big-tooth aster.

Mid-layers  May apple, wild geranium. Most of the other plants fall into this group.

Ground-hugging, short layer  Pennsylvania sedge, wild geranium, sweet woodruff, lamium.

Leaf litter  We don't remove all the fallen leaves, which adds more shade, erosion control, while preserving nutrients and moisture.

Early bloomers

The earliest bloomers (in April) are called "spring ephemerals."  They emerge early before the tree canopy leafs out, so they can get enough light to quickly flower and store food, often in a bulb.  Then they die back when the trees leaf out.  Others, like may apple, bloom a bit later but die back in the summer--so the amount of shade at ground level changes over season.

Terraces and ditches

The garden is designed to catch runoff from storms, prevent flooding by storing the water, then let it seep into the ground.  The berm at the lower end of the garden acts like a dam to create a temporary pool.  There are channels to route water into the garden.  Terraces create smaller pools or slow the runoff to prevent erosion.

Borders

Borders are attractive and provide a low-key message to "stay out."  They keep grass from creeping in or garden plants from escaping.  In the past, we've used a row of commercial mulch or rocks.  The problem with mulch is that it attracts weeds and tends to disappear from mowing or raking.