The Thinking (and Action) Behind a Successful Raingarden

In this post, we explore the thinking behind a successful raingarden. Thought has power, in that it sometimes turns into action, and so we will explore the thought behind the action that has made this large native planting thus far succeed.

In this case, it is a detention basin in Smoyer Park that was converted from turf grass into a wet meadow dominated by native prairie grasses with some wildflowers mixed in. Planted by Partners for Fish and Wildlife, this large-scale raingarden appeared sparse its first couple years, but has now grown thick and subtly colorful with time and attention.

Thick, that is, with intended plants, which is not a given when a raingarden is planted. How many gardens of all sorts, no matter how lovely the vision that brought them into being, have been taken over by mugwort, Canada thistle, and other invasive weeds? Having been this raingarden's volunteer caretaker for its first three seasons, as part of my work for Friends of Herrontown Woods, I visited it recently to see what needed to be weeded, and found nearly nothing requiring my attention. For a gardener, this state of affairs is almost unimaginable.

How could this be? Well, it helps to have planted a strong backbone of tall native grasses:

Indian grass

and big bluestem, to claim the space.

The caretaker's job then becomes a matter of influencing what other species come up in the spaces unclaimed by the grasses. Given limited time, a caretaker must learn to recognize each species and know from experience what to weed out and what to leave. Balance, beauty, and diversity are the goals. Some weeds like mugwort, Canada thistle, and crown vetch are notorious for taking over, and so were weeded out early in the game, before they could become too numerous and ruin any chance for diversity and beauty.

Others, like this fluffy-seeded, native but weedy pilewort that is growing lustily around the edge of the meadow, are judgement calls. I left it in, hoping it will behave like lambsquarters--another annual that makes a powerful showing one year but largely disappears the next.

(An example in this raingarden of a species that was numerous one year, gone the next, is black-eyed susan, which was in the original seed mix and generated lots of color before disappearing this year.)

There are a few native perennial wildflowers that have moved in on their own. At this stage, they are relatively few, and add spots of color and late-season nectar. But I don't entirely trust that they will continue to "play well with others." These include late-flowering thoroughwort,

a goldenrod species with narrow leaves,

and frost aster.

Providing bright color earlier in the season was one large specimen of false sunflower (Helianthus helianthoides)--commonly found in seed mixes for wet meadows. In another raingarden some years back I found it to be overly aggressive, but how it will behave here is yet to be seen.

A native annual I wish would spread around more is jewelweed, whose tubular orange flowers serve hummingbirds all summer long, and whose explosive seedpods delight anyone who takes one in their palm and touches it. Usually, the plant is rambunctious enough to survive the insistent browsing of deer, but for now it's huddling against the concrete outlet, chewed down before it can flower.

A "seed bombing" conducted by a girl scout troop in late spring may have been the origin of these young rose-mallow hibiscus (Hibiscus moscheutos),

and some ironweeds. These are local wildflowers we added to get more color and cater to the pollinators. Both of these can get tall, but may end up being kept low by deer browsing.

One of the pleasures in this wet meadow thus far, surely unnoticed by passersby and hard to photograph, is the clarity, by which I mean one can see inbetween plants all the way to the ground. Low-growing weeds like stiltgrass, carpgrass, ground ivy, and many others tend to muddle the planting, obscuring the ground. This clogged condition, the visual equivalent of listening to a scratchy record, has ecological as well as visual impacts. If the spaces between the bunch grasses are clogged with weeds, ground feeding birds have a harder time navigating, and can become more susceptible to disease from rubbing up against wet foliage.

An encounter with a garden that needs no weeding brings, for an experienced gardener, a mixture of feelings. There's surprise, some quiet pride and elation (that past interventions could have had such a positive effect), and a sense of foreboding. Surely I was missing something.

As it turned out, there was good reason for the foreboding. Getting ready to leave, I spotted a small patch of stiltgrass invading along one edge. Unlike lambsquarters or blackeyed susan, which can be prominent one year, gone the next, stiltgrass is one of the uber-invasives, an annual that grows ever more numerous with time, producing billions of seeds each year. Inedible to wildlife, it creates a stifling appearance, making a planting or whole woodland appear to be blanketed with green cobwebs.

Since its seeds were not yet ripe, I pulled as much as I could find out of the ground, then collected them all and threw them in a nearby woods where stiltgrass had already invaded. The source of the stiltgrass was actually the mowed lawn that surrounds our wet meadow. Given that stiltgrass can grow and seed even though only a few inches tall, the lawn is likely to remain a source of invasion for the foreseeable future. Since the weed prefers moist conditions, the more frequent and heavier rains we are experiencing due to global warming will only make it more aggressive. There continues to be hope that something--a fungus, an insect--will figure out how to consume it and thereby begin to bring stiltgrass into ecological balance, but that wait could be long.

These, then, are the thoughts that fill a wild gardener's mind when visiting a raingarden. Because the aggressive weeds were caught early--nipped in the bud, so to speak--the visits need only come once or twice a month for an hour, even for a wet meadow nearly an acre in size. Knowledge and experience make for strategic, efficient interventions. There's time left over to appreciate the bright flowers of partridge pea and the growing diversity. And it's satisfying to know that in a landscape dedicated to ballfields, a lowly bumblebee or the occasional monarch butterfly will still find some sustaining habitat when visiting Smoyer Park.