Wooly thyme (Thymus pseudolanuginosus) (foreground), yellow corydalis (Corydalis lutea) (center), Kenilworth ivy (Cymbalaria muralis) (background) and sedum (below) all do their parts to cover the ground. The corydalis has a lovely habit of seeding into the most interesting location and forming a rolling carpet on top of open ground. It blooms for nearly the full season.
Ideally, good, aggressive garden plants are tough, spread nicely and can be controlled easily by pulling, cultivation or herbicides. The thicker and taller they are, the better they suppress weeds. But what exactly are ‘good’ aggressive plants?
Yellow corydalis (Corydalis lutea)
“The difference between you and me,” snarls the villain to the hero, “is not so great.” It’s a classic moment in action movies that forces us to process in our minds the sometimes razor-thin, but important, differences between good aggression and evil domination. In the garden, knowing the difference between good and appreciated vigorous plants versus bedeviling invasive ones can mean the difference between a bountiful and vibrant garden or a disgusting mess.
While no one could or should understate the woeful impact invasive plants have had on our ecosystems, I sometimes worry that a lot of ground has become merely a showcase for mulch rather than a place for diverse and beneficial plant life simply because people have become fearful of aggressive plants. Remember, ecologically turf is always better than asphalt or mulch and richly planted gardens are always — if maintained — better than turf.
Two tough competitors tumble over a wall in a battle for space. Snow in summer (Cerastium tomentosum) in the foreground and cranesbill geranium (Geranium x cantabrigiense ‘Biokova’) in the background are both spectacular in bloom and provide rich carpets of foliage the rest of the season.
Aggressive or Invasive —Define Please
There’s no way around it. Any mention of “aggressive plants” will stomp all over the thin ice of the native-versus-exotic debate. So let’s define some terms.
Invasive Plant: An exotic plant that can jump spatial barriers, escape into wild places and grow in large enough numbers to create monocultures or near monocultures. A native plant cannot be “invasive.” It can be weedy and not something you want in your garden, but it cannot be “invasive.”
Monoculture: A situation where a plant is aggressive to the point it excludes a significant percentage of indigenous plant life in the wild.
Regionality: A plant that is very desirable in one region may turn out to be invasive in another. If in doubt, check with your local extension agents or expert nursery staff before introducing potentially invasive plants in your garden.
Stylistically, filling the voids between plants with more plants helps increase multi-season interest, provides color and textural contrast, increases habitat and food for beneficial insects and wildlife, increases repetition and is simply good design. Functionally, vegetation performs all the roles we would typically assign to mulch better than mulch. Growing more plants is a win-win.
Just because plants used for such purposes are often called “filler” or “ground cover” doesn’t mean they must be banal! A swath of black-eyed Susan (Rudbeckia fulgida var. fulgida) in midsummer bloom is a show stopper. A carpet of plumbago (Ceratostigma plumbaginoides) in bloom and fall color in late September is a sight to behold. A river of hardy geranium (Geranium cantabrigiense ‘Biokova’) flowing between blooming azaleas is capable of upstaging them.
Arkansas bluestar (Amsonia hubrichtii) gradually becomes a big, sturdy plant. If you move seedlings or plant it in sizeable numbers as in here at The Scott Arboretum in Swarthmore, Pa., you can cover large amounts of real estate with beautiful, beneficial plant life.
Cranesbill geranium (Geranium x cantabrigiense ‘Biokova’)
Ideally, good, aggressive garden plants are tough, spread nicely either by seed or vegetative growth, can be controlled easily by pulling, cultivation or by glyphosate or pre-emergents. The thicker and taller they are, the better they suppress weeds. Filler plants can be any height, but they should not exceed one-third the height of your nearby specimen plants. Needless to say, such plants can go a long way towards allowing you to garden more on a small budget.
So don’t be afraid. Don’t be shy. Put some rambunctious things in your garden, spur your horse, and ride off into the sunset.
Examples of Good, Aggressive “Filler” Plants
|Japanese anemone (Anemone hupehensis ‘September Charm’)||Spreads|
|Arkansas bluestar (Amsonia hubrichtii)||Seeds|
|‘Birch Hybrid’ bellflower (Campanula ‘Birch Hybrid’)||Spreads|
|Plumbago (Ceratostigma plumbaginoides)||Spreads|
|Snow-in-summer (Cerastium tomentosum)||Spreads|
|Lily of the valley (Convallaria majalis)||Spreads|
|Yellow corydalis (Corydalis lutea)||Seeds|
|Ostrich fern (Matteuccia struthiopteris)||Spreads|
|‘Biokova’ hardy geranium (Geranium cantabrigiense ‘Biokova’)||Spreads|
|Hellebore (Helleborus x orientalis)||Move seedlings|
|Dwarf crested iris (Iris cristata)||Spreads|
|Creeping mazus (Mazus reptans)||Spreads|
|Phlox (Phlox subulata)||Spreads|
|Black-eyed Susan (Rudbeckia fulgida var. fulgida)||Seeds|
|‘Goldsturm’ black-eyed Susan (Rudbeckia fulgida ‘Goldsturm’)||Seeds|
|Brown-eyed Susan (Rudbeckia triloba)||Seeds|
|Sedum (Sedum spp.)||Spreads|
|Thyme (Thymus spp.)||Spreads|
|Veronica or speedwell (Veronica spp.)||Spreads|
A version of this article appeared in the January/February 2013 print edition of State-by-State Gardening.
Photography courtesy of Scott Beuerlein.