Good groundcover can be a bad actor
Will plants come through groundcover when it gets very
thick? I am afraid of it choking out everything. - Rita
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In time, every groundcover can out-compete at least some other
plants. This category of plant is supposed to fill and hold space
to keep weeds from taking hold, so it wouldn't fit the bill if it
wasn't at least a little aggressive.
Some groundcovers take over faster and hold on more tightly to
space than others. Some plants in the path of a groundcover succumb
faster than others. Many persist but can't grow any larger, replace
bulk they may lose to pest or poor growing years, or find open
ground where replacements can begin from seed.
Predicting dominance when plants grow on a collision
When two or more plants are going to tangle, we assess each
plant's chances this way:
- Height wins. So if the groundcover is taller than another plant
in or near it, the groundcover will win.
- Evergreen wins. If one of an equal-height pair must vacate the
premises during winter but the other can stay on and
photosynthesize, it should be no surprise when the evergreen gets
the upper hand.
- Wood wins. Trees, shrubs and vines can add a bit of height each
year and start from that point the next spring. Herbaceous
perennials and annuals have to start at ground level each year.
Sooner or later the woody plant wins.
Below, left: English ivy (Hedera helix) has crept
into the area held by creeping Jenny, a.k.a. golden coins
(Lysimachia nummularia 'Aurea'). There is no question in
our minds, the woody evergreen ivy will "win" this spot unless a
gardener plays referee.
Below, right: Deadnettle (Lamium maculatum, with the
white leaves) is a tough groundcover but it won't be able to hold
this territory against the taller, evergreen variegated
Euonymus (E. fortunei).
Quilt sounds comfy, is really a battleground
When we ask two or more plants to coexist in what we call a
quilt -- a sizable planting that provides a bit more variety but no
more and maybe a bit less work than standard groundcover. Using
more than one species is my hedge against the bad times that come
to every species now and then. In the best quilt, if one plant
develops a problem, its comrades are not affected and can close
ranks where the wounded goes thin.
When we plant a quilt we try to keep intervention to a minimum.
We call it "see who wins." We may plant the west half of a quilt
area with Ajuga, the east with Lamium, then allow
each to fill its own space and mix it up along their shared
boundary. These two plants have proved to be compatible.
Right: Groundcover junipers (Juniperus squamata
'Blue Star' here) and creeping thyme and creeping pinks
(Dianthus gratianopolitanus) will "play nice" even when
they knit together as a groundcover quilt.
Best quilters have energy to grow together, contrast to please
In choosing plants for a quilt:
- Look for players with compatible energy levels. If one is
Pachysandra terminalis, which is a steamroller but rather
slow, don't pair it with a speed demon like Ajuga. If one
is an ericaceous plant such as Rhododendron that struggles
in Midwest soil, don't handicap it with an evergreen groundcover
that will monopolize the rhodo's root space all year and make the
growing even slower.
- Aim for pleasing contrast within my self-imposed constraints of
height, evergreen and woodiness. Pachysandra procumbens is
our native species and it's semi-evergreen. Using that allows more
partners to choose from. You might try snow in summer
(Cerastium tomentosum) for its finer texture and to blend
its gray with the Pachysandra's green. Or use companions
that have already proven themselves to be equal to the task, such
as Japanese painted fern (Athyrium g. 'Pictum') and lenten
rose (Helleborus x orientalis).
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