Invasive Exotics

Posted by on November 10, 2008 in Uncategorized | Comments Off on Invasive Exotics

A friend called recently to tell me he’d just bought his
first house. He had never planted a
garden before, but he couldn’t wait to get started. “I just love the ivy,” he told me. “I know I’ll keep that.”

 Well, maybe
not. Most gardeners, even beginning
gardeners, quickly come to realize the pitfalls of invasive, non-native
plants. English ivy (Hedera helix),
for example, may be a quick fix on unstable slopes or anywhere a
low-maintenance groundcover is needed, but it is nearly impossible to eradicate
and when it spreads into wild areas, it can smother less vigorous native
plants.

 A native
plant is generally defined as any plant that was here before European
contact. Planting a native-only garden
might mean excluding too many beloved favorites: what would my garden be, for instance, without its lavender or
its daffodils? But even if you’re not
committed to planting an all-native garden, you should still think twice before
introducing a plant into your garden that is not only exotic, but
invasive. You run the risk of
inadvertently introducing the plant into a nearby wilderness area, and you may
find it very difficult to pull out if you ever decide you wanted a change.


 The
definition of “invasive exotic” varies by region, so check with your local native
plant society to be sure. And don’t
assume that all plants you buy from the nursery are non-invasive. I often find ivy, pampas grass, and the
much-detested Scotch broom for sale at local nurseries with no warnings on them
whatsoever. It can be tempting to bring
them home, too. After all, part of the
reason these plants are so widespread is because they have some quality that we
admire, but would be better off learning to live without. California gardeners
might want to think twice before planting any of the following attractive but
invasive non-natives, all of which are listed as A-1 invasive plants by the
California Exotic Pest Plant Council.

 Iceplant
(Carpobrotus edulis):
 I’ll
be the first to admit that nothing says “California beach” like this non-native
succulent with brilliant yellow and pink flowers. Gardeners who own beach homes are probably especially tempted to
introduce this sturdy, colorful groundcover into their sandy seaside gardens. But this plant, along with European beach
grass (Ammophilia arenaria), has taken over one of California’s most
fragile and beautiful ecosystems—our sand dunes. Ironically, these plants were sometimes introduced as part of
public works projects to stabilize and save the dunes, and instead they have
destroyed life on the dunes. 

Fortunately, there are some lovely
alternatives for beach gardens. The
dune tansy (Tanacetum camphoratum) thrives on dunes and produces yellow,
button-like flowers. Yarrow (Achillea
millefolium
) is another native that does well in shifting sands, and there
are many non-native, but well-behaved cultivars that offer a wide range of
colors. Yarrow can also be mowed as a
turf substitute. Finally, don’t forget
the native beach strawberry (Fragaria chiloensis), which does well as a
groundcover on slopes and provides berries for the birds.


Pampas Grass (Cortaderia
selloana and C. jubata
) and Fountain Grass (Pennisetum setaceum):
 Ornamental grasses look so very right
in a California garden, where they blend well with native plants, Mediterranean
gardens, and Southwestern-style landscapes. But it’s important to avoid pampas and fountain grass, both of which
have taken over the coast ranges. Pampas is so aggressive that it is able to out-compete plants that are
much taller than its own six-foot height, and fountain grass, although lovely
when it blooms and produces silken seed heads, has spread to roadsides,
canyons, and grasslands. 

Look no further than native
California grasses for alternatives. Deer grass (Muhlenbergia rigens) grows up to four feet high and
about as wide, producing purple or yellow flower spikes in fall. I also like California fescue (Festuca
californica
), which forms slightly smaller clumps of silver-green leaves
and produces feathery flower spikes in spring and summer. 


Scotch Broom (Cytisus
scoparius
)
: If you want to see
a northern California gardener bristle, just ask her what those lovely
yellow-flowering shrubs are alongside the highway. Scotch broom grows up to ten feet tall, produces wands of
yellow-gold flowers, and makes itself at home in coastal scrub areas, in oak
woodlands, and in the Sierra foothills. To my astonishment, it is widely sold in nurseries. One of the reasons it is so hard to
eradicate is that the seeds can remain viable for decades. It’s also highly flammable, making it a
particular liability in areas at risk for wildfires.

Gardeners who are looking to
introduce color into their garden have plenty of alternatives to Scotch
broom. There’s the purple and white
Pacific Coast iris (Iris douglasiana), the orange Humboldt lily (Lilium
humboldtii
), and of course the golden California poppy (Eschscholzia
californica
). Larger shrub
alternatives include Cleveland sage (Salvia clevelandii), with its
silver foliage and long-lasting blue flowers, and the deeper blue California
lilac (Ceanothus impressus). For contrast, try sticky monkey flower (Mimulus
aurantiacus
), which blooms in shades of yellow, orange, and red, and
California fuchsia (Zauschneria californica), which stands out in desert
gardens or in fog-bound coastal gardens with its grey foliage and red,
trumpet-shaped flowers that lure hummingbirds.

 

Himalayan
blackberry (Rubus discolor)
: I can understand why it would be tempting to let a little of this wild
blackberry sneak under your fence and form a thicket in your garden. I’ve eaten
many a home-baked Himalayan blackberry pie myself, and the vines provide
shelter and food for birds in the neighborhood. But this berry vine has completely covered riparian areas, oak
woodlands, and marshes. In fact, on a
walk this morning, I realized that the entrance to a wild riparian area near my
home was entirely impassible thanks to Himalayan blackberry.

 If you’re
interested in growing berries for your own breakfast table, choose one of the
better-behaved, cultivated varieties available from your nursery as bareroot
stock in winter. You’ll have the option
of choosing thornless varieties, varieties that are disease resistant, and
you’ll find plenty of interesting choices that aren’t available at the grocery
store, like tayberry and nectarberry. Plant them in raised beds so that it is easy to keep them confined, and
train them up a trellis to make pruning easier.

 And if
you’re looking for something that produces berries for the birds, there are
plenty of choices among California natives. I like the red huckleberry (Vaccinium parvifolium), which grows
slowly, reaching about ten feet tall in the right conditions, produces lovely
white flowers in spring, and offers plenty of edible (for humans and birds) red
berries in fall. Another good choice is
coffeeberry (Rhamnus californica), a tidy, mounding shrub that usually grows
five to six feet tall and about eight feet wide, although it can reach up to 15
feet tall. The berries undergo a
transformation from green to red to black throughout the fall.

 Eradication
of invasive exotics is nearly impossible. I’ve seen community groups spend all weekend on a beach clean-up to rid
the dunes of ice plant, and I’ve seen landscapers with bulldozers trying to
wrestle pampas grass out of the ground by its roots. I know a couple who bought a home on a piece of property that was
completely smothered in ivy; they spent four years pulling it up, let the dead
ivy dry out, and running it through a chipper-shredder for mulch. Eventually, they were able to start planting
and have restored the property to the woodsy creekside environment it once was.

All of these methods take time and
persistence. Smothering invasive
exotics with plastic weed barrier doesn’t work; they find a way to get through
eventually. Herbicides can be
effective, but many gardeners would rather avoid the use of chemicals when they
are creating a natural setting for wildlife. And as the recent California wildfires have shown, even fire doesn’t get
rid of invasive exotics. Sometimes it
encourages seeds to germinate even faster. In home gardens, cutting down invasives and digging out the roots, year
after year, is the best approach.

Once they’re gone, though, you’ll
find that the natives you bring in to replace them are virtually
maintenance-free. Most actually prefer
poor soil, they need little in the way of pruning, they are less likely to
suffer diseases and pest infestations and, once established, they can tolerate
long summer droughts. Best of all,
you’ll attract a surprisingly diverse community of birds, butterflies, and
beneficial insects to your garden.

 

As always,
remember that California is a diverse state. A plant that thrives on the Mendocino coast might whither away in a Palm
Springs backyard. Check with a local
native plant nursery to find just the right plant for your microclimate. The California Native Plant Society
(www.cnps.org or 916 447-2677) can refer you to a nursery or local expert.