National Wildlife Federation/ San Diego County

Invasive Plants

by Deb Coon, Habitat Steward

What are Invasive Exotics and What’s the Problem? “Invasive exotics” (or simply “Invasives”) are plants that are introduced from other geographical regions and begin to displace the species native to a given region. [Animal species can also become invasive and problematic when introduced outside their usual range, but on this page we will restrict the discussion to plant species.]

The topic of invasive exotics is somewhat controversial. Plants have always spread geographically via wind, water, and animals (including humans), so some people argue that we’re just continuing that process when we introduce species from other regions into our gardens. The problem as others see it is that some plants and animals are almost too successful in their new surroundings – they escape from our gardens via wind, water, and animals, and then compete with and replace numerous native species, disrupting local ecosystems in unforeseen ways. The net effect, according to this view, is a decrease in global biodiversity, and the disruption of regional ecosystems at an unprecedented pace. Think of the rapid spread of Kudzu throughout the southeastern U.S. or Purple Loosestrife in any of the moister regions of the U.S., and you have an idea of the concern.

My personal feeling is that perhaps we gardeners would do well to adopt Hippocrates’ words of caution to physicians: “To help, or at least, to do no harm.” There are so many beautiful plants available that we can easily choose not to buy plants known to be invasive, and may even choose to avoid the plants that are not yet invasive but are causing concern among ecological experts. Many of us go a step further and choose to substitute native plants typical of our region in our landscapes whenever possible, in hopes of ameliorating at least a bit of the ecological damage our suburban developments are doing. When there are beautiful native plants available, why not plant them?

It’s important to note that not all exotics are invasive. An example of a non-invasive garden plant would be the common gardening shrub Raphiolepsis. It hasn’t spread to our wildlands in southern California (at least to my knowledge), probably because our local birds don’t seem to like its berries. However, the same quality that makes it a safe choice for ornamental landscaping, makes it a relatively poor choice for wildlife gardening – i.e., it doesn’t spread because local critters dislike its berries. One could instead plant a handsome native berry-producing plant such as California Coffeeberry that would look good and provide cover while also feeding the berry-eating birds and critters.

It is also the case that some exotics are invasive only under certain conditions. An example of this is the Butterfly bush (Buddlea spp.), which is very invasive in wetter regions of the U.S., but has not yet shown to be invasive here in San Diego County. It seems to need the year-round moisture of the garden setting or riparian area to thrive, and perhaps hasn’t spread to our riparian areas because it lacks appealing seeds or berries to be eaten by wildlife. Melaleuca quinquenervia (Paperbark Melaleuca) is another example. Melaleuca trees have taken over hundreds of thousands of acres in the Florida Everglades, but they don’t seem to be invasive in our dry southern California climate. However, conservationists are monitoring our riparian (streamside) areas closely to see whether Melaleucas eventually become invasive in those areas, as Eucalyptus have already done here.

It’s important for us to learn which plants are invasive in our locale so that we can make our gardening choices appropriately. Several examples are discussed in the next section, while the websites listed below have an abundance of helpful information. The California Invasive Plant Council (formerly California Exotic Pest Plant Council) is dedicated to research and education regarding invasive plants. Members of the general public are invited to become members and help support the cause.

Some Commonly Used Invasives and Alternatives to Them

The California Invasive Plant Council (Cal-IPC) has published an excellent brochure called “Don’t Plant a Pest.” It has descriptions and pictures of some of the most invasive landscaping plants and, most important, suggests good garden substitutes for them. Their suggestions include both native plants and exotics known to be non-invasive. What follows are some excerpted suggestions that will perhaps whet your appetite to explore their website for further information (see “Cal-IPC’s Landscaping Alternatives” hotlink below). I hope these examples illustrate that we can continue to create aesthetically pleasing landscapes while avoiding plants that can escape into the wildlands and destroy them.

Suggestions from Cal-IPC’s “Don’t Plant a Pest” brochure:

Instead of the invasive groundcovers Periwinkle (Vinca major), English Ivy (Hedera helix), and Algerian Ivy (H. canariensis), plant the non-invasive Pachysandra (Pachysandra terminalis) or Serbian bellflower (Campanula poscharskyana).

Instead of the invasive groundcover Hottentot Fig or Iceplant (Carpobrotus edulis), plant a non-invasive variety of Iceplant such as Delosperma cooperi with its electric purple flowers, or plant Showy Dewflower (Drosanthemum floribundum) with pink flowers.

Instead of the terribly invasive ornamental Pampas Grass, plant another beautiful but non-invasive grass such as Lindheimer’s muhly grass (Muhlenbergia lindheimeri) or San Diego Sedge (Carex spissa).

For bird-lovers, instead of Cotoneaster, plant the handsome and adaptable California native shrub/tree Toyon (Heteromeles arbutifolia). Toyon’s bright red berries are loved by berry-eating birds (local and migrant) such as Thrushes, Orioles, Cedar Waxwings, Mockingbirds and others.

Where to Learn More

In addition to the websites listed below, two helpful books for learning more about invasive exotics and aiding in their identification are Tom D. Whitson (Ed.), Weeds of the West (Revised ed., 2002) and Carla Bossard et al. (Eds.), Invasive Plants of California’s Wildlands (2000).

Invasive Plant Information on the Web

California Invasive Pest Plant Council ?– Cal-IPC
Cal-IPC?’s List of Worst Invasives (1999)
Cal-IPC?’s List of Lesser Invasives (1999)
Cal-IPC?’s Landscaping Alternatives
CA. Dept. of Food & Agriculture?’s Encycloweedia
Western Ecological Research Center of the USGS, Projects
San Diego Natural History Museum -- ?“Pretty Pest Plants?”