Next week, July 21-25, is California Invasive Weeds Awareness Week. While no projects to eradicate plants or educate the public are planned in our area, it isn’t because there aren’t any invasive plant species threatening our native ecosystems. According to the California Horticultural Invasives Prevention (Cal-HIP), a coalition of horticulturists, botanists, and nursery people, there are at least 10 plants that have become so noxious that they are recommended to be removed from nurseries and eradicated wherever they are growing. These species have escaped the confines of gardens and are reproducing rampantly in the wild to the detriment of native vegetation.
Arundo donax, often called giant reed, is a large-scale grass that is native to the Mediterranean where it grows along water courses. It’s no wonder that it finds our south coast climate to its liking as well. It has become a serious nuisance, however, now that it has become established in our river and stream beds. It out-competes native plants by creating dense stands of tall stems, sucking up the water and shading natives of lesser stature. When it gets really entrenched, it even flowers and sets seed that waft off to start new colonies where conditions are suitable.
Fountain grass (Pennisetum setaceum), grown for its fluffy, ornamental plumes, is also making its way into the native landscape. It thrives even in some of the most inhospitable places. You can readily see it along roadways throughout the county, but it also is making its way into sensitive areas such as area creeks. Thankfully, the decorative red varieties do not set seed (and may even be a different species), so they present no threat.
Another water-sucking weed is tamarisk or salt cedar, Tamarix chinensis. It is a tough, salt- and drought-tolerant shrubby tree that was widely planted in desert and other marginal habitats as a wind break. It has escaped those fence lines to choke out natives along perennial streams, where its root system can reach deep underground to tap into subterranean sources. It has done much to reduce the flow of water in the Colorado River (and many other lesser rivers and creeks) as it passes through the drier parts of the state.
Now such a ubiquitous part of the landscape that many outsiders consider it a native is Eucalyptus globulus. It, too, was widely planted as a windbreak, but its fecundity has soured its charm now that seedling trees are taking over wild lands. Not only is it a thirsty tree that can grab moisture away from other plants, it is a serious contributor to fuel loads in areas subject to wild fires. Brittle wood also adds to the hazard of having this eucalypt growing nearby.
Tough and fast-growing would probably describe at least half of the invasive plant species in our area. Myoporum laetum is another one of them. Often planted as an evergreen hedge, it can spread into adjacent wild areas with ease, stifling less vigorous natives.
Beloved for their sunny yellow flowers and drought-tolerance, a group of plants collectively known as brooms were widely planted for years, until biologists began to see them in places they shouldn’t be. Canary Island broom (Cytisus canariensis), Spanish brooms (Genista hispanica and Spartium junceum) are all escapees in various areas.
Pampas grass (Cortaderia selloana) was intentionally grown for its decorative seed heads in Santa Barbara as early as 1874. Original plantations contained only female plants and without their male counterparts, set no seed. Only after later growers started propagating new plants from seed, did the males make their appearance. In short order, their offspring were invading coastal communities and have made their way all along the Pacific bluffs. There are sterile cultivars that can add drama to the garden, but do your homework before buying any.
A few more, unprepossessing groundcovers, have also wreaked their own versions of havoc in the landscape. There are two succulent species that can have a place in gardens, but if they escape are extremely detrimental. Ice plant is the common name for each of them. Mesembryanthemum crystallinum is native to South Africa, but it has spread all over California and even Baja-Norte and Sur. In areas of limited rainfall, such usurpers are a real threat to native species. Carpobrotus edulis is often planted on slopes to offer protection from erosion (mistakenly, it happens, in the case of steep slopes where its own water-logged weight can create a slide anyway). In fragile sand dune communities, it easily overruns natives. Periwinkle grows in the shade and has sweet blue flowers, but turn your back and next thing you know it is halfway up the trail and threatening to climb into the trees as well as smother the native annuals.
There are some dishonorable mentions, too. These are species that are under still investigation. In some areas, they appear to be supplanting natives and are known to seed easily and widely. They include several species of ivy and cotoneaster. A common herb, fennel (Foeniculum vulgare) is another suspect. Some species may only be a threat in specific areas. The Mexican fan palm (Washingtonia robusta), Brazilian pepper (Schinus terebinthifolia), and even the edible fig (Ficus carica) also may need to be watched carefully.
The unfortunate thing about all these plants is that they can spread widely, establish quickly and easily, and survive through the worst droughts. Some of them even resist fire (like Carpobrotus, they may have been planted to do just that), so these survivors are ideally suited to usurp natives. To help you make better choices, Cal-HIP has set up a very useful website at plantright.org to remind you which species not to plant and suggest many alternatives for your garden. Instead of giant reed, plant a clumping bamboo, for example. Bamboos bloom so seldom that they probably won’t do so in your lifetime, and the clumping varieties do not spread far or fast. In place of that messy, dangerous eucalyptus, plant a Catalina ironwood. Australian willow (Geijera parviflora) can fill in for tamarisk, yellow bush daisy (Euryops pectinatus) for broom, and Serbian bellflower (Campanula poscharskyana) for periwinkle. There are plenty of other options listed as well. Please plant responsibly