|What are native, non-native, and invasive plants?
You may have heard the terms native, non-native, naturalized, exotic, invasive, and weed being applied to plants without being sure exactly what these mean. Yet these concepts are basic to the theory and practice of land conservation and habitat restoration as applied by a wide range of organizations and agencies. Whole organizations are built upon these concepts. It’s also important to understand them if you garden and would like to grow plants appropriate to the local area.
Native plant species are those that have been living and growing in a particular region for a very long time, often for millions of years. Generally in California, ecologists consider a plant native if it existed in a given area before European settlement.
They are ideally adapted to the local environment, including to the soils, climate, and weather conditions. Here, they are adapted to California’s Mediterranean climate with its cool, wet winters and long, dry summers. Different plants are adapted to different elevations, drier or wetter areas, even to the north or south facing slopes of hills. At the same time, they provide just the right habitat – including food or shelter – for native animals. Sometimes, they have co-evolved with particular species of insects or birds, each giving each other something that they need.
The swallowtail pipevine and the pipevine butterfly are dependent on one another. The milkweed provides food for the monarch butterfly caterpillar. The plant is poisonous and confers on the caterpillar a bitter taste, which helps it to ward off predators. Plants with tubular flowers like the California fuschia are a perfect fit for the small beak of the hummingbird, which takes precious nectar and at the same time helps pollinate the flowers.
Coast Redwoods have been growing along the northwest coast for millennia. Today, they are confined mainly to a narrow strip that roughly corresponds to the extent of coastal fog, which they rely upon for summer moisture. They are a “keystone” species, in that a whole ecosystem has grown up around them, with plants and animals interdependent in an interrelated web of connections and interactions. In the Laguna de Santa Rosa and Santa Rosa Plain, the native valley oak (Quercus lobata) is a keystone species, providing habitat for hundreds of different creatures from insects and fungi to the acorn woodpecker to deer and squirrels.
Some plants are not only native, but also endemic, meaning that they grow in one area alone and nowhere else in the world. The endangered Sonoma sunshine, Burke’s goldfields, and Sebastopol meadowfoam are endemic to the vernal pools of the Santa Rosa Plain.
Non-native (or exotic) plants are those introduced from other areas, in our case initially by European settlers 150-200 years ago, and non-native plants are showing up from escapees from the nursery trade and other sources. Sometimes they were brought intentionally, to feed cattle or grow food or as ornamentals. At other times they were transported unintentionally in ships, on boots or clothing, or in the guts, hooves, or hides of domesticated animals. Soil, containing seeds, was used to ballast trading ships, then dumped on the shores of the new land, and weeds proliferated. Some of these plants have become invasive, wreaking havoc on the natural ecosystems where they were introduced. In their own natural setting, herbivores would keep them in check, but released where there are no natural predators they can take over and crowd out native species, reducing biological diversity by reducing habitat for other native species.
An example is the eucalyptus, a native to Australia, which was introduced in 1853 as an ornamental, for fuel, and in the mistaken belief that it would provide good building timber. It didn’t, but many people grew it, and it is now ubiquitous in Sonoma County, often crowding out native plants. True, they can provide resting places for hawks and monarch butterfly roost sites as they are now often the only remaining tall trees in a given area, but the roots exude a toxin which impedes the growth of other plants, and they are extremely flammable.
Some of the most invasive plants in the upper reach of the Laguna de Santa Rosa in Cotati and Rohnert Park include pampas grass (tall, white, bushy grasses from South America), the pretty purple-blue periwinkle, originally from southern Europe and Northern Africa, English ivy, the brilliant yellow flowered French broom, and Himalayan blackberry.
There’s a native California blackberry too, which has different features. Sometimes they grow side by side, but mostly the Himalayan crowds out the native and most of the blackberries we see in urban areas are non-native.
Some non-native plants have been here so long they have become a common part of the vegetation we see in wild areas and some people regard them as “naturalized.” While it’s subjective which non-native plants are “naturalized,” naturalized plants do not, over time, become part of the native flora. Wild oats, originally brought by the Spanish with their livestock in the 1700’s, is one of the many non-native grasses that turns the hills golden in the summer. And lastly, the term “weed” is often used to refer to non-native, invasive plants. Generally, a weed is a plant, native or non-native, that simply is not wanted in the place where it is growing.
Jenny Blaker is Outreach Coordinator of Cotati Creek Critters, a local habitat restoration group fiscally sponsored by and working in partnership with the Laguna de Santa Rosa Foundation (a 501c3 nonprofit) and in collaboration with the City of Cotati and Sonoma County Water Agency.