Most Napa Valley gardeners have long embraced the idea that planting native species is the way to go. These locally evolved species tend to be more acclimated to our long, dry summers and less thirsty as a result.
But there is growing evidence that native plants do far more than just save water. A study by scientists at the University of Delaware and the Smithsonian Migratory Bird Center, recently published in the journal Biological Conservation, underscores another way that native plants can support local ecosystems in urban and suburban settings.
The study was conducted over a four-year span. The observation team identified where breeding birds foraged for food in the yards of 203 homes in the Washington, D.C., metropolitan area. They documented which plants provided the most bird food, such as insects and caterpillars.
Among the findings were that native plants provided more caterpillars than non-native plants, that the birds preferred foraging in native plants and were more likely to breed in sites with native plants.
While these findings may be limited to the specific parameters of this study, the idea that the natural world is deeply interconnected comes as no surprise. We now understand that most insect species evolved over time with a specific plant species or group of plants. The insects adapted to the chemical defenses of its host plants.
Often these adaptations are very precise. Caterpillars, an important food source for birds, tend to be picky about what they eat. For instance, the monarch butterfly larva must have milkweed (Asclepias fascicularis) to survive. Most insects and animals avoid milkweed because of the toxins and latex it secretes, but the monarch has adapted to it. In fact, the monarch will lay its eggs only on milkweed. Without milkweed along its migratory path, the monarch cannot reproduce and the creatures that feed on monarch caterpillars must look elsewhere for a meal.
Last year, the Audubon Society launched the Plants for Birds program, which encourages and provides resources for people to support birds by planting native species (http://www.audubon.org/plantsforbirds). The site also provides a database by ZIP code that helps to identify local native plants and the types of birds they tend to attract (https://www.audubon.org/native-plants).
According to the Audubon Society, the human-dominated landscape “no longer supports functioning ecosystems, and the remaining isolated natural areas are not large enough to support wildlife.” This situation is the result not just of urbanization and expanded agricultural production, but also of our desire for manicured lawns and exotic ornamental plants. Research by the entomologist Doug Tallamy has shown that native oak trees support more than 500 species of caterpillars, whereas the gingko, the commonly planted and beautiful landscape tree from Asia, hosts only five species of caterpillars.
ReScape California, also known as the Bay-Friendly Landscaping & Gardening Coalition, is a resource for those who want to learn more about incorporating biodiversity into their landscape. Among its guiding principles are water and energy conservation, improving the “soil food web” and reducing the landscaping impact on landfill.
In this organization’s view, maintaining a Bay-friendly landscape means recognizing that “biodiversity is crucial to the health of natural ecosystems and that by using native plants and increasing the diversity of plant palettes, our built landscape can provide food, water and shelter for birds, butterflies, beneficial insects and other creatures.”
According to U.S. Census data, there were nearly 66,000 people living in Napa County in 1960. By 2010, the county’s population had more than doubled. That increase mirrored the growth of the Bay Area as a whole, which almost doubled in population in those five decades. Our ancestors needed space to live, just as we do, and so will those who come after us.
I fuss over my lilacs because they demand it and because I love them. But the fact is, they contribute almost nothing to everything else that lives in my yard, or might want to live here. Nothing ever eats them. The same goes for the Australian tea tree, even though it’s never thirsty.
I’m not going to pull these plants out, not yet anyway. But the next time I have the chance, I’m going to think about the birds and the bees and the caterpillars. After all, I’m not the only one who lives here.
Workshop: U. C. Master Gardeners of Napa County will host a workshop on “Rose Pruning” on Saturday, Jan. 13, from 10 a.m. to noon, at the UC Cooperative Extension, 1710 Soscol Ave., Napa. Why do we prune roses? Is winter the only time to do it? What will happen if we don’t? Should hybrid teas be pruned differently from floribundas?
Join the Master Gardener Rose Team at this popular forum where resident experts will answer your questions about basic rose pruning fundamentals with research-based information. Topics include rose types, how and when to prune, what tools to use and how to care for them, safety and sanitation. Each Master Gardener will have some suggestions for new plantings, too. Online registration (credit card only); mail-in/walk-in registration (check only or drop off cash payment).