By Laura Lovett
If you love to garden, Marin County is a great place to live. Cold freezes are rare, winters are short, low hills provide wind protection and rains fall more frequently here than inland, making hospitable growing conditions for a huge variety of plants. Those same conditions, however, make it a good host for what are known as “invasives.”
Since settlers arrived in California, we have been importing and growing plants here from all corners of the world, especially those that thrive in similar Mediterranean climate zones. The vast majority of these are well-behaved visitors. Invasive plant species are those visitors with characteristics such as fast growth and multiple methods of propagation. When combined with a lack of natural predators and diseases, these plants can take over major areas of land and water, forcing out native plants and creating monocultures. It is this ability to suffocate and replace other native vegetation—creating environmental degradation—that makes a particular plant an invasive weed.
Some invasive species were introduced with good intentions and then ran wild, but there are several that we still bring home from nurseries and plant in our garden that subsequently spread rapidly beyond them. If you live along a stream or in an area that’s foggy and damp, the potential for uncontrolled spreading is increased. While you may be careful gardener, wind, birds and other animals will help plants migrate to where they cause havoc. Below are a few of the plants that we hope are not in your garden and some suggestions on what else to plant that will grow to a similar size and form.
Cotoneaster (Cotoneaster lacteus, Cotoneaster pannosus). A very pretty shrub that produces thousands of berries each year. The problem with cotoneaster is that those seeds are widely dispersed by birds, small mammals, water movement and human activities. Cotoneaster is very competitive and has an aggressive root system that displaces native plants. It appears quickly in disturbed sites and gets established before preferred native shrubs like Coffeeberry (Frangula californica) and Toyon (heteromeles arbutifolia) that will provide far more ecological value to the habitat. Good options to use in shadier spots include Pink-flowered Currant (Ribes sanguineum var. glutinosum) and Snowberry (Symphoricarpos albus).
Carpobrotus edulis – ice plant
Highway Ice Plant (Carpobrotus edulis). This vigorous groundcover forms impenetrable mats that compete directly with native vegetation. It regenerates through seed and from small fragments of vegetation that blow or wash elsewhere. Introduced as an ornamental plant, Ice Plant now inhabits our coastal scrub, grasslands, and bluffs, and covers large areas of Point Reyes dunes and beaches. If you must have it, plant Trailing Ice Plant (Delosperma cooperi), a non-invasive (and non-native) Ice Plant with smaller succulent leaves and brilliant violet-pink flowers that’s salt tolerant and fire resistant. Native groundcover options include Beach Strawberry (Fragaria chiloensis), Seaside Daisy (Erigeron glaucus), and Dwarf Coyote Bush (Baccharis pilularis ‘Twin Peaks’ or ‘Pigeon Point’).
Pride of Madeira (Echium candicans). Very little will grow under an established Echium candicans. It is especially problematic in damp areas and along watercourses, which provide moisture that enables it to spread. It should not be planted near natural open space—new seedlings can sprout as much as 30 feet from the parent plant. Cut off the flower stalks before seeds mature to avoid this. But why would you plant something from across the Atlantic when you can enjoy a lovely native Ceanothus ‘Concha,’ Cleveland Sage (Salvia clevelandii, try varieties ‘Pozo Blue’ or ‘Winifred Gilman’), Island Bush Poppy (Dendromecon harfordii) or Flannel Bush (Fremontodendron californicum)? All of these are gorgeous shrubs in their own right.
Mexican Feather Grass (Stipa tenuissima). A feathery and attractive grass that’s easy to grow, this pest appears in gardens all over Marin. It is particularly regrettable to see it in gardens near waterways as the seeds are easily dispersed by water. Seeds are also carried by livestock, humans, and wind; they adhere to clothing and fur; and can lay dormant for more than four years. This plant is now found in all types of landscapes around the county. Recommended alternatives include Blue Grama Grass (Bouteloua gracilis ‘Blonde Ambition’), Prairie Dropseed (Sporobolus airoides), Red Fescue (Festuca rubra), and Slender Hairgrass (Deschampsia elongata).
Big stands of Pampas Grass clog the Corte Madera marsh. Photo: Laura Lovett
Pampas Grass (Cortaderia selloana). We introduced this plant to California for ornamental use before it took over many areas of the state. The fluffy plumes produce 100,000 seeds (or more) that blow away in the wind, making it nearly impossible to control. It tolerates winter frost, warmer summer temperatures, moderate drought, and produces significant amounts of extremely flammable biomass, increasing both the frequency and intensity of fire. For a substitute with the same striking stature, try Giant Sacaton (Sporobolus wrightii), which grows up to 10 feet tall and has showy, feathery seed heads in late summer. Also try Lindheimer’s Muhly Grass (Muhlenbergia lindheimeri) or Giant Wild Rye (Elymus condensatus).
Giant Reed Grass (Arundo donax). This bamboo-like tall grass is a serious problem in damp areas and along streams. Its dense growth crowds out native plants, damages habitat, and creates a fire and flood hazard while providing little shade for fish populations in the stream. Giant Reed threatens riparian ecosystems by modifying the hydrology of the river, retaining sediment, and constricting flow. Alternatives include Lindheimer’s Muhly Grass (Muhlenbergia lindheimeri), Deer Grass (Muhlenbergia rigens), Arroyo Willow (Salix lasiolepis), and Giant Wild Rye (Elymus condensatus).
Running Bamboo and English Ivy will both spread far beyond where they were planted. Photo: Laura Lovett
Running Bamboo (usually Phyllostachys species but also Pseudosasa, Chimonobambusa, Arundinaria, Semiarundinaria, etc.). Technically a giant grass, Running Bamboo is one of the world’s most invasive plants. Once established, it is literally next to impossible to control. Many homeowners plant bamboo to create a fast-growing privacy screen. Before you do, however, keep in mind it will not stay on your property but will invade much of your neighbor’s as well. Bamboo grows particularly vigorously when near irrigated lawns and gardens. Bamboo barriers eventually break. Trust me, mine did! It took 30 hours of work sifting the soil to remove every shred of root that could sprout a new plant. If you already have it on your property, use a backhoe to remove as much root and soil as possible, then comb through the remainder for root fragments.
Better options to plant include Island Mountain Mahogany (Cercocarpus alnifolius) which makes an evergreen shrub 4 to 6 feet wide and 12 feet tall; Holly-leaf Cherry (Prunus ilicifolia), also evergreen with white flowers and red fruit in fall that birds love; or California Cypress (Cupressus goveniana), very drought tolerant, to 15 feet tall and evergreen.
Woodland Forget-me-not (Myosotis sylvatica). A native of North Africa, this charming little plant grows like wildfire in shady, moist areas and in fields, meadows, woodlands and coastal forests. Be sure to pull or hoe the plants before they go to seed as they spread by seeds and by roots at the leaf nodes. Plants will soon re-sprout if you don’t remove all of the roots. Good replacement choices include the very similar native Woodland Phacelia (Phacelia bolanderi), as well as Western Columbine (Aquilegia formosa), Creeping Snowberry (Symphoricarpos mollis), and Candy Flower (Claytonia siberica).
English Ivy (Hedera helix). This European import shows up along roadways, on the coast, and outcompetes almost everything in California’s forests as well. Ivy can smother understory vegetation, kill trees, and inhibit regeneration of understory plants, including new trees and shrubs. Replace it with any native groundcover and you’ll immediately increase the biodiversity of your garden. Excellent choices include groundcover varieties of ceanothus like ‘Centennial,’ ‘Anchor Bay’ or ‘Carmel Creeper,’ Bearberry (Arctostaphylos uva-ursi), Beach Strawberry (Fragaria chiloensis), and Yerba Buena (Satureja douglasii).
Periwinkle (Vinca major). Periwinkle has trailing stems that root wherever they touch the soil. Riparian zones are particularly sensitive. Fragments of periwinkle vines can break and wash downstream, allowing it to spread rapidly in shady creeks and drainages where it smothers the native plant community. Any of the alternatives suggested for English ivy will grow where periwinkle has been removed. If you have a sunny site, try Bee’s Bliss Sage (Salvia ‘Bee’s Bliss’) or Seaside Daisy (Erigeron glaucus).
Native plants may not seem showy like your favorite roses or hydrangeas, but they are essential to our natural world. These plants have had millennia to adapt to our region’s local climate and soils, co-evolving with insects, fungi, and microbes to form complex relationships that create the foundation of our ecosystem. Imported invasives that push these plants out create “green deserts” that do not provide any resources for pollinators or food for birds or wildlife. State agencies, parks and regional groups spend more than $82 million a year to control them. We encourage you to help this effort to conserve our natural biodiversity by making thoughtful choices for your home garden. See calscape.org for a handy tool to help you plan.