Invasive Species to Know for the California Supplemental Exam
Invasive plants are non-native species that infest natural ecosystems.
Scotch Broom (Cytisus scoparius) invades California landscapes throughout the state. Photo by Schaefer Rudolf
California’s mild climate is suitable for a lot of plants. Once introduced, these exotic plants survive and spread on their own without further human help. Frequently, invasive plants do not provide any benefit to the state’s wildlife. As a result, native plants must compete with these foreign invaders and habitat is lost.
As landscape architects, we take advantage of the enormous plant palate available in the state.
Although there is a growing state-wide preference for using native species in the landscape, many adapted exotic species serve important roles in contemporary landscape architecture. The state has not banned any of the following landscape plants. However, use caution when planting near natural areas.
Top 8 Statewide Invasive Plant Species for the CSE
Fennel is extremely invasive and well-adapted to coastal and foothill sites. Photo by John Tann.
The following landscape plants are widely adapted to California’s diverse habitats and landscapes. These species can naturalize in nearly any part of the state.
- Tree-of-heaven—Ailanthus altissima
- Scotch Broom—Cytisus scoparius
- Tall fescue—Festuca arundinacea
- Fennel—Foeniculum vulgare
- English Ivy—Hedera helix
- Dalmatian toadflax—Linaria genistifolia ssp. Dalmatica
- Kentucky bluegrass—Poa pratensis
- Black Locust—Robinia pseudoacacia
Invasive Species In Coastal Landscapes
Pampasgrass is invasive and adds fuel volume for wildfires.. Photo by Macleay Grass Man/Flickr.
California’s mild coastal climate is a particularly troublesome area for invasive species. Many common exotic species of landscape plants thrive a little too well along the coast and invade native landscapes. All of these plants are available from California nurseries. Use the following plants with caution near natural areas:
- Black acacia—Acacia melanoxylon
- Capeweed—Arctotheca calendula
- Hottentot fig, iceplant—Carpobrotus edulis
- Giant dracaena—Cordyline australis
- Pampasgrass—Cortaderia selloana
- Cotoneasters—Cotoneaster spp.
- English hawthorn—Crateagus monogyna
- Portuguese broom—Cytisus striatus
- Cape ivy-Delairea odorata
- Pride-of-Madeira—Echium candicans
- Red gum—Eucalyptus camaldulensis
- Tasmanian blue gum—Eucalyptus globulus
- Edible fig—Ficus carica
- French broom—Genista monspessulana
- Licoriceplant—Helichrysum petiolare
- Yellowflag iris—Iris pseudacorus
- Ox-eye daisy—Leucanthemum vulgare
- Sweet alyssum-Lobularia maritima
- Crystalline iceplant—Mesembryanthemum crystallinum
- Myoporum—Myoporum laetum
- Olive—Olea europaea
- Red fountaingrass—Pennisetum setaceum
- Canary Island date palm—Phoenix canariensis
- Castorbean—Ricinus communis
- Peruvian peppertree—Schinus molle
- Brazilian peppertree—Schinus terebinthifolius
- Spanish broom—Spartium junceum
- Athel tamarix—Tamarix aphylla
- Big periwinkle—Vinca major
- Mexican fan palm—Washingtonia robusta
- Calla lily—Zantedeschia aethiopica
There are a few questions about invasive species on the California Supplemental Exam for landscape architects. Know the definition of an invasive species, the most noxious offenders, and the situations where plants that can invade native landscapes should not be used.
For more information on invasive horticultural plant species, check out How to Manage Pests
Pests in Gardens and Landscapes: Invasive Plants. You can find the page here.
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John is a landscape architect who is currently preparing to take the California Supplemental Exam to become licensed in California. He is currently a licensed professional landscape architect in Georgia and Florida. John graduated from California State University, Pomona with a BSLA degree in landscape architecture in 1997 and has extensive practice experience in residential and commercial landscape design.