Research

PlantRight is guided by the California Horticultural Invasives Prevention (Cal-HIP) Steering Committee, which uses objective, scientific criteria and direct observations to make recommendations about invasive plants. When there are unanswered questions about a specific invasive plant, Cal-HIP has partnered with scientists and/or launched investigations to better understand the plant's potential to invade California's wildlands.

This careful consideration assures gardeners and businesses alike that the recommendations from PlantRight are based on credible research and thoughtful analysis.

Each invasive plant undergoing research has a unique, unanswered question about its biology or invasiveness. Read below for updates on research that provides scientific information on the following plants:

• We know that some forms of pampas grass (Cortaderia jubata and C. selloana) have invaded natural areas, especially along the coast. Research by UC Davis determined that many forms of pampas grass have spread to the wild and even “sterile” cultivars can sometimes produce seed.
• A familiar wildland invader is a form of fennel (Foeniculum vulgare). Cal-HIP members investigated the commercial production of fennel seed (a culinary spice) and fennel bulb (for eating) and found that these crops are not the source of the invasions: plants grown for seed are raised in isolated plots to guarantee purity, and all of the seeds are harvested before they can disperse. In 2007, the California State Herbarium examined fennel collected from invasive populations around the state for Cal-HIP but could not determine conclusively which cultivar(s) are in the wild. However, we believe that all or most invasive fennel is spreading from existing wild populations, not from home gardens, and therefore it is safe for planting.
• Some form of ivy is invasive in many natural areas across the state. But there are three species of ivy: Algerian ivy (Hedera canariensis), English ivy (H. helix), and Irish ivy (H. hibernica). English ivy alone has hundred of varieties that are for sale. Ivy can be impossible to correctly identify in the wild, even to the trained eye – and no one knows which type of ivy is the one invading natural areas! It could be that only a few species/cultivars are invasive while others are safe for planting. We hope that future research will discover which plants are invasive, in hopes that we can discover the bad players and still use the benign forms in our gardens and landscaping. Please consider using one of the featured alternatives to ivy if you live along the coast or near inland waterways.

Questions remain on some species. For some plants, we need more information on the extent of their invasions. We know that others are invasive in certain areas of the state and are working with the horticultural industry to examine the level of their importance to the industry and to determine appropriate alternatives. Washington palm (Washingtonia robusta) and Brazilian peppertree (Schinus terebinthifolia) are invasive along waterways in Southern California but we are uncertain of the impact to the industry of recommending against them. Licorice plant (Helichrysum petiolare) will require mapping of the invasive populations and sampling to assess the existence or absence of a connection to horticultural varieties. Cal-HIP would also like to determine which species or varieties of cotoneasters (Cotoneaster franchetii, lacteus, and pannosa) are responsible for the invasions of cotoneaster in natural areas, and if they are a significant part of the horticultural trade.

Research is in progress to examine sweet broom (Genista racemosa or Cytisus spachianus), which was developed as a sterile hybrid and might provide a safe alternative to invasive Scotch or French broom. Questions on sweet broom include whether it is truly sterile and whether plants labeled as sweet broom in nurseries are truly this hybrid rather than the invasive species.

Didn't find what you were looking for? Try reading our Frequently Asked Questions. You can also find more sources of invasive plant research in our library of peer-reviewed articles. If you still have questions, please contact us.