Where invasive plants are sold in California — and how we know

October 7, 2016
Survey volunteer with pampas grass

This past spring, they were all over California. With clipboards, cameras, and data sheets in hand, they were searching at a nursery or garden center near you — for invasive plants.

The nursery survey is how we measure what is happening in California’s retail nursery industry, and none of it would happen without our survey volunteers. This year 175 volunteers(many of them UC Master Gardeners) were trained by PlantRight to survey for invasive plants at retail locations throughout the state.

“Helping with the survey gives the UC Master Gardeners not only a chance to further their knowledge of invasive plants, but the opportunity to work with wonderful people at PlantRight. We find most nursery managers want to know more about the PlantRight program and are eager to get the helpful information on alternative plants which we are able to leave with them to help them educate their customers about growing choices.”

— Linda Haque, Ventura County Master Gardener and survey volunteer

New volunteers watch a webinar that discusses invasive plants, how to conduct the survey, and includes a guest speaker each year. This year’s webinar included a presentation by Dan Gluesenkamp from the California Native Plant Society about gardening with California natives and Calscape, their tool to help people find native plants for their garden. Returning volunteers can watch a refresher video but are welcome to join the webinar. After demonstrating their newfound knowledge and completing a quiz, volunteers have access to survey materials and are able to claim their survey store (or multiple stores).

This year thanks to extra outreach efforts and dedicated volunteers, we were able to get more data than ever before — from 302 nurseries compared to 265 nurseries in 2015. One of those volunteers was Melanie Keeley, a board member of the Alta Peak Chapter of the California Native Plant Society. Melanie surveyed 7 stores in Kings and Tulare counties where we were unable to get data in past years.

“As a board member of the Alta Peak Chapter of CNPS, I receive a variety of correspondences and requests. This spring, I had an interesting proposal from an organization called PlantRight. From taking their informative training, I was shocked to learn that “In California, the percentage of invasions of horticultural origins is 48% (Bell et al. 2007).” Considering almost half of the state’s invasions are horticultural introductions, it is pretty obvious that rather than being a malevolent conspiracy, the cause is simply a lack of information and awareness, which has become PlantRight’s compelling mandate.”

— Melanie Keeley, CNPS Board Member and 2016 survey volunteer

Information volunteers gather is vital to our program for three main purposes:

1. Tracking the sale of invasive plants currently on our list, and retiring those that have been largely phased out of retail.

The first PlantRight invasive plant list was developed using the California Invasive Plant Council (Cal-IPC) Inventory as a starting point and included 19 plants, focusing on the highest priority invasive plants still sold in garden centers.

“At the time that many of these invaders were introduced decades ago, they were considered novel, gorgeous, functional plants for the landscape. Back then, nobody imagined the costly damage that invasive plants would cause. Today we understand more clearly the adverse impacts invasive plants have wrought on wild lands, waterways, native species as well as on the economy. Weed science has also developed to better identify and further predict potential explosive plant species’ population growth.”

— Melanie Keeley

After several years of working to phase out invasive plants from the industry, we decided in 2012 that the list needed an update. As plants from the original list are no longer sold, they are “retired.” To retire a plant, it must be found at less than 1% of nurseries surveyed for 3 years in a row. This doesn’t mean that they are gone from our landscapes. Plants like Scotch broom (Cystisus scoparius) and arundo (Arundo donax) have already invaded wildlands in California, and there’s still much work to be done to control their spread. Since our invasive plant list was first published in 2006, 15 plants have been retired, which is excellent progress. You can help with these retired plants by removing them if they are on your property or volunteering with groups that remove invasive plants.

There are 7 plants currently on PlantRight’s list of invasive plants. The 2016 survey results for these 7 plants can be found in our Fact Sheet and Full Report. Four of these plants were part of the original list of 19: green fountain grass (Pennisetum setaceum),pampas grass (Cortaderia selloana), highway iceplant (Carpobrotus edulis), and big-leaf periwinkle (Vinca major).

Three plants were added in 2014: Mexican feathergrass (Stipa/Nassella tenuissima), yellow flag iris (Iris pseudacorus), and water hyacinth (Eichhornia crassipes). We’ll go into that process next.

Mexican feathergrass (Stipa/Nassella tenuissima)
Rows of Mexican feathergrass (Stipa/Nassella tenuissima) at a nursery. While green during the wetter part of the year, this grass will turn golden brown at the top as its seeds ripen — its many, many seeds. Each plant can produce thousands of seeds, which are dispersed by wind, water, contaminated soil, automobiles, and animals. Photo by Jan Tilford.

2. Helping us determine whether or not to add candidate plants to the plant list.

In late 2012, PlantRight recruited several horticultural experts (all volunteers, from diverse sectors of industry, academia and conservation) to establish our Plant List Committee. These individuals engage in the complex, involved, and time-intensive process of working with PlantRight to decide which surveyed plants to add to our invasive plant list and which non-invasive alternatives we should suggest. Each fall, PlantRight solicits input from our Plant List Committee and an extended network of experts to see what plants might be hopping the garden fence and escaping into wildlands.

Once we have a list of possible candidate plants, we evaluate them using the Plant Risk Evaluator (PRE) and look at whether or not they are included in the Cal-IPC Inventory. Together with the Plant List Committee, we decide which candidate plants to include in the nursery survey. This year we surveyed for 8 candidate plants.

Once the survey data is collected by our volunteers, checked by PlantRight staff and a botanist, and analyzed by Learning for Action, it’s ready to present to the Plant List Committee and then the Steering Committee. We share the results at our annual Steering Committee meeting, which took place this year on August 4th at the UC Botanical Garden at Berkeley. Here are the survey results and outcomes for candidate plants this year:

2016 Nursery Survey Candidate Plants

To appear on PlantRight’s invasive list, a plant must meet the following criteria:

  1. When evaluated by PlantRight’s Plant Risk Evaluator (PRE), it rates as being “high risk” or is on the Cal-IPC Inventory
  2. It is found in 3% or more retailers surveyed in our annual Spring Nursery Survey
  3. Its invasive range is large enough to warrant action, and its inclusion is supported by both our Plant List Committee and Steering Committee

If we were to add a new invasive to the list, we would work with our Plant List Committee to develop a list of non-invasive alternatives, inform our retail nursery partners and the industry through the California Association of Nurseries and Garden Centers (CANGC), and determine a phase-out timeline based upon input from our partners, and our Plant List Committee.

Why were no plants added to the invasive plant list this year?

English holly (Ilex aquifolium)
An English holly (Ilex aquifolium) cultivar with variegated leaves. Photo by Mary Gish.

Only two candidate plants this year met the first two criteria — English holly (Ilex aquifolium) and Canary Island date palm (Phoenix canariensis). Thankfully, the other 6 plants were found at less than 3% of retail nurseries.

English holly was found at 4.3% of stores, but nearly all the plants found were cultivars (a cultivated variety) and not the straight species, which is what’s on the Cal-IPC Inventory. Some of these cultivars may be sterile and aren’t what has invaded wildlands. For many California gardeners there are better options, such as toyon (Heteromeles arbutifolia) for those who want a plant with red berries and it’s not a popular plant throughout the state. For these reasons, it was not deemed a high enough priority for addition to the plant list.

Canary Island date palm (Phoenix canariensis)
A young Canary Island date palm (Phoenix canariensis). Photo by Arthur Tilford.

Canary Island date palm has a long history in California, and commands a high price for large specimens. Although it has invaded riparian areas in Southern California, it’s not invasive in large geographic areas of the state, and has a Limited rating on the Cal-IPC Inventory, which means it is invasive, but the impact is minor on a statewide level. It’s a plant with cultural and architectural significance, and a lack of readily available alternatives that are as hardy or grow at a similar rate. There’s also a disease killing these palms called fusarium wilt. It often spreads when rows of the trees are trimmed at once, transferring the disease from one plant to another. It can also enter through the roots — surviving in soil for at least 25 years.

We aim to keep the plant list manageable (<20 plants) and focused on the highest priority invasive ornamental plants. As Steering Committee member and Senior Botanist at the California Department of Food and Agriculture (CDFA) Dean Kelch said, “We need to choose our battles carefully.”

Although we aren’t adding any new plants to the list or retiring any plants for 2017, we will be pulling back the curtain on our plant considerations over the years and sharing more information about these candidate plants. Look for this information and more on our new website (coming in 2017). Visitors to will learn why the plant wasn’t added to the list and the particular habitats or regions of California where the plant is invasive and should be avoided. We look forward to making this available and becoming an even more robust source of information about ornamental invasive plants in California.

There’s one more way PlantRight uses the survey information, helping us use our energy and time strategically.

Deal Kelch
Dean Kelch in a photo that was included in a presentation during the 2016 Steering Committee meeting.

3. Informing our industry outreach strategy with information about sales in different regions and store types.

We have worked across the nursery supply chain to raise awareness about invasive plant issues and benefits of planting right. In recent years we’ve been recruiting retail nursery partners. These partners commit to sell exclusively non-invasive plants — you won’t find any plants on our plant list at a retail nursery partner’s store. Project Associate Christina Kohler leads our outreach to nurseries, and explains how the survey informs our work.

“The annual spring nursery survey allows our team to focus outreach efforts in high impact areas. California is very large state, and knowing where to prioritize is key. Many thanks to the countless hours of work the survey volunteers put in to making the survey a success!”

Partnerships with big box stores have measurably decreased the sale of invasive plants at that store type. The Home Depot California became a partner in 2015, and the sale of invasive plants at big box stores decreased from 43% in 2014 to 16% in 2015. Lowe’s and OSH in California joined in 2016 and the percentage of invasive plants sold at big-box stores decreased once again to 6% in 2016. These stores also sell a large volume of plants, so partnering with them affects sales throughout the state and their leadership as problem solvers on this issue cannot be overstated.

We appreciate our independent and small chain partners just as much. So many are educational hubs on planting right in their community, so it’s a natural fit. We will continue our outreach to nursery and landscape professionals and consumers to keep the momentum going, so that year by year, the sale of invasive plants in California will decrease. You can help by sharing what you’ve just read and our Plant List, by shopping at our retail partner locations, and speaking to someone at your favorite nursery about why planting right matters to you.

Monkeyflower growing in a container garden in San Francisco.

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