Open Letter to California Native Plant Society

A diverse garden of native and non-native plants will be the most resilient to climate change. Photo credit Marianne Willburn, Garden Rant

In a recent edition of Nature News, Jake Sigg published an announcement of a new publication and a brief description of it that was apparently written by Susan Karasoff, Outreach Chair of the Yerba Buena Chapter of the California Native Plant Society:

San Francisco Estuary Institute (SFEI) developed Making Nature’s City: A Science-based Framework for Building Urban Biodiversity, which summarizes the key indicators supporting urban biodiversity. Local native vegetation is the base of the food web for our wildlife, including our local native pollinators, our native bees, butterflies and hummingbirds.

Biodiversity is biosecurity. Biodiversity only applies to locally native plants and wildlife. Introduced, invasive and non-local native plants contribute to landscape diversity, not to biodiversity. Our pollinators eat local native plants and pollen as caterpillar food. Introduced plant leaves feed few, if any, caterpillar species. Caterpillar species feed the rest of our local food web. Healthy urban ecosystems are measured by the health of contributors to their biodiverse food web and habitat.

Native plants planted in plant communities are resilient to climate change. Introduced and non-local native plants and trees are not resilient to climate change. San Francisco benefits from native plants planted in plant communities. San Francisco Estuary Institute (SFEI)’s Hidden Nature project, done in conjunction with the Exploratorium, maps the locations of plant communities in San Francisco prior to European arrival.

Jake Sigg’s Nature News, February 18, 2022

I compared the description to the SFEI document because the description contains several counterintuitive statements.  Although the SFEI document reflects a clear preference for native plants, it does not corroborate these specific statements:

  • “Biodiversity only applies to locally native plants and wildlife. Introduced, invasive and non-local native plants contribute to landscape diversity, not to biodiversity.”
  • “Introduced plant leaves feed few, if any, caterpillar species.”
  • “Native plants planted in plant communities are resilient to climate change. Introduced and non-local native plants and trees are not resilient to climate change.”

Here are a few studies, references, and public policies that explicitly contradict these counterfactual statements.

Biodiversity is not confined to native plants

“Biodiversity is the biological variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic, species, and ecosystem level.” (Wikipedia)  The Simpson index and the Shannon-Wiener index are the two most commonly used measures of biodiversity by ecological scientists.   Neither index makes a distinction between native and non-native species.  In fact, such a distinction is difficult to make and is often hotly debated. 

In San Francisco, home of the Yerba Buena Chapter of the California Native Plant Society, public policy explicitly acknowledges that non-native species contribute to local biodiversity:  “Parks and open spaces in San Francisco include both native and non-native species, both of which can contribute to local biodiversity.” (Policy 4.1, Recreation and Open Space of San Francisco General Plan)

 Non-native plants are host to many butterflies in the Bay Area

Butterflies lay their eggs on plants called their host plants. The eggs develop through several larvae stages into caterpillars that feed on the host plant that is often confined to a particular plant genus or family.  Few butterflies are confined to a single plant species. For example, plants in the milkweed genus are the host of monarch butterflies.  There are many species within the milkweed genus and many are not native to the San Francisco Bay Area.  An introduced milkweed species, tropical milkweed, is a particular favorite of monarchs and it has the advantage of being available throughout the year, unlike native milkweed species that are dormant during winter months. Some have attributed the recent comeback of the California monarch migration to the widespread planting of tropical milkweed in residential gardens.

This article from the UC Davis Bug Squad says they plant tropical milkweed and two species of native milkweed in their experimental garden. Monarchs show a strong preference for tropical milkweed in their experimental garden: “In July, we collected 11 caterpillars from the narrowleaf [native] milkweed; we rear them to adulthood and release them into the neighborhood. But in the numbers game, the tropical milkweed [A. curassavica] won. From July through today, we have collected a whopping 43 eggs or caterpillars from A. curassavica. How many from [native] A. speciosa? Sadly, none.”

Anise swallowtail butterfly is another common butterfly species in the Bay Area that is dependent upon a non-native host plant.  Before non-native fennel was introduced to California, anise swallowtail bred only once each year. Now it is able to breed year around on non-native fennel and is therefore more plentiful than it was in pre-settlement California.

These butterfly species in the San Francisco Bay Area are not unique with respect to their need for non-native plants:  “California butterflies, for better or worse are heavily invested in the anthropic landscape [altered by humans].  About a third of all California butterfly species have been recorded either ovipositing [laying eggs] or feeding on nonnative plants.  Roughly half of the Central Valley and inland Bay Area fauna is now using nonnative host plants heavily or even exclusively.  Our urban and suburban multivoltine [multiple generations in one year] butterfly fauna is basically dependent on ‘weeds.’  We have one species, the Gulf Fritillary that can exist here only on introduced hosts.  Perhaps the commonest urban butterfly in San Francisco and the East Bay, the Red Admiral is overwhelmingly dependent on an exotic host, pellitory.  And that’s the way it is.” (1)

During the butterfly phase of life, butterflies eat pollen and nectar of many different plants, not just its host plant.  When native plant advocates eradicate important sources of food for butterflies, they aren’t helping butterflies.  For example, butterfly bush (buddleia) is as popular with butterflies as they are unpopular with native plant advocates because they aren’t native.  Butterflies don’t care if they are native because they are an important source of food. 

Butterfly bush is the host plant of Variable checkerspot butterflies. It is also an important source of nectar for butterflies and bees. It is being eradicated on public land because it is not a native plant.

Native plants are NOT more resilient to climate change

The most dangerous of the counter-factual statements by the spokesperson for the California Native Plant Society is the claim that native plants are resilient to climate change, but non-native plants are not.  That claim defies reality and it prevents us from responding effectively to climate change. 

Here are a few samples from scientific literature that contradict this inaccurate claim about native plants.  There are many others, just Google “Are native plants more resilient to climate change.” 

  • “As spring advances across the Midwest, a new study looking at blooming flowers suggests that non-native plants might outlast native plants in the region due to climate change.”
  • “Warming temperatures affect native and non-native flowering plants differently, which could change the look of local landscapes over time, according to new research.”
  • ”Whether in natural areas or in our gardens, climate change is affecting native plants. According to the Maryland Climate Summary, our temperatures are expected to increase 5⁰ F to 11⁰ F by 2100.
    1. “Higher temperatures cause native plants to experience more heat-related stress. Heat stress causes higher water demand, a situation made worse by longer droughts.
    2. “Higher atmospheric carbon dioxide (CO2) levels preferentially promote the growth of invasive plant species, decreasing the space needed to support natural areas.
    3. “Elongated growing seasons cause earlier leaf out and bloom times, which in turn affects the animal species synchronized to the life cycles of native plants, especially pollinators.”

An appeal to native plant advocates

I am publishing this article today as an open letter to the California Native Plant Society as an appeal to their leadership to make a new commitment to accuracy.  CNPS and native plant advocates enjoy a vast reservoir of positive public opinion.  However, they put their reputation in jeopardy by advocating for policies that are not consistent with reality, with science, or with public policy.  CNPS can’t distance itself from the Yerba Buena Chapter of CNPS because Susan Karasoff has made several presentations for CNPS that are available on its website.  Native plant advocates can best promote their agenda by providing accurate information.

14 thoughts on “Open Letter to California Native Plant Society”

  1. Tropical milkweed (Asclepias curassavica) is a non-native milkweed that has exploded in popularity in response to the demand for milkweed. It is simple to propagate, allowing growers to rapidly produce the plant for quick sale. The plant is also attractive, both to humans and monarchs, providing flowers and lush green foliage throughout the growing season – and that’s a problem.

    Tropical milkweed becomes a problem when planted in temperate areas where it does not die back in winter. A protozoan parasite of monarch butterflies, Ophryocystis elektroscirrha or OE for short, can travel with monarchs visiting the plants and become deposited on leaves. When caterpillars hatch and start eating the plant, they ingest the OE. High OE levels in adult monarchs have been linked to lower migration success in the eastern monarch population, as well as reductions in body mass, lifespan, mating success, and flight ability.

    Webmaster: Scientific research debunks the myth that tropical milkweed is harmful to monarchs. Leiling Tao (“Disease ecology across soil boundaries: effects of below-ground fungi on above-ground host—parasite interactions,” Proceedings of Royal Society of Britain, 282: 2015.1993.) studied monarch lifespans when they fed on a variety of milkweed species. They looked at both resistance to monarch parasite (O. elktroscirrha) infection and tolerance once infected. They found a complex interaction between species of milkweed the monarchs fed on and the amount of mycorrhizal fungi on the roots of the milkweed. But one result was clear: monarchs raised on tropical milkweed (A. curassavica) lived as long, or longer than, monarchs raised on other species of milkweed. They were less likely to be infected, and once infected, tolerated the infection well. In short, there is nothing about tropical milkweed as a host that is detrimental to monarch survival in the presence of parasites.

    When native milkweeds die back after blooming, the parasite dies along with them so that each summer’s monarch population feeds on fresh, parasite-free foliage. In contrast, tropical milkweed that remains evergreen through winter allows for OE levels to build up on the plant over time, meaning successive generations of monarch caterpillars feeding on the plant can be exposed to dangerous levels of OE.

    A monarch butterfly infected with OE may not be able to eclose from its chrysalis, or may emerge with wings it cannot extend and flatten. (Photo: Dara Satterfield, Project Monarch Health) Tropical milkweed can also interfere with monarch migration and reproduction. When grown in northern areas, where it can grow later in the year than native species, the presence of tropical milkweed may confuse monarchs into breeding at a time when they should be migrating. In California, where this milkweed is widely planted, it can be growing near overwintering sites along the coast and may spur monarchs to breed when they should be overwintering.

    Webmaster: Speculation of deformity is contradicted by an empirical study. More importantly, the suggestion that the monarch migration might be disrupted in some way by the presence of milkweed is not only speculative, it does not acknowledge that the climate has changed and will continue to change the timing of migration and breeding. We should expect the monarch to respond to the changing climate and we should celebrate the fact that it can respond because that will ensure its survival. The survival of all plant and animal species on Earth will depend on their ability to make whatever changes in their ranges and life patterns are needed to live in the changed and changing environment. The fundamental error of nativism in the natural world is that it assumes that life is immutable. Demanding that species live only where they lived in the distant past will doom many species. Nativism in the natural world has become a form of climate change denialism.

    The history of the global dispersal of monarchs should reassure us that they will survive changes in the climate. The dispersal of monarchs from their original range in North America is approximately 200 years old, according to molecular analysis of populations across the Pacific Ocean (Hawaii, Samoa, Fiji, New Caledonia, New Zealand, Australia) and across the Atlantic (Spain, Portugal, and Morocco). These dispersals are assumed to have been aided by human transportation of both milkweeds and monarchs and extreme weather events. “For example, monarchs were recorded in Australia in 1870 and were most probably carried there on cyclonic winds from a source population in New Caledonia.” (Amanda Pierce,, Proceedings of the Royal Society of Britain, 281: 2014.2230.) These populations do not migrate and are therefore genetically distinct from the ancestral population of North American monarchs as a result of genetic drift.

    In many of the homes of new populations of monarch butterflies there was no native species of milkweed before being introduced simultaneously with the monarch populations. Although there are numerous members of the milkweed family native to Australia, monarchs do not appear to utilize the native species, preferring the introduced species of milkweed.

    Monarchs and other animals know more about what they need to survive than we do. They will make better decisions for themselves than we can make for them about where they will find what they need.

    With tropical milkweed so readily available, what’s a gardener to do? Some advice has suggested plants can be cut back to the ground twice during the growing season to limit the spread of disease, and that plants should be removed late in summer so as not to interfere with migration. In practice however, we’ve found it’s been a hard-sell to get anyone to cut back plants that are actively supporting monarch eggs or caterpillars, or remove lush plants in full flower.

    Climate Change Could Make Matters Worse

    In addition to the concerns over OE and disruption of migration behavior, emerging research suggests that tropical milkweed may actually become toxic to monarch caterpillars when the plants are exposed to the warmer temperatures associated with climate change. Under these conditions, tropical milkweed produces higher cardenolide concentrations. Monarch caterpillars are tolerant of these chemicals⁠—in fact, cardenolides are the very compound that protects the monarch from predation. But when the cardenolide concentrations are high enough, not even monarch caterpillars can withstand them. In contrast, native swamp milkweed (A. incarnata) has naturally lower cardenolide levels, and when used as a control in the study mentioned above, it did not exhibit the same radical changes in toxicity as tropical milkweed.

    Another climate change consideration is that tropical milkweed is often sold as an annual in places where it is expected to die back in winter. With climate change warming our winters, tropical milkweed may begin sailing through winter in places it wouldn’t have in years past, carrying OE along with it.

    While milkweed is needed in large numbers to support and expand the monarch butterfly population, we do not recommend planting tropical milkweed, and further suggest milkweed of any species not be planted within 5–10 miles of monarch overwintering sites in California.


    1. It is notable that the author of the nativist position regarding introduced milkweed mentions “recent studies “ and other language that implies their positions are facts, but there are no citations. Not one. Yet everyone of your replies contains a citation to back up your factual statements. I’ve noticed this is usually the case. Plant Nativists continue to pass around beliefs as facts.

      1. That is an astute observation. In my long experience with native plant advocates, they use the word “science” as though it is a magical talisman that confers credibility without evidence. Science seems to be a word they do not understand.

        Take a look at the comments on this post to see an extreme example of the use of a rhetorical device that attempts to turn opinion into fact.

        I post these negative comments, hoping that they expose the empty rhetoric of nativism in the natural word. So, I thank you for noticing the difference between information and opinion.

    2. And, I meant to say, as far as a lack of citations… the original post is another example! Lots of factual statements, but no citations. Yet your rebuttal is full of them.

        1. Oh! Yes, I was meaning that the quotes you were presenting in the o.p. written by Susan Karasoff had lots of sciency sounding statements, with Zero citations, and your rebuttal was Full of citations- backing up every factual statement that you made.

          1. Thanks for the clarification. Native plant advocates are without the evidence needed to support their ideology. They share their opinions among themselves for so long that it becomes reified. They are isolated in their communities and are rarely exposed to other viewpoints, let alone scientific studies. A US Forest Service social scientist who visited UC Berkeley spent a year studying our local native plant movement. Before he left he made a presentation in which he said they are victims of incestuous amplification.

            The source of the opinions of Susan Karasoff were originally published in Jake Sigg’s Nature News. I have sent Jake Sigg every article that I have published that is contrary to articles in his Nature News, including this Open Letter to CNPS. In an email reply he said he “never reads anything” I send to him. The easiest way to maintain our beliefs is to shut off all other sources of information. That’s how ignorance thrives.

  2. Native Plant enthusiasts should spend their energies preserving native plants and their evolving progeny. This is similar to preserving artistic and cultural treasures. Just as protecting art created in the past does not benefit from discouraging artistic activity by those living in the present, preserving native plant species should not be conflated with natures expressions as Life adapts to present conditions.

  3. Thank you for this wonderful and so needed article. Already so many plants are being killed by humans expanding into areas that once homed and fed animals, including insects. And then the nativists want to deprive them of what is left, which is often “non-native” plants and trees. (Yet the nativists are rarely native themselves and often plant or buy to eat non-native plants.)

    I’m horrified to see Fennel targeted when it feeds so many animals, including birds and butterflies. In some areas, like Cesar Chavez park in Berkeley, where the Fennel has helped camouflage the vulnerable Burrowing Owls and provides food for the small animals they eat, it’s heart-breaking to see it cut down seemingly for no rational reason.

    Fear of fire is also why so many people are reducing their yards to empty wastelands, which only makes fire risk from wind more likely.

    A few days ago, I saw amazing rare and beautiful birds at San Leandro Marina/Marsh that is in a heavily sprayed area (EBRPD again) right next to the bay where the birds are searching for food in garbage. The tide was incredibly low and I saw what usually is hidden: hundreds of tires sticking up out of the mud. They are extremely toxic now and as they degrade, yet they are left to poison the bay and wetlands. Instead of nativists targeting the surviving trees and other plants to kill, why not put that energy into removed the tires? Same for the EBRPD spraying poison everywhere they can, which is not done in Marin.

  4. Often the nativists not only kill “non-native” plants but they kill with herbicides. These nativists just don’t get it and it’s like trying to discuss religion or race-people are set in their ways and can’t listen and won’t listen. So the answer is in the elections.
    With that in mind, we are circulating a petition to allow a proposal to BAN HERBICIDES in SF and get this proposal on the ballot this November. We need 8500 signatures. Will you help by signing our petition? You can find a link to the Nextdoor site and other info here:


    including where you can find a petition to sign. We also desperately need volunteers to help collect signatures (the deadline is in mid-May.)

    1. This is the website about the petition to reduce herbicide use in San Francisco’s parks:
      These are the locations where you can find the petition and sign it if you want this measure on the ballot in San Francisco. Residents of San Francisco are qualified to sign the petition:
      There will be signature collection on Saturday, March 12, at the Noe Valley Farmers’ Market, from 10:30am-12:30pm.
      The pet stores listed below kindly agreed to have our form for signing for those who would like to do so. You need to ask for it at the counter.
      The Animal Connection at 3401 Irving St. (If a person at the counter doesn’t know about the petition ask for Isabella or Jennifer.)
      The Animal Company at 4102 24th St.
      The Cal’s Pet Supply at 5050 California St.

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