Open Letter to California Native Plant Society






Jake Sigg’s Nature News, February 18, 2022




Biodiversity is not confined to native plants










Native plants are NOT more resilient to climate change




An appeal to native plant advocates


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.


    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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