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What to Know About Monarch Butterflies and The Milkweed in Your Garden

What to Know About Monarch Butterflies and The Milkweed in Your Garden
Resource: Joshua Siskin - OC Register
Published: August 20, 2022

In 2020, around Thanksgiving, a total of fewer than 2,000 monarch butterflies were counted at around 250 overwintering sites where they have traditionally been counted in the state of California. A year later, nearly 250,000 overwintering monarchs were counted at those same sites. I was curious about this change and wondered if more milkweeds, which have been increasingly planted by gardeners in recent years, could have anything to do with this 1,000 percent increase in monarch numbers. Milkweed is the only plant where monarchs lay their eggs.

In search of a solution to this conundrum, I turned to Ron Vanderhoff, general manager of Roger’s Gardens in Corona del Mar. Vanderhoff is deeply involved in monarch conservation efforts and personally counts monarchs in their various overwintering sites in Orange County.

“Monarch populations can vary wildly from one year to another,” he wrote, “due to various factors, nearly all of which cannot be confirmed as determinative in the monarch numbers we see from year to year. Populations of almost all short-lived annual insect species are accustomed to wild fluctuations in numbers which are an adaptation to changing conditions in their environment. If food is abundant, weather is ideal, competition is minimal, predators are reduced or some other ‘advantage’ is present, these critters are wired to increase their populations quickly in order to exploit that immediate opportunity.

“Conversely, if food is scarce, drought is significant, there are lots of predators or competitors, or a debilitating disease is present, the species will conserve its energy and decrease in abundance. It’s sort of nature’s way of bringing insect or other animal populations into balance with their environment. Birds of prey are excellent examples of this and will lay more eggs when there are lots of mice and less when there are fewer. The last couple of years, because of the drought, there have been fewer plants and flowers growing in our wildlands. That means fewer pollinating insects and, with less pollination, fewer seeds are produced. Since seeds form a large part of a mouse’s diet, fewer seeds mean fewer mice and other small animals for the hawks and falcons and eagles to eat. So these birds have not been laying very many eggs. Similarly, tuna will breed more prolifically when there are lots of anchovies, grasshoppers will go crazy when plants to their liking are abundantly available, and so on.

“What all this means is that populations of animals, especially those with a short life cycle, will fluctuate sometimes wildly. Researchers often cannot pinpoint precisely what the reasons are for these fluctuations since there are usually many factors at play at once. But scientists and ecologists do not put too much credence in irregular spikes and dips in populations, paying more attention to the longer-term trends which paint a better picture of how the animal is doing. In this context, the conservation and research community is pleased with last year’s monarch counts, but warns not to get too optimistic – it’s just one year.

“That being said, 250,000 monarchs in California are a whole lot better than 2,000 monarchs. Let’s just hope this trend holds and it’s not a one- or two-year blip. I do think that the recent commitment to planting native milkweed in our urban gardens is at least a contributing factor to these better monarch numbers although, of course, I can’t prove this. Roger’s Gardens shoppers alone added well over 7,000 native milkweeds to our urban landscape last year, and a similar number will probably go in this year. Tree of Life Nursery in San Juan Capistrano is another large contributor of milkweed to urban gardens and supplied a similar amount. While I don’t think anyone else came close to the two of us in terms of milkweed numbers, that’s still about 15,000 more native milkweed plants that attract monarchs and I have to believe all these additional plants are helping in some way.

“While you may have heard that the monarch butterfly was elevated to endangered species status last month, this declaration was made by the International Union for Conservation of Nature, which has limited influence. In the United States, it is the U.S. Fish and Wildlife Service that designates a species as endangered. When this happens, funds are made available for protection, research and conservation. However, the USFWS has not yet added the monarch butterfly to its endangered species list and we are pushing for this to happen soon.”

What to Know About Monarch Butterflies and The Milkweed in Your Garden

It is important to remember that while native milkweed species are good for monarchs, imported tropical species are not. Still, some nurseries carry tropical milkweed and exclude native milkweed from their stock of plants. This is probably due to the fact that tropical milkweed is evergreen with bright orange and yellow flowers, while native milkweed does not flower nearly as brilliantly and also goes dormant in the fall, looking completely dead, factors that make it less appealing as nursery fare. (In the garden, if you cut back the dead milkweed stems, it’s a good idea to leave the bottom six inches as a marker so you do not inadvertently dig in that area; new shoots will start growing up from the roots the following spring.)

I asked Vanderhoff about the tropical milkweed problem.

“Most of the scientific research is in agreement that tropical milkweed does indeed interfere with monarch migration habits,” Vanderhoff noted. “This is due to the tropical species’ propensity to offer forage (leaves) all year, including the winter months.

“The historical ecology of the western monarch butterfly is to migrate to the immediate coast of California in the fall as an adult, not breed for the next three or four months, and then leave again in the late winter to begin a new migration, primarily in search of mates and milkweed.

“This strategy has evolved over a long period of time and is reinforced by a historical absence of milkweed in coastal California during the monarchs’ winter stay as well as during the beginning of its migration the following year. All of our native milkweeds are summer growers and are completely leafless and dormant from fall through early spring. Edible foliage starts showing up again on our native milkweed species no sooner than April and often not until May.

“No food plant for monarch larvae means there is no reason for monarchs to stay in the area and lay their eggs; hence, they migrate. However, when a larval food plant (tropical milkweed) is available year-round, including in the winter and early spring months, the monarchs continue breeding and laying eggs all through the year and have no need to migrate.

“Migration promotes insect fitness (migrating butterflies are much stronger than sedentary ones), genetic diversity in the population (the foundation of adaptation to changing conditions), and resilience to catastrophic environmental pressures (local disease outbreaks, wildfires, droughts, predators). Tropical milkweed, in preventing migration, is an example of an ‘ecological trap,’ a poor-quality habitat which, when available, may attract an animal over a high-quality habitat.”

Vanderhoff also mentioned a protozoan parasite (OE) that lives on tropical milkweed and is consumed by monarch larvae (caterpillars). An OE spore carried by an adult monarch may rub off onto an eggshell or leaf on a tropical milkweed. If a monarch larva consumes that eggshell (and the larva does often eat the eggshell that covered it before it hatched) or leaf, it will become infected with OE. The adult that this larva eventually becomes will have a shorter lifespan than a healthy monarch, be impaired in its flying capacity, and spread OE within the local monarch population.

Most California native nurseries carry native milkweed plants and seeds and, of course, you can always acquire the seeds, at least, from online vendors.

The monarch butterfly is thought to have been named in honor of a monarch, the English King Wiliam III (1650-1702). It could only have been named for an English king after the colonization of America began since monarch butterflies are not indigenous to Europe, with one species native to North America, one to South America, and one to the Caribbean. Incidentally, King William was also known as William of Orange so there was that color’s connection to the royal butterfly naming, too.

Narrowleaf milkweed (Asclepias fasciculatus) is the milkweed of choice for the greater Los Angeles area and everywhere else in California, for that matter. It is simply the best host plant for monarch butterflies. However, it is also visited by painted lady, dogface, and Acmon blue butterflies, as well as by tiger moths. Although narrowleaf milkweed will grow in some shade, it flowers more abundantly in the sun. It has a reputation for tolerating a wider range of garden conditions than other native milkweeds and has been found growing in both clay and saline soils. Narrowleaf milkweed handles drought well and, according to advice offered at, the best website for detailed information on California natives, we should “mulch heavily or, better yet, plant next to a boulder, water well the first month, and ignore.”

Although narrowleaf milkweed has its charms, it may look drab in the eyes of some. Surround it with natives whose vivid flowers are also rich in nectar and so they will help attract monarchs, too. A list of these plants is provided by the California Native Plant Society at and includes: Indian mallow (Abutilon palmeri), Ceanothus spp., California fuchsia (Epilobium canum), buckwheat (Eriogonum spp.), mint (Monardella spp.), monkeyflower (Mimulus spp.), Penstemon spp., sages (Salvia spp.), and apricot mallow (Sphaeralcea ambigua). The following non-native plants, as recommended by Roger’s Gardens, will also pull monarchs into your garden: lantana, butterfly bush (Budleja davidii), yarrow (Achillea millefolium), and moss verbena (Verbena tenuisecta).