Thistle Lovers All: The Cobwebby Thistle as Habitat

By: Charlotte Torgovitsky

Charlotte Torgovitsky is a naturalist and amateur lepidopterist who loves to garden, both at home in Novato, California, and at…

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Early spring foliage of cobwebby thistle (Cirsium occidentale). Photographs by Phil Van Soelen

Early spring foliage of cobwebby thistle (Cirsium occidentale). Photographs by Phil Van Soelen

The beautiful cobwebby thistle (Cirsium occidentale) is nearing the end of its life cycle in my Novato garden. With the approaching heat of summer, the spiny involucre is splayed open, offering up an abundance of soft, downy chaff. It’s almost June, and I watch as the goldfinches gather in my sunny borders, flying off with beaks full of thistle chaff, a perfect soft and silky lining for their tiny open-cup nests.

The American goldfinch (Carduelis tristis), the “sad thistle eater,” apparently in reference to its song, is obviously fond of all that the thistles offer. Goldfinches breed late in the season, their reproductive strategy timed to coincide with the thistle’s life cycle. Unlike most songbirds, goldfinch nestlings are raised on seeds, not insects.

The female builds her tiny nest, sometimes in a bushy plant, often at a fork of several upright branches within a shrub. Goldfinches pair up for the breeding season; the mated pair learns special shared notes to keep their bond strong. They are loosely colonial when nesting, but flock, roost, and forage together at all other times, utilizing contact calls to keep the group together.

The female incubates the eggs while her mate feeds her, returning again to the stand of thistles in the garden. Nutrient-rich seeds are on offer, though tightly embedded in the receptacle. The goldfinch’s conical bill is the perfect tool for removing the seed. Many are eaten on the spot, but a number of them are scattered, providing forage for ground-feeding birds. Various sparrows plant next year’s drifts for me by their typical double-scratch foraging technique. Those that happen to land near a drip emitter may germinate in summer, but most seedlings appear after the rainy season has begun.

The seeds are not easy for me to extract. I work a few heads with a tweezer, my container at the ready for the smooth brown achenes. I will take just a sampling of seed to grow and share with friends, leaving the majority of seeds for the goldfinches.

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A hummingbird feeds on the flowers of cobwebby thistle (Cirsium occidentale)

I have seen this California native thistle only occasionally in the wild, growing in the back dunes on the Point Reyes Peninsula and, once or twice, on Mt Tamalpais. Thankfully, it is adaptable to garden culture, as long as the drainage is good. Containers can also provide ideal conditions. Full sun is required for vigorous growth.

A mature specimen will be multi-branched and attain a height and breadth of about two to three feet. The foliage is crisp and prickly, leaves pale green underneath and distinctly white on the upper surfaces. Pink disk flowers are held within spiny bracts, all laced with a tracery of white, woolly “webbing.” The plants behave as annuals or biennials, flowering over a long period. Fresh buds appear even as the older flowers are fading, releasing their bounty of seed and chaff.

In my south-facing sloping borders, cobwebby thistles mingle beautifully with the electric blue flowers of foothill penstemon (Penstemon heterophyllus) and the ubiquitous, in my garden at least, California poppies (Eschscholtzia californica). Who knows where they will show up next year? I love the “amiable disorder” that these volunteers create and generally let them grow where they choose, with just a minimum of editing.

My gardens are totally organic and pesticide free, designed to enhance the habitat and add diversity to the oak woodlands that surround my home. The majority of the plantings are California natives, many selected specifically because they provide for the wildlife (insects included) that shares our land. I am entranced by these wild visitors, and find I spend as much time in my garden with binoculars and a lens as I do with my fork and trowel.

Thistle seed can simply be sown in containers in late summer, guarded from seed predators by a gossamer layer of frost cloth, and left outdoors in a protected place through the rainy season. Pre-germination techniques can help speed the process: soak the seed overnight in warm water, mix into a moist soilless medium, and stratify in the refrigerator. Germination occurs early in spring, and vigorous seedlings can be planted out whenever the soils are workable.

A close-up view of a flower head of cobwebby thistle (Cirsium occidentale) shows clearly the reason for the common name

A close-up view of a flower head of cobwebby thistle (Cirsium occidentale) shows clearly the reason for the common name

The Interconnectedness of Life

Last year, I paid particular attention to the remarkable series of relationships that center on my patch of cobwebby thistle. As soon as the seedlings were of a certain size, Argentine ants started to excavate at the root zone, piling fine, light, and loose soil at the crown. The ants then brought in aphids, a greenish type, protecting and tending them for their honeydew. Aphid colonies can expand quickly, utilizing the tender new growth; if they become too unsightly I simply dislodge a number of them with a gloved hand or a strong spray of water.

The aphid colony attracts a host of other insects. Ladybird beetles come in to hunt, and soon their neat rows of orange eggs are evident on the plants. Before long every stage of their life cycle—eggs, larvae, pupae, and adult—are present on the thistles, and many other plants in the garden as well.

Green lacewings lay their eggs in clusters, each one at the end of a threadlike stalk. The adults and their larvae, known as “aphid lions,” are voracious predators; the aphid colony provides amply. Syrphid flies also visit, the adults feeding on pollen and nectar and laying their eggs where their predaceous larvae can find plenty to eat; they pupate in the mulch at the base of the plants.

Aphid “mummies” appear amongst the tightly packed colonies, evidence that parasitoid wasps are present. A minute braconid wasp lays her egg inside an aphid, and both larval and pupal stages occur there, the body of the aphid providing all the food and protection needed. The mummies are of a different color than their living sisters; close inspection reveals a hollow case with a circular hole from which the adult stage of the braconid wasp has emerged.

All this insect life attracts insectivorous birds. Ruby-crowned kinglets hover at the foliage, picking off tiny insects. Flocks of bushtits twitter through the border, carefully working through the stand of thistles as they glean insects from leaves and buds.

Once the buds open, bold pink disk flowers produce enough nectar for Anna’s hummingbirds to visit regularly. No doubt the abundance of small insects adds to the attraction; hummers regularly feed on arthropods close to nectar sources.

A pipevine swallowtail butterfly rests on the broad head of cobwebby thistle (Cirsium occidentale) as it feeds on nectar

A pipevine swallowtail butterfly rests on the broad head of cobwebby thistle (Cirsium occidentale) as it feeds on nectar

Butterflies, Too

Various butterflies, as well as bees and bumblebees, visit for nectar. Painted lady butterflies (Vanessa cardui) utilize the tender new growth as larval food. Though this extremely cosmopolitan butterfly will take advantage of numerous plants as larval hosts, it is often thought of as the “thistle butterfly.” Indeed, its species name refers to one of the genera within the tribe Cynareae, which encompasses the various thistle-like plants in the sunflower family (Asteraceae).

Painted ladies experience cyclical variations in their local populations; 2005 saw perhaps the highest numbers in recent memory. That year, my garden was alive with ladies in all stages; adults nectared at a multitude of flowers, and the larvae utilized a wide range of host plants. Though the vast majority will be found on thistles, the larvae also feed on other composites and plants in the mallow and borage families.

The larvae were literally everywhere, and I was obsessed with rescuing them from the weedy roadside stands of Italian thistles (Carduus pycnocephalus) before the road crews came in with their weed-whackers, oblivious of all the life they were destroying. Painted lady larvae are easy to rear indoors, if one has enough of their host plants to feed them, and make excellent subjects for classroom studies.

Silky hairs on the individual seeds of cobwebby thistle (Cirsium occidentale) allow them to travel on the gentlest of breezes

Silky hairs on the individual seeds of cobwebby thistle (Cirsium occidentale) allow them to travel on the gentlest of breezes

Late in the season, when the first rains encourage the growth of thistle seedlings, the fall flying adults of mylitta crescent butterfly (Phyciodes mylitta) will lay their eggs on the fresh young leaves. The eggs hatch, and the larvae begin feeding, but then hibernate at a halfgrown stage until warmer weather, when new growth ensues.

I wait until late in the season to do a little cleanup in the borders. The old, weathered stems are cut to make way for the new seedlings. Much of what is trimmed becomes mulch, cut into smaller pieces and left to decompose on the spot, thereby recycling nutrients back into the soil and providing over-wintering sites for a host of creatures who will fill the garden with life in the following year.