Cobwebby Thistle

Cobwebby thistle (Cirsium occidentale) is the only native thistle in the Reserve. It is tall, and stately, very prickly and utterly charming. Cobwebby thistle looks like its name. The plant has the very prickly leaves, and the purple pom-pom flower of a thistle and the entire plant is covered with long white hairs, like cobwebs. On a foggy day, the plant has a pale, ghostly appearance.

The fierce prickles protect the plant from large grazers such as deer. The flowers attract birds, butterflies and bees.

Description ^11,26,35,59

Cobwebby thistle is a stately herb that may reach five feet (1.5 m) in height. Generally, a taproot and  basal rosette of leaves are produced the first year, followed the next year by a stout upright stem, which is unbranched or has several ascending lateral branches. All parts of the plant are covered with long, pale, silky hairs, which resemble cobwebs. The ovate to lanceolate leaves are 15 inches (40 cm) or less in length, smaller on the stem. The leaves are pinnately lobed with triangular lobes cut half-way to the mid-vein; their margins are undulate with sharp prickles; their bases clasp the stem, often with prickly wings.

Flowers are composite. Flower heads are rose-purple produced at the ends of the stems, mostly at the top of the plant. The receptacle is globose. Sharp phyllaries radiate outward and are knit-together silvery  hairs. Ray florets are absent. Bisexual disk florets are confined to a small tuft on top of the receptacle, about one inch (2.5 cm) high. The petals of a flower are united into a narrow tube with five purple lobes. There are five stamens with purple anthers fused into a cylinder around the two-branched, purple style. The pollen is white. Plants bloom once, generally in the second year, and then they die (they are “monocarpic”).  The main flowering season is March through July.¹

The brownish seed is about 1/4″  (0.6 cm) long, attached at one end to a white, plumose pappus that carries it away on the slightest breeze.

Classification

Cobwebby thistle is a dicot angiosperm in the sunflower family, the Asteraceae.^2,11 This is one of the two largest families of vascular plants in the world, second only to the orchid family (Orchidaceae).^44,143 “Flowers” of the sunflower family are made up of one or both of two types of flowers (florets): symmetrical disk florets and strapped-shaped ray florets. These are crowded onto a common base (receptacle); the whole is called a flower head, which is often assumed to be a single flower.^11,44,49  Although a few plants in this family, such as lettuce and artichokes, are used as food plants,  their main economic value is in their use as ornamentals: sunflowers, daisies, zinnias, marigolds, chrysanthemums, and many more.¹⁴³ Many plants in this family are serious agricultural pests.⁴¹ Asteraceae that occur in the Reserve include bush sunflower (Encelia californica), goldenbush (Isocoma menziesii), and coyote brush (Baccharis pilularis).

Plants of the sunflower family are divided into tribes on the basis of flower and fruit morphology.¹¹ Perhaps the most easily recognized is the thistle tribe, distinguished by its prickly nature (although not all thistles have prickles) and by the tuft of purple to white disk flowers atop the large receptacle.

There are seven varieties of cobwebby thistle reported in California, primarily separated by characteristics of the phyllaries.² The thistle in the Reserve is variety occidentale. Several years ago, there was a white-flowered cobwebby thistle in East Basin. This may have been variety californicum, but until it reappears, we will not be certain.

Ecology ^41,268

Thistles have neither spines nor thorns. Thistles have prickles – and there is a difference. A thorn is a modified branch, and a spine is a modified leaf. Both thorns and spines have vascular tissue. In contrast, a prickle develops from the surface tissue and may occur anywhere on the plant – on the stems or leaves or flower heads. Thorns and spines are not easily removed from the plant; prickles often are. Spiny redberry (Rhamnus crocea) has thorns, not spines, roses (Rosa spp.) have prickles, not thorns, and thistles have prickles. Even botanists tend to be casual about their use of these terms.

Thorns, spines and pickles are all thought to serve the same function, to deter grazing by large herbivores. Livestock not only do not eat thistles, but they will not forage around them. Cobwebby thistle is deer resistant.²⁴⁶

In spite of it’s fierce defences, cobwebby thistle has significant wildlife value. It is an important source of pollen and nectar, especially for native bees and butterflies.^59,83 The painted lady, which occasionally forms huge migratory swarms in Southern California, both feeds and nectars on cobwebby thistle,¹¹⁶ and goldfinch relish the seeds. Rarely have we examined a cobwebby thistle without finding one or more ladybugs on patrol, suggesting a supply of prey insects hidden beneath the “cobwebs.”

The long cobwebby hairs may also have adaptive advantages. For a discussion of the ecological role of hairs, see the Ecology discussion of California croton, Croton californica.

Human Uses ¹⁵

There are no reports of thistle being used by the local Kumeyaay, although the Chumash in Santa Barbara and the Channel Islands used several species as food. The Cahuilla to the east (Riverside county) ate the bud at the base of the plant, while the Ohlone in central and northern coastal California ate thistle stems raw or boiled, and used the stems and roots medicinally.

Cobwebby thistle is occasionally sold by native plant nurseries for habitat gardens because it attracts birds, butterflies and bees and because it is deer resistant.²⁴⁶

Interesting Facts ²⁶⁹

From 793 A.D. onwards, the Vikings made repeated raids on the British Isles.⁴¹ According to Sottish legend, during one night invasion, as Norse invaders were stealthy approaching a Scottish camp, one of the raiders stepped on a thistle. His cry of pain woke the Scots who successfully drove the raiders away, thereby saving their country. That is how the thistle became the national emblem of Scotland.