Canada Thistle

Beware of tall, attractive wildflowers with prickly leaves growing in fields. Most have beautiful magenta or lavender-colored heads, but the leaves, and sometimes the stems, are edged with numerous, sharp spines to defend against herbivores looking for an easy meal. These are the thistles. They are composites, related to daisies and goldenrods in spite of the obvious differences. Most are biennials, starting as a ground-hugging rosette of prickly leaves the first year, then growing tall and flowering the second.
 

The Canada Thistle (Cirsium arvense), a misnamed alien invasive from Europe, is one of the less-obvious species here [in the northern Finger Lakes region of NY] because its light-purple heads are less than an inch high. The whole plant grows four to five-plus feet high and often forms dense colonies after spreading by underground stems. In Europe and northern Asia, it is known as Creeping Thistle. One plant's roots can spread to a three-foot diameter (or more) in one year and continue growing at this rate for many years, making it a difficult plant to eradicate. A single thistle plant can be considered biennial, but the root system is perennial and continues to put its energy into new clonal [cloned] shoots.
 

Canada Thistles form colonies from a spreading underground
 root system. Less energy goes into producing flowers.

Less energy goes into producing flowers which are usually dioecious (either male or female). Male flowers may be slightly smaller, but otherwise similar. Both are very fragrant, which is energy spent to attract pollinators and NOT humans. Female flowers produce 40 to 80 seeds per head, but the fuzzy pappus is loosely attached and often ineffective in seed dispersal.

Canada Thistle's main stems are smooth, but the deeply-lobed leaves are designed to keep you at a distance. In the spring, wear heavy gloves and protective clothing so you can return to the location and collect the young leaves, young stems and roots for food. One of the plant's nicknames is "lettuce from hell" (and it actually is related to lettuce). After removing the spines, add young leaves to salads or eat them cooked. The pithy, young stems are easier to deal with; just peel and eat raw or cooked. The roots of first year plants, before the tall stems begin, are a survival food, but they also can cause flatulence.


A crab spider is camouflaged against a Canada Thistle flower bud.

Canada Thistle is also food for wildlife, especially goldfinches and other finches that eat the seeds. Some butterfly and moth larvae eat the leaves. Look for Red Admirals, Viceroys and Painted Ladies on this thistle. The larvae of the Orellia ruficauda fruit fly parasitize the fertile seed heads, making them a somewhat-effective biological control.

In the past, a tea made from the leaves was used as a general tonic (stimulant) and diuretic to increase the loss of water from the body. Externally, it was used for skin sores and poison-ivy rash. Native Americans used a root tea as a digestive tract stimulant and dewormer. (I wonder what they used before Europeans introduced this wildflower.) At the other extreme, it was said to cause inflammation and irritation (Jacobs and Burlage). Go figure.
 

Cirsium comes from the Greek word, kirsion, from kirsos, a "swollen vein" remedied by one of the species in that genus. The species name, arvense, is Latin for "in the field." Among its other common names are "field thistle," "perennial thistle" and "cursed thistle." It was originally native to the Mediterranean and southeast Europe and likely arrived in North America during the 1600's.

Corrections, comments and questions are always welcome at northeastnaturalist@yahoo.com or connect through my Facebook page and photo page. There's also a community-type page for The Northeast Naturalist. Other nature and geology topics can be found on the parallel blogs Adirondack Naturalist and Heading Out.

Goldenrod Ball Galls -- © Dave Spier

Pair of ball galls on Canada Goldenrod stem at Montezuma Audubon Center - © Dave Spier

In August and early September, the field had been bright yellow with goldenrod flowers. Now it’s turning brown, although this too can be attractive when the plants are frosted and backlit by the rising sun. In November, most of the goldenrod leaves will shrivel and drop, exposing abnormal swellings on many of the plant stems. The common ones found on Canada Goldenrod are ball-shaped, resembling miniature basketballs about an inch or more in diameter. Not surprisingly they are usually called "goldenrod ball galls."

To fully understand galls we need to cut one open and look inside. Carefully pry and twist while cutting into the pithy core and try to pop the gall open. At the very center is a small cavity with the even smaller white larva of the peacock fly, Eurosta solidaginis, a relative of the fruit fly. Originally injected into the stem as an egg, it hatched and began feeding. Chemicals in its saliva stimulate the host plant to produce a pithy bulge in an effort to heal the "wound" caused by the parasitic larva. This actually benefits the insect by providing endless food and later an insulating shelter with a hard, weatherproof shell.

The gall, however, is not impermeable. Downy Woodpeckers and Black-capped Chickadees have learned that galls contain a nutritious morsel, and these birds will peck and chisel until they get it. You can recognize their work by the rough, funnel-shaped excavations they leave behind.

Gall fly larvae that escape this fate will chew a tunnel almost to the surface and there they will pupate until they emerge. Winged adults lack chewing mouth parts and can only pop the lid. They do this by inflating a balloon-like bladder (called the ptilinum) between the eyes. You can recognize their work by the small, sharp-edged hole that appears to have been drilled into the gall. The only function of the adult stage is to find a mate and lay more eggs in more goldenrod stems.

Goldenrods are prone to several other kinds of galls. An irregular-shaped growth called the "knotty goldenrod gall" is caused by the gall midge Lasioptera solidaginis. It’s somewhat globular, but not pith-filled. Less obvious is a long, spindle-shaped bulge called the "elliptical goldenrod gall" caused by the caterpillar of a moth.

A third type is very different in structure and occurs at the top of the stem. The "goldenrod bunch gall" is actually a globular head of deformed leaves about two inches in diameter. It is caused by a gall midge, a type of fly related to the mosquito. Its egg-laying apparatus, called an ovipositor isn’t strong enough to pierce the tough stem, so it contends with laying its egg on the tip of the plant.

Questions and corrections may be sent to northeastnaturalist@yahoo.com or connect with on Facebook.