In the Company of Plants and Rocks: Salsola

Friday, September 25, 2015

Fall Colors: Pink & Green

Salsola kali, one of many synonyms (experts are still debating).

They’re among the most notorious characters of the western US … have been since the 1870s when they entered illegally. Locals will tell you: they take over pastures, fuel wildfires, clog fences, and either poison livestock or starve them to death by driving out palatable plants. These scoundrels are Russian thistles—the iconic tumbleweeds of the American West.

I'll keep rolling along

Deep in my heart there's a song

Here on the range I belong

Drifting along with the tumbling tumbleweeds

(Tumbling Tumbleweeds by Bob Nolan)

The first Russian thistles arrived in the US in a batch of flax seed carried by Russian immigrants to South Dakota. They spread quickly, and now are abundant in drier parts of the American West. They're also common along the Atlantic and Gulf coasts, and anywhere with regular disturbance—roadsides, ag fields, overgrazed pastures and waste areas. Throughout its North American range, Russia thistle is designated noxious: “a weed designated by an agricultural authority as one that is injurious to agricultural or horticultural crops, natural habitats or ecosystems, or humans or livestock” (source).

But this is too harsh. Immature Russian thistles are palatable to livestock, and show promise as hay crops in semi-arid regions. During the Dust Bowl—the great droughts of the 1930s—ranchers fed tumbleweeds to their cattle, thereby saving the industry. Deer and elk will eat small amounts of Russian thistle, until it becomes too prickly. Pronghorn antelope feast on it, and it’s an important food for prairie dogs. Small mammals and birds eat the protein-rich seeds. Dead tumbleweeds serve as nursery plants for seedlings of other species, providing shade and protection. Ironically, these same species may well out-compete Russian thistle, eradicating it from the site.

Some Russian thistles near my house are now bright red and prickly.

Others are still green, with some linear leaves of youth.

The widespread success of Russian thistle is intriguing, for it’s a very poor competitor. A Russian thistle seed contains no endosperm—no nutritious tissue to sustain the embryonic plant until it can photosynthesize on its own. Seeds that don’t germinate within a year die, so there’s no Russian thistle seed bank. A seed won’t germinate in compacted soil, or if it’s buried more than 5 cm (2.5 in). If a seedling is shaded, it will die. Russian thistle can’t grow in soils with mycorrhizal fungi—normally beneficial partners of plants. These fungi invade and kill them root by root.

But chances are that at least one of the 250,000 seeds scattered by a single tumbleweed (!) will land on a favorable spot—unshaded loose soil that's poor in mycorrhizal fungi, and with few competitors. In other words, Russian thistles require recently disturbed habitat, and we have plenty of that.

The seeds are programmed to refrain from germination until spring, but then … they go crazy. A seed needs only a little moisture to germinate, and can tolerate “virtually any seedbed temperature.” Germination is fast—in as little as 15 minutes! Each seed contains a fully-formed coiled embryo, complete with a care package of chlorophyll (remember, there’s no endosperm). The little plant unwinds, driving a tiny root into the ground, and starts growing.

Russian thistle seedling. From UC Davis.

Young Russian thistles have soft linear leaves, and seedlings look a bit like pine seedlings. Initially, plants grow and branch into a roughly conical shape. Later the branches grow longer and curve up, creating rounded forms ready to roll. The largest are five feet in diameter. Leaves on mature branches are short and stiff, with a sharp spine at the tip. This is why livestock and wildlife don’t eat mature Russian thistles unless they’re starving, and why we wear sturdy gloves to remove them.

Flowers and spine-tipped leaves (bracts).

Russian thistle flower is about 5 mm across; from Britton and Brown (1913).

The inconspicuous petal-less flowers in the leaf axils are beautiful up close, with their translucent veined sepals and Victorian colors.

Striped stems are characteristic. In younger plants, they're red and green.

At the end of the season, when Russian thistle plants are fully dry, they break off at the base. Now they become tumbleweeds, scattering seeds as they roll before the wind, and finally coming to rest in fences, in trees and under cars, as well as in some rather surprising places.