Thistle amaze you

by Helen M. Scott
Tuesday, June 17, 2008 2:40 PM MDT

On windy days in Wyoming in the late summer or autumn, one is fascinated by the curious spectacle of hundreds of strange round objects rolling and bouncing along the ground and piling up along fences.

This is the unique plant known as the tumbleweed. Russian thistle and wind witch are other common names for this symbol of the West.

Although it is native to the arid steppes of the Ural Mountains in Russia, it first was reported in the United States around 1877 in South Dakota, apparently transported in flax seed imported by Ukrainian farmers.

Within a short time, it had tumbled into a dozen surrounding states, and by 1900, it had reached the Pacific Coast.

There are five species of tumbleweeds in the U.S. The common tumbleweed, a member of the Amaranth family, is native and most abundant in arid regions from Canada to New Mexico.

The worst of these is the Russian thistle, which is not a thistle, but a member of the Goosefoot family. A native of Asia, it thrives on land too dry for most plants and is the last survivor in drought years.

There are two other tumbleweeds, both members of the Goosefoot family: the bug seed and the winged pigweed. These plants thrive in salty and alkaline soils, but can be out-competed by natives in undisturbed habitats.

Seeds are unusual in that they lack any protective coat or stored food reserves. Instead, each seed is a coiled, embryonic plant wrapped in a thin membrane.

To survive winter without a warm coat, the plant does not germinate until warm weather arrives. When moisture falls, the plant is ready to uncoil and germinate.

All that is required are temperatures of at least 28 degrees, which triggers the plant to quickly send up two needle-like leaves that shoot skyward.

By autumn, the plant has reached maximum size, flowered and begun to dry out. The tumbleweed, after it dies, breaks away from its root, commonly known as stem base snap, and the whole plant goes tumbling over the fields and prairies, blissfully scattering seeds everywhere.

A specialized layer of cells in the stem facilitates the easy break between plant and root, and the journey begins. This appears to be something that has evolved due to the benefits of having the seeds dispersed along with the dead plant.

As the tumbleweed rolls, it usually drops seeds easily, but sometimes there is a tendency for seeds in the middle of the ball to get trapped there, but a good rainfall will cause the dried branches to unfold a bit and a large number of seeds will get dumped at once - and watered at the same time.

Virtually everyone recognizes the mature thistle or tumbleweed, which looks like the skeleton of a normal shrub. Plants may be as small as a soccer ball or as big as a VW beetle.

Most people, however, would fail to recognize the seedling and juvenile plants’ bright green, succulent, grass-like shoots, which are usually red or purple striped. Inconspicuous green flowers grow at axils (where leaf branches off from stem) of the upper leaves, each one accompanied by a pair of spiny bracts. Mice, sheep and antelope eat the tender shoots.

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