It only stands to reason that drier conditions would produce more tumbleweeds, right?
Not so, say Caltrans landscape architects. Weed growth overall is down this year, a result of insufficient water to germinate seeds, even those species requiring very little.
The Russian Thistle (Salsola iberica), or tumbleweed, is an invasive weed in arid natural areas. The blowing skeletons can interfere with traffic and lodge against fences. Growing to three feet tall or higher, Russian thistle flower in summer and early fall. Once the seeds are set in ground, they will germinate the following spring and seeds can survive up to three years.
Their habitats include disturbed sites (from construction and/or maintenance activity), bare ground and roadsides. They are competitive and opportunistic, their oblong shape and ability to pre-generate seeds internally giving them an advantage. When they blow, their shape causes frequent banging against the surface, dislodging seeds.
Caltrans controls them mechanically, by pulling, cutting, tillage, or chemically. There are downsides to each method: pulling is not feasible for large areas; cutting or mowing can be done when the Russian thistles are still small but must be followed up with herbicides to prohibit further growth; tillage (also called disking) will control both seedlings and larger plants but must be repeated frequently which in turn creates more disturbance and hence more weed growth. That leaves chemical control as the most practical method provided it is done at the right time, frequency (not too much or -- again -- disturbance) and using the proper herbicides.
As in all processes involving nature, a balance must be maintained between abating tumbleweeds for optimum roadway operation and over-abating, which ironically creates more pests.