I'm sure you've noticed that different years have dominant weed species. For example this year in Southern California Russian thistle (aka tumbleweed Salsola tragus among other Salsola species) is abundant in many areas. Other years it is only moderately abundant and some years it seems to hardly hang on. The obvious question is why? Why is there so much variation in abundance between years?

Fortunately for us people have been working on this issue for a while and have a few general answers for us. Since the bulk of weeds in California are annuals I'll limit my discussion to those plants.

The seeds of annuals germinate only under a certain range of environmental conditions. For example, many winter annuals in California germinate when temperatures are cool or cold. Sahara mustard (Brassica tournefortii) does not germinate well when the temperatures are above about 85F. Other plants do not germinate when the temperatures are cold. You can see this pattern on THIS page of common garden vegetables by Cooperative Extension in Alabama. There are cold season plants (onion) and warm season plants (tomato, melon).

Although we have only one rainy season in California, and sometimes less than that, we still have cool season weeds and warm season weeds. Most of our weedy grasses (like bromes) are cool season, and our late season weeds (like tumbleweed) are warm season. Of course there can be overlap, warm temperatures in the fall can provide suitable conditions for warm season weeds, even though they ‘normally' germinate in the spring.

Our ability to predict which weeds will be abundant is hampered by many things; most of all is insufficient data. We know plants respond to soil moisture, soil temperature, oxygen levels in the soil, light levels (seeds can ‘see' light!), scarification (abrasion of the seed coat), soil salinity, age of seed, age of parent plant, and many other factors.

Imagine we tested to the germination conditions of 3 weed species for 3 factors (moisture, salinity and light) and each factor at just 3 different levels (high, medium or low), and we replicated this test in 3 different soil types (sandy, loam, silt). In the end, we would have 81 different trials for this simple experiment (weed #1 at high light in loam = 1, …). Barring a large infusion of money to conduct experiments to predict weed germination, and thus examine one factor in the spread and increase in weeds, we will be using coarse measures to figure out the population of weeds.

Back to my original question, why was there a lot of tumbleweed this year? Because tumbleweed germinates better in conditions of high light or disturbed soil, with relatively warm temperatures and the seed germinates rapidly, effectively using small amounts of moisture. Given there was limited rainfall early in the winter, which limited production of many cool season weeds, and moderate moisture in the mid spring coupled with a relatively warm winter, conditions were rife for the establishment of tumbleweed.

UC IPM page for tumbleweed is HERE