Do weed populations change during a drought? Does drought favor certain species? Does annual or perennial species matter? During this four year period of drought in California, have they changed? What are the populations of annual and perennial weeds? With a limiting growth factor, in this case water, weeds become more prominent and which of them will or could disappear?

In urban landscapes, where turf grass areas are being renovated, or in non-irrigated land that has been farmed, but furloughed, or in non-cropped roadsides or wasteland, are we seeing life-cycle species shifts? Or is it that we see a loss or decreased competitiveness of annual species, thus perennial species can be observed?

I will give the example of several landscape sites, two of which started as a lawn with tall fescue, a cool season perennial and the third was a small vineyard in the landscape. With the assistance of the homeowner, the background was established. Three scenarios:

1. when the homeowner cut off watering of the turf, the grass died in spots and left a relatively bare ground of dead vegetation
2. the turf was smothered with cardboard and mulch or chips (two locations)
3. a vineyard area where winter weed species were controlled. Results are observations in the spring and summer of 2015.

The result in the first site was that annuals may have germinated after a rain, but died from lack of moisture. Also when there was a small area of bermudagrass or nutsedge, they grew and spread, though slowly. If field bindweed was present, it grew slowly but bloomed and set seed. Dandelion, dallisgrass, narrow and broadleaf plantain (all perennials) also survived and grew.  

In the second scenario, the area remained free of weeds at both sites for a period before field bindweed and/or bermudagrass emerged in several areas of the site. It grew well and spread. In the second location with a similar mulching operation, the grass was treated with glyphosate (Roundup) before close mowing. This was followed by an application of 2 inches of red chips. Weeds that grew back were bermudagrass and yellow nutsedge. This home owner sprayed at least two additional times. They were not eradicated.

In a small vineyard site next to an uncultivated area, annuals were sprayed during the dormant season (glyphosate and oxyfluorfen). The dominate weeds in the vineyard became Johnson grass, yellow nutsedge, perennial pepperweed, cheeseweed and willowherb.

In reduced, intermittent irrigated landscapes, I am finding more yellow nutsedge, bermudagrass and dandelion, but still plenty of smooth crabgrass, prostrate and creeping spurge or knotweed. In places where tree species are dropping seed into the turf, tree seedlings are present as weeds.

In my three examples, plant management practices are having a distinct effect on subsequent species and growth in this drought environment. Mowing allows bermudagrass to seriously compete over annual species. It is apparent that under drought stress, seed head production of bermudagrass and flowering of field bindweed are common. There seems to be a proliferation of perennial species at the expense of most annuals, except deep rooted annuals and biennials (purple star thistle, prickly lettuce, knotweed).

Weed populations do change when drought stress occurs. This stress seems to favor perennial species over annual species. Depending upon the species present at the beginning of the drought, these perennial species are more tolerant than annuals and thus become a greater part of the landscape. Annual species will probably not decline in the area over time unless the drought continues for more years because of the seed banked in the soil.

Are we seeing a life-cycle shift during this long drought? Are there any places where annual weeds are becoming larger populations; with a reduction of perennials?