It's October and it's weeding season! That's not a phrase we regularly hear. In many years in California October can be a month where weeding is a low priority task. With the late spring rain and then the remnants of the August hurricane that came through, coupled with some wet, cool misty days lately, we had a lot of summer annuals germinate and survive. However, at least in Southern California quite a few winter annuals have also germinated. And then with recent rains in October those annual weeds are starting to put on a lot of new growth.
As I've been doing some driving lately, I've been seeing quite an interesting blend of weeds in Southern California. I'm seeing tumbleweed, puncturevine or goat's head (so much puncturevine), spurges, even a few sorghums (likely from bird seed), crabgrass, portulaca, and plenty of wild sunflowers, all of which are summer emerging plants. I'm also seeing mustards, exotic annual grasses (oats, bromes, etc.), storkbills, a few California poppy seedlings, artichoke thistle, henbit, and oxalis, all of which emerge in the winter.
There are many different triggers for seeds to germinate. In some species soil temperature is a key factor, on other cases the seed coats needs to be scarified (scratched, digested, etc.), some seeds germinate in the presence of light, others in the shade. This year we had enough rains this summer for both the summer annuals to germinate, and those winter annuals that tolerated the temperature of the summer rains to also germinate.
This year is an excellent example of phenology-based weed management. This idea is nothing new, it's been around for decades, where managers use the weeds' natural cycle (phenology) to determine timing of management. Most of us are doing this, often without realizing it. This year it's quite obvious, we have a busy fall for weed management. We are controlling both summer and winter emerging weeds. A task that is not on the typical weeding calendar. In most cases our standard weed management practices will be robust to this unusual combination of weeds. In other cases, the weeds may have emerged on sites because pre-emergent herbicides applied in the spring may have degraded in the soil as the half-life of some products is too short to provide fall control. In other cases, it may currently too hot to apply some herbicides that readily volatilize in the heat, even though they work well in the cool winter. And products that which are more safely applied in the heat will need to be used instead.
While the weeds have thrown out their calendars this year, unfortunately for us we are going to have to put a little thought into controlling weeds when its 95 degrees in October and henbit, tumbleweed, goat's head and oats are all flowering at the same time. Coming off of last winter's deluge of atmospheric rivers, and with a very high probability an El Niño is on the way this winter (https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml), we could have two years of above average rainfall in California, which is a rare treat. And that might be something that causes us to throw out our calendar next year too and manage based on the phenology of the weeds.