Our annual grasslands need 0.5-1 inch of rain to germinate in the fall. For the 40 years (or so) that folks have kept track of such things at the Sierra Foothill Research and Extension Center (SFREC) in Browns Valley, CA, a germinating rain has arrived sometime between late September and early November (on average). In other words, a soaking rain is needed to get the seeds of our annual grasses to sprout. The required amount depends on soil type, aspect, topography, and other factors. For most of the lands that we graze near Auburn, 0.75" of rain is enough to get the grass started.
Continued growth depends on a number of other factors. Once the grass has germinated, we need a good mix of sunny days and continued rainfall to keep it going. We also need conditions that help keep moisture in the soil - dry north wind, for example, will dry out our soils. Cooler weather - and cold storms - also cool the soil. We need soil temperatures of greater than 50 degrees F to keep the grass growing. And finally, we reach a point in early December where we simply don't have enough daylight to grow grass - the days are too short for photosynthesis.
Based on these considerations, an ideal autumn for me is one in which we get a germinating rain before our irrigation water shuts off in mid October. This first rain is followed at regular intervals (perhaps once a week) by moderate rainfall (0.33-0.75 inches) interspersed with sunny days. These conditions allow for enough grass to grow before we reach winter dormancy. At that point, we have to manage our grass carefully to get through to the resumption of growth that usually occurs in February. If the grass gets an early start in the fall, we have more to work with through the winter!
Here in Auburn, we had our 2015 germinating rain on November 1-2. Last weekend, we had another nice storm. The grass is starting to grow! On the other hand, these storms ushered in cooler weather - our daytime highs here at home have been in the mid 50s. At SFREC, soil temperatures have dropped into the lower 50s in the last several days. Past experience and a look at the weather forecast suggest that we'll probably go into our winter dormancy period in the next 3-4 weeks.
Early germination isn't always ideal, nor is fall precipitation always a good indicator of how the year will turn out in terms of forage production. In 2013, we had a germinating rain in early September, followed by a lengthy dry spell. This early germinated grass died for lack of moisture. We then had another germinating rain in mid October, again followed by a dry spell. The grass germinated again, but didn't grow much. Finally, we had a very cold storm in early December, followed by 50+ days of no rain at all. When it finally rained again in late January 2014, it took 45 days before we actually had enough green grass to graze our sheep. By comparison, last fall was outstanding in terms of precipitation and warm temperatures - we had good grass growth by Christmas. Once again, the storm door slammed shut - we had one of the driest January-February periods on record.
Those of us who rely on Mother Nature's provenance are conditioned to uncertainty - we never know for sure what the rainy season holds in the Sierra foothills (or anywhere else, for that matter). Despite this uncertainty - or perhaps because of it - I am always excited when we get our germinating rain in the fall. I always celebrate Germination Day!