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Weed control, management, ecology, and minutia
by Steven Fennimore
on January 23, 2018 at 7:22 AM
Hi Pratap  
this is an interesting pattern that i have never seen before. We published a paper over 10 years ago on seasonal emergence patterns of annual bluegrass here on the coast. Emergence seems to peak in December when days are short and the soil is wet. In the literature search for this paper we found some references to the niche of bluegrass - in winter when days are short, when soil is often saturated and there is less competition from other weeds. Could it be that the section of the bed where you have the most emergence is saturated for longer periods of time?  
the paper i refer to is below. Bluegrass is certainly an interesting little weed! thanks for the post  
Shem-Tov, S., and S. Fennimore. 2003. Seasonal changes in annual bluegrass (Poa annua L.) germinability and emergence in California. Weed Science. 51:690-695.
by Guy B Kyser
on January 23, 2018 at 8:14 AM
Interesting observation. I'm going to guess that the south-facing banks are warmer, stimulating germination.
by John Roncoroni
on January 23, 2018 at 8:24 AM
Was there a herbicide appplication and was the wind blowing from the south?
by Jeff Quiter
on January 26, 2018 at 9:17 AM
In the cooler months, we see our native seeds germinate first sometimes in the south rows on our bed tops. Our beds are oriented east-west and have 4 rows seeded on top. I have measured soil temperatures. The soil temps in the row furthest to the south can be 5+ degrees warmer.  
In the summer months with long days, we have seen the opposite pattern with seed germinating first in the north sides of the beds, where conditions are cooler and more favorable for germination. We have had trouble germinating anything in our fields in 95+ degree weather. If the soil is moist, the soil temps can get 10 - 20 degrees higher than the outside temp. Not good for germinating seed.
by Carl Bell
on January 29, 2018 at 11:37 AM
Its the difference in warmth from the more direct winter sun that is low on the horizon hitting the south facing slope of the furrow. It also has a bit to do with moisture levels; the south facing slope dries out quicker, making more oxygen available to roots which encourages faster growth.
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