2012 Weed emergence in Wisconsin: uncharted territory

Mark Renz Extension Weed Scientist, Agronomy Department, University of Wisconsin-Madison

Time of first weed emergence is fairly consistent from year to year. For example Jerry Doll monitored yellow foxtail emergence over a three year period in Arlington Wisconsin and found that on average it began to emerge on April 30^th, differing by no more than seven days between years. This reliability has been a huge benefit to growers as we can predict when these weeds will emerge and apply our PRE herbicides at the correct time. A summary of these data along with the ability to create a graphic that estimates emergence and flowering time in your area can be found at this link (weedometer):

http://weedecology.wisc.edu/weedometer/

Realize that these average timings for emergence and potentially flowering presented on the weedometer will be several weeks late this year due to the warm weather this spring. I visited the Arlington weed garden recently and documented emerged plants, and summarized what I saw compared to average first emergence for common weeds.

Refer to the table below for my observations of common weed species in Wisconsin.

Briefly, perennials are emerging and flowering three to four weeks ahead of typical timing. For example yellow rocket should be flowering at Arlington in 1-2 weeks this year which is 3 weeks ahead of the average timing. I expect the same thing to occur with biennials as there emergence was several weeks ahead of schedule. Annual weeds’ response is much more variable, as species that had cotyledons present on April 4^th on average emerged 19 days earlier, but the range was between 3 and 35 days depending on the species.

So you should plan on weed emergence to be several weeks earlier. We can expect it to also speed up the development of these weeds and they will flower sooner, but other factors besides temperature can affect this change. Also don’t forget we are one frost/freeze away from resetting the clock which is not uncommon in Wisconsin. What will happen in this coming year is really not known as this is truly an atypical spring.

Weed species	Average date of first emergence 1998-2000 (range observed in days)	Weed emergence as of 4/4/2012	Days earlier emergence than average^
PERENNIAL WEEDS
Canada thistle	April 16^th (15)	Y
Curly dock	April 10^th (24)	Y
Dandelion	March 29^th (6)	Y
Field bindweed	April 26^th (4)	Y
Field horsetail	April 28^th (7)	N
Hemp dogbane	April 28^th (1)	N
Common milkweed	May 6^th (5)	N
Quackgrass	March 23^rd (24)	Y
White cockle	March 30^th (12)	Y
Yellow nutsedge	May 12^th (3)	N
Yellow rocket	April 2^nd (12)	Y
BIENNIAL WEEDS
Burdock	April 4^th (24)	Y
Plumeless thistle	April 7^th (7)	Y
Wild carrot	April 7^th (12)	Y
ANNUAL GRASS WEEDS
Barnyardgrass	May 3^rd (8)	N
Fall panicum	May 5^th (6)	N
Giant foxtail	April 27^th (2)	Y*	23
Green foxtail	May 3^rd (3)	N
Large crabgrass	May 14^th (9)	N
Shattercane	May 13^th (10)	N
Wooly cupgrass	April 30^th (6)	Y*	26
Yellow foxtail	April 30^th (7)	Y*	26
ANNUAL BROADLEAF WEEDS
Bur cucumber	April 29^th (5)	N
Common ragweed	April 7^th (28)	Y*	3
E. black nightshade	April 30^th (16)	Y
Giant ragweed	April 8^th (32)	Y
Common Lambsquarter	April 18^th (22)	Y*	14
Redroot pigweed	May 9^th (2)	Y*	35
Waterhemp	May 5^th (1)	Y*	31
Wild cucumber	April 16^th (10)	Y*	12
Wild radish	April 12^th (18)	Y*	8

*If only cotyledons present on weeds

^ Only calculated if weed species appeared to have just emerged (only cotyledons present)