Mark Renz Extension Weed Scientist, Agronomy Department, University of Wisconsin-Madison
Knowledge of weed emergence is very important as it help in the selection and optimization of early season weed control. While typically weed emergence is consistent from year to year, the last two years have not followed this trend. During the 2012 growing season we saw weeds emerging 2-4 weeks earlier than typical. But in 2013, many species emerged 2-3 weeks late. So we can’t assume that weed emergence will be the same every year.
So what is happening in 2015? I visited the Arlington Weed Garden last week to determine what has emerged and it appears most species are on target for typical germination/emergence in 2015. Of the 100+ species at the Arlington weed garden 40% of the common Wisconsin weeds have emerged. Of these 41 species, all of the biennial weeds have emerge, while 40% of the perennial and 29% of the annual weeds have broken through the soil surface.
Winter annuals (shepherd’s purse, fleabane, chickweed, downy brome, Field pennycress, prairie pepperweed, pineapple weed, henbit) are well developed and some are beginning to flower as you can see from this picture of henbit.
Most early emerging summer annuals have just germinated and are at the cotyledon to first true leaf stage (Russian thistle, knotweed, kochia, common ragweed, and giant ragweed). These are close to historical average emergence times for these species. For example common and giant ragweeds typically emerge on April 5th or 6th respectively and I viewed cotyledons present on my visit to the weed garden on 4/16 indicating that they emerged the prior week (see pictures below). Common lambsquarter is the only exception to the list, but I have noticed it emerged elsewhere.
We are still waiting for pigweeds and foxtails to emerge, which typically around May 1st. With the current and projected weather for the next couple of weeks I expect their emergence to be on time as well.
A summary of average emergence along with the ability to create a graphic that estimates emergence and flowering time can be found at this link (weedometer):
This tool allows you to even change the location and estimate emergence at other locations throughout the state (and United States). A common rule of thumb however, is that what we observe at Arlington is occurs 2-4 weeks later in northern Wisconsin, and 1-2 weeks earlier in southern Wisconsin.