Knowing When to Look For What:
Weed Emergence and Flowering Sequences in Wisconsin

Jerry Doll

Knowing the sequence of weed emergence is useful weed management information. It can help producers and ag professionals estimate when certain weeds may appear, particularly after the early emerging species arrive. Such weed emergence sequences have been reported for primarily annual species by several Midwestern states. Many of you have seen the Iowa State (2000) flyer that presents the emergence sequence of 16 common annual weeds for the upper Midwest. An earlier extension bulletin from Iowa State (Buhler et al. 1997) reported on the emergence patterns of 35 annual and 13 perennial weed species.

What has not been done routinely is to monitor the time when perennial and biennial species begin to regrow in the spring. Nor has information on the flowering sequence of a wide array of species at one location been observed and reported. Flowering information for perennial species is important to be able to synchronize tillage, cultivation and systemic herbicide application with the onset of flowering for maximum effectiveness. It is important information for both biennials and perennials in pastures, roadsides, CRP fields and other non-disturbed sites that will be mowed so that this operation is done before weeds produce viable seeds (usually 10 to 15 days after pollination). This paper provides information on the emergence and flowering times and sequences of 82 weed species in a range of life cycles for a three-year period in southern Wisconsin.

Methods

From 1998 to 2000, we monitored and recorded the sequence of emergence and flowering of annual, biennial and perennial species in the Wisconsin Weed Garden at the Arlington Agricultural Research Station. The Garden was established at its present location in 1995 and contains self perpetuating populations of more than 100 weeds common in Wisconsin fields and other habitats (gardens, roadsides, fence rows, etc.). Biennial and perennial species were considered as "growing" when 2 inches of new growth was present. The date of annual broadleaf emergence was recorded when they reached the cotyledon stage; annual grasses were considered as emerged when they were 0.5 inch tall. Dates of emergence for annuals are only of the first cohort as subsequent germination events were not recorded. We continued observing each species once or twice weekly until all reached anthesis (pollen production).

Data were summarized for the three years and analyzed in several ways. First, we placed the species into seven emergence and flowering groups. We did this because Iowa State used seven groups to report their emergence observations (Iowa State University, 1997). For weed emergence, the seven groups fell into 10-day intervals from March 20 to May 31; for the start of flowering, the species fell into seven groups using a 20-day interval, from April 20 to September 20. The exact dates for each grouping system are found in Table 1.

Results

The three-year average of the growth/emergence data found that among the 28 perennial species, quackgrass (Mar. 23), leafy spurge (Mar. 24), dandelion (Mar. 29) and white cockle (Mar. 30) resumed growth the earliest (Table 2). Yellow nutsedge (May 12) and horsenettle (May 26) were the last perennials to appear. Four perennials emerged early April; five in mid-April and seven in late April. Purple loosestrife, hedge bindweed, common milkweed, smooth groundcherry and wild four o’clock appeared in early May. The time interval between the appearance of the first and last perennial weed was 64 days.

In sharp contrast, all six biennial species resumed growth between April 2 and 7, a spread of only six days. This has several explanations. First, compared to the perennials, there were relatively few biennials in the Weed Garden. There may well be biennials that resume growth earlier or later than those observed. Also, five of the six biennials are in one plant family (Asteraceae) while the perennial  are from 17 different plant families. Morphologically, biennial plants are similar in that growth resumes from the crown which is at the soil surface. In contrast, perennials regrow from buds on the roots, rhizomes or tubers, or from crown buds. These structures may be at different depths and may also have different heat unit requirements for resuming growth between species.

Downy brome is a winter annual grass species. Seeds of this weed germinated the previous summer and plants resumed growth on Mar. 26. Among the 13 summer annual grasses, giant foxtail emerged first (Apr. 27) and stinkgrass last (May 28), a spread of 32 days. Woolly cupgrass emerged at the same time as yellow foxtail (Apr. 30). The three panicum species (fall panicum, witchgrass and wild proso millet) essentially emerged at the same time (May 5 to 7). It was surprising to see fall panicum emerge this early and this is in contrast to the observation in Iowa where this species emerged considerably later than the foxtails, woolly cupgrass and wild proso millet. Sandbur, shattercane and both crabgrass species germinated in mid-May.

The time between new growth initiation and flowering for perennials varied from 31 days for dandelion to 144 days for Jerusalem artichoke: a range of 113 days (Table 2). Horsenettle was the last perennial to appear (May 26) but flowered in 33 days (June 28). Yellow nutsedge flowered 58 days after resuming growth, quackgrass in 82 days and wirestem muhly in 110 days. Many perennial broadleaves flowered 45 to 70 days after resuming growth. As expected, Cruciferae species flowered in comparatively short times. The range of flowering times for biennials was 43 days (musk thistle 75 days and bull thistle 118 days). For annual grasses the range was 55 days (43 days for stinkgrass, the latest emerging annual grass, and 98 for giant foxtail, the earliest emerging annual grass). The range for annual broadleaves was 115 days (24 days for shepherd’s purse and 139 for common ragweed). Kochia emerged first but took more than 100 days to flower.

When the species are examined by emergence group (as defined in Table 1), we see that 17 to 24% of the weeds emerged in groups 1, 2, 4 and 5, with less than 10% emerging in groups 3, 6 and 7 (Table 3). A similar assessment of flowering times finds nearly a normal distribution (bell-shaped curve) with most species (36%) flowering in group 4 (June 21 to July 10).

Interesting Observations

Species in some families grow and/or emerge similarly (Cruciferae, Chenopodiaceae, Amaranthaceae); some don’t (Asteraceae, Poaceae, Polygonaceae, Solanaceae).

Some close relatives emerge differently: wild cucumber 13 days before bur cucumber.

Some close relatives emerge similarly: eastern black nightshade and hairy (1 day); common and giant ragweed same day.

Some close relatives flower differently: musk thistle flowered 9 days before plumeless; plumeless thistle flowered 20 days before bull thistle.

Fall panicum and witchgrass emerge in the middle the annual grasses but flower later than the rest.

Woolly cupgrass and giant foxtail are early emerging annual grasses but are the last to flower.

Giant foxtail was the first annual grass to emerge; stinkgrass the last. Stinkgrass flowered in the least time (43 days) and giant foxtail took the most (98 days).

Milky-sapped perennials flower in ~ 50 days; common milkweed 52 days; hemp dogbane 50 days; leafy spurge 53 days.

Horsenettle was the last weed to appear (May 26) but flowered quickly (33 days).

Jerusalem artichoke is a short day plant; flowered on Sept. 20, 144 days after emergence.

Annual dicots broke into two distinct groups regarding variation in emergence dates over years: those in groups 1-3 averaged 20.8 days variation; those in groups 4-7 averaged 9.5 days variation.

References

Buhler, D.D., R.G. Hartzler, F. Forcella and J. Gunsolus. 1997. Relative emergence sequence for weeds of corn and soybeans. Iowa State Univ. Pub. Pest Man. File 9 (SA-11).

Iowa State Univ. 2000. Weed Emergence Sequences. Univ. Exten. Flyer IPM 64a.

2001 Proc. Wisconsin Fertilizer, AgLime, and Pest Management Conf. Vol. 39: 96-103.