In wet and mesic tallgrass prairies of central North America, the perennial forb Zizia aurea starts growth early in the spring and flowers in May and June; seeds are shed in autumn and germinate the next spring after exposure to cold, moist winter soil (Rock, 1981, Mohlenbrock, 1986; Swink and Wilhelm, 1994, pers.
Zizia aurea was one of 20 grasses and forbs planted in 60, 9 x 9 m plots in a wet-mesic sandy loam near Viola (43 [degrees] 32[minutes]N, 90 [degrees] 20[minutes]W), Wisconsin in late June and early July 1990.
This aggressive [C.sub.3] grass is native to Eurasian and North American wetlands and wet prairies (Sampson, 1921; Curtis, 1959; Galatowitsch and van der Valk, 1996; Mergliano and Lesica, 1998), and formed the matrix with which Zizia aurea interacted.
Zizia aurea plants were censused before and after seasonal burns.
In 1996 Zizia aurea occurred in 10 of 12 plots burned in August 1995, six burned in May, and four left unburned ([X.sup.2] = 6.17, 2 df, P [less than] 0.05).
High fire intensities may encourage Zizia aurea. August fire intensity in 1995 had been higher in the six plots with more than one Z.
In 1996, the 12 plots mowed in May 1995 had a total of 77 Zizia aurea; in May 1993 (5+ wk after the April 1993 fires) 76 were counted.
Twenty-four of 121 Zizia aurea flowered in plots mowed in August, whereas five of 77 flowered in plots mowed in May.
- Candidates for suppression of Zizia aurea are growing canopies of the dominant grass ([greater than] 95%) Phalaris arundinacea in the spring, mature canopies in the summer and litter from the previous (or earlier) seasons.