Poa annua L. (annual bluegrass) includes biotypes ranging from true annuals to long-lived perennials. It is widely thought that turfgrass management practices can be altered to promote certain biotypes and minimize undesirable seed head development. This study examined the effect of irrigation frequency on lateral spread and inflorescence development of annual, perennial, and intermediate P. annua biotypes. Three treatments were imposed by watering to field capacity and allowing depletion to 12, 8, or 4% volumetric water contents (VWC) before rewatering. Plants were mowed 5 d a week at 6 mm and grown for 3 mo. Plant lateral spread and number of inflorescences were measured weekly. Lateral spread of the perennial and intermediate biotypes was four times faster than the annual biotype, but the annual biotype had a sixfold higher density of inflorescences than either of the other two biotypes. Lateral spread of all biotypes was reduced by more than 60% at the lowest water level (4% VWC), but, unexpectedly, changing irrigation from the highest frequency (12% VWC) to an intermediate frequency (8% VWC) increased lateral spread of the annual biotype 30% and decreased inflorescence density 25%. These results suggest that less frequent irrigations have the potential to increase growth and decrease seed head development.
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