Multiple experiments conducted over the past 5 years suggest that greenhouse-grown bedding plants, particularly fall-propagated cool-season annuals, may exhibit reductions in vegetative growth and flowering in response to plant growth regulators (PGRs) applied at rates commonly used by growers. Studies using Viola ×wittrockiana Gams as a model system indicated that paclobutrazol applied at production stages and rates reportedly used by growers could result in significant postharvest residual responses that adversely impacted landscape performance. Most of these rates were also included within the recommended ranges on the agricultural chemical labels. Multiple applications to the same plants during production increased the severity of the residual responses and decreased the rates at which residual responses were detectable in landscape plantings. Tests with additional taxa, Brassica oleracea L. var. acephala DC, Calendula officinalis L., Ipomoea carnea Jacq. subsp. fistulosa (Mart. ex Choisy) D. Austin, Lantana urticoides Hayek `L.S. Red', Lupinus texensis Hook., Plumbago auriculata Lam., Salvia greggii Gray, and Verbena canadensis Kunth `Homestead Purple', PGR formulations and at various times of the year indicate that the postharvest landscape responses to PGRs vary among taxa and seasons. These results strongly suggest that in order for researchers to make responsible recommendations on PGR use, studies must include not only greenhouse or nursery production data, but also subsequent testing for residual responses to the PGRs in landscape settings.
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