Response of Herbaceous Perennials to Growth Retardants Applied at Different Developmental Stages when Grown Under Night-Interrupted Lighting Outdoors in the Southern United States

Abstract

A study was conducted in 2004 and 2005 to determine how ‘Early Sunrise’ coreopsis (Coreopsis grandiflora Hogg ex Sweet. ‘Early Sunrise’), ‘Moonbeam’ coreopsis (Coreopsis verticillata L. ‘Moonbeam’) and ‘Goldsturm’ rudbeckia (Rudbeckia fulgida Aiton ‘Goldsturm’) responded to two plant growth retardants applied at three stages of plant development (SOD) when plants were exposed to night-interrupted lighting (NIL) while grown outdoors under nursery conditions in the southern United States. With few exceptions, height of all species was suppressed by 2 weeks after first treatment (WAT) compared to NIL only, regardless of whether 5000 ppm B-Nine or 20 ppm Sumagic was applied at the beginning of a period of rapid shoot elongation (SOD 2) or 2 weeks prior to (SOD 1) or following this stage (SOD 3), and continued throughout the plants’ vegetative phase, although the level of suppression varied with SOD and PGR. Applying either PGR at SOD 2, when rapid shoot elongation first began, was most effective in suppressing vegetative height of ‘Early Sunrise’ coreopsis and ‘Goldsturm’ rudbeckia, whereas vegetative height suppression of ‘Moonbeam’ coreopsis was not affected by SOD when Sumagic was applied, but suppression was greatest when B-Nine was applied at SOD 1, two weeks before rapid shoot elongation began. ‘Early Sunrise’ coreopsis and ‘Goldsturm’ rudbeckia were shortest at first flower when PGRs were applied at SOD 2 and SOD 3, respectively, but plant height of ‘Moonbeam’ coreopsis at first flower was not affected by SOD. Where differences in height suppression occurred at first flower, B-Nine was more effective than Sumagic. Plant SOD when PGRs were applied had no effect on flowering or plant quality of ‘Moonbeam’ coreopsis and minimal effect on ‘Early Sunrise’ coreopsis or ‘Goldsturm’ rudbeckia.