PERSISTENCE OF PLANT GROWTH REGULATOR EFFECTS ON PERENNIAL PLANTS IN THE NURSERY

There are still many unknowns in the production of containerized herbaceous perennials. We are screening herbaceous perennials for their response to several different plant growth regulators (PGRs). Under early summer conditions, the grower would like to know how long PGRs will hold back plant growth in the container. Young, uniform plants of Heliopsis, Gaura, and Hypericum selected from nursery stock in May 2000, were treated with one of the following foliar applications: 5,000 ppm daminozide (applied twice), a tank mix of 5,000 ppm daminozide and 1,500 ppm chlormequat, 500 ppm ethephon (applied twice), paclobutrazol at 0, 40, 80, 120, or 160 ppm or uniconazole at 0, 15, 30, 45, or 60 ppm. Plant height of Heliopsis helianthoides ‘Summer Sun' was not significantly affected by paclobutrazol or uniconazole treatments, but plants treated with daminozide or the tank mix were less than half the height of the untreated controls. These height reductions persisted through 12 weeks after treatment (WAT). Ethephon caused a 15% to 18% reductions in plant height that persisted through 12 WAT, but did not delay flowering. Gaura lindheimeri ‘Corrie's Gold' was very responsive to all the tested PGRs but the effects of paclobutrazol were more persistent in the nursery container than those of uniconazole. Ethephon reduced plant height over 25% at 4 and 6 WAT, but was no longer significant at 8 WAT. Hypericum calycinum was not responsive to daminozide, the tank mix, ethephon or paclobutrazol treatments, but uniconazole treatments resulted in linear or quadratic reductions in plant height that persisted through 8 WAT. These results suggest that PGR rates may be selected to provide growth control for up to 12 weeks under nursery conditions.

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