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

Abstract

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. INTRODUCTION Although additional research is increasing, there are still many unknowns in the production of containerized perennials. Increasingly, they are being grown alongside the annual bedding plants as a larger proportion of the Spring plant mix. However, when perennials start their rapid growth phase and the grower starts looking for effective methods of growth control, there is still a large information gap between annuals and perennials. Here at Virginia Tech, we are continuing to screen popular perennials for their response to several different plant growth regulators (PGRs) including B-Nine (daminozide), Cycocel (chloromequat), tank mixes of B-Nine and Cycocel, Bonzi (paclobutrazol), Sumagic (uniconazole) and Florel (ethephon). We evaluate several different rates of application in an effort to find the linear range of plant response to the PGR. This approach helps us to narrow down the recommended application rates in fewer steps. Generally, for Spring production and sales, excessive persistence of the growth reduction in treated plants is unacceptable (Latimer et al., 1999; Thomas et al., 1998). The customer wants the plant to grow out when planted into the landscape. However, the grower would like to know how long the PGR will hold back plant growth in the pot for those plants that do not get sold in the Spring. Effective use of PGRs would reduce or delay pruning of perennials over the late Spring and Summer. To evaluate the length of effective growth control, the following study was undertaken at Riverbend Nursery in Riner Virginia. Proc. XXVI IHC – Elegant Science in Floriculture Eds. Th. Blom and R. Criley Acta Hort. 624, ISHS 2003 Publication supported by Can. Int. Dev. Agency (CIDA) 230 MATERIALS AND METHODS Uniform plants were selected from nursery stock in May 2000. Plants were growing in 12-cm plastic pots filled with Scott’s Perennial Mix (O.M. Scotts, Maryfield Ohio). Plants were treated with one of the following foliar applications: 5,000 ppm BNine (applied twice), a tank mix of 5,000 ppm B-Nine and 1,500 ppm Cycocel, 500 ppm Florel (applied twice), Bonzi at 40, 80, 120, or 160 ppm or Sumagic at 15, 30, 45, or 60 ppm. B-Nine and Florel were applied at the initiation of the experiment and again at 2 weeks after the initial treatment (WAT). All other treatments were applied once. All treatments were applied to young, rapidly growing plants using a handheld CO2 pressurized sprayer at the label-recommended volume of 210 ml/m. After treatment, the plants were maintained by Riverbend Nursery staff under normal nursery conditions, in full sun on ground cloth beds equipped with overhead irrigation. Each species was set up in a randomized complete block design with 10 single plant replications for each PGR treatment. Vegetative plant height was measured at 2week intervals up to 12 WAT or until height reductions were no longer significant at a probability of P = 0.05. RESULTS AND DISCUSSION Two of the species tested, Nepeta faassenii ‘Six Hills Giant’ and Saponaria ocymoides ‘Snow Tip’, showed no response to the PGRs applied. Asclepias incarnata was not responsive to Bonzi or Sumagic at the rates tested, but B-Nine, the BNine/Cycocel tank mix and Florel treatments reduced plant height moderately (13% to 18%) at 2 and 4 WAT with no significance beyond this (data not presented). Height of Heliopsis helianthoides ‘Summer Sun’ was not significantly affected by Bonzi or Sumagic treatments. However, Heliopsis was very responsive to the B-Nine, BNine/Cycocel tank mix and Florel treatments (Table 1). At 4 WAT, plants treated with BNine or the B-Nine/Cycocel tank mix were only half the height of the untreated controls. Height reductions persisted through 12 WAT. In addition, B-Nine, and to a lesser degree the B-Nine/Cycocel tank mix, delayed flowering. Lower application rates of B-Nine should be tested for less persistence. Florel caused a 15% to 18% reduction in plant height but did not delay flowering. Hypericum calycinum was not responsive to B-Nine, the B-Nine/Cycocel tank mix, Florel or Bonzi treatments, but the higher rates of Sumagic reduced plant height 20% to 25% through 8 WAT (Table 2). Gaura lindheimeri ‘Corrie’s Gold’ was very responsive to all the tested PGRs but the effects of Bonzi were more persistent in the nursery container than those of Sumagic (Table 3). However, maximum height reductions for Bonzi treated plants were only 20% to 24%. The B-Nine and B-Nine/Cycocel tank mix treatments caused significant but small (~15%) reductions in plant height, but Florel reduced plant height over 25% at 4 and 6 WAT. These results provide starting rates for testing PGRs under a producer’s specific growing conditions. These results show that even the less active compounds like B-Nine can significantly hold back plant growth over 12 weeks in the container for a responsive plant. It has been widely stated that plants grown outdoors under nursery conditions require higher rates or more frequent applications of plant growth regulators to control growth. Rates and PGR selections are dependant on the plant species and sometimes even on the cultivar selected. A grower must first decide how long he wants the plants to be held back i.e., how much persistence is he looking for? What is the anticipated sale date? Growers should be reminded that excessive persistence in customer’s hands is not desirable. Growers should select their PGR and the rate applied based on the information available on persistent responses to the chemicals. There continues to be much research on perennial plant responses to PGRs. Current results have been recently summarized and should be used as initial test points for grower trials (Latimer et al., 2001).