Problem: Container grown hybrid rhododendrons produced for garden center and spring landscape sales have their value enchanced by the presence of flowers or flower buds. However, certain cultivars are difficult to "bud" as young plants under commercial production conditions.

Possible Solution: Spray with an anti-gibberellin to limit late season vegetative growth, increase foliar pigmentation and the number of flower buds per plant.

The Test: The same test was conducted at different commercial container nurseries in the piedmont (USDA Hardiness Zone 7b) and mountains (Zone 6b) of North Carolina. All plants were sprayed with either 0, 50 or 100 ppm uniconazole-P (Sumagic). Plants in the piedmont were larger and growing in 3 gallon containers while those in the mountains were growing in 2 gallon containers.

Previous research had demonstrated that mid to late July is the best time to spray. Earlier spraying does not allow plants to conclude a growth flush. Later spraying will stop the second growth flush plus no flower buds will develop. Plants were sprayed as the first growth flush had finished and just as the second growth flush was starting. Cultivars tested were 'Chionoides,' 'English Roseum,' 'Nova Zembla,' 'Purpureum Elegans' and 'Roseum Elegans.' There were 10 indivdual plant replicates per treatment (30 plants per cultivar per nursery).

Results: There was no phytotoxicity from any of the treatments and no significant height increase for any of the plants following treatment. All flower buds opened normally with no deformity of flowers the following spring.

1. Similar results occurred at different nurseries under different growing conditions.
2. All five cultivars had a significantly increased number of flowers when sprayed with as low as 50 ppm Sumagic in mid July.

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North Carolina State University