Responses of potted tulips to novel growth-retarding chemicals and interactions with time of forcing

The effects of the growth retardants ancymidol and 2,3-dihydro-5,6-diphenyl-1,4-oxathiin (coded UBI-P293) were compared on early-, mid- and late-season forced tulip bulbs of cultivars ‘Apeldoorn’, ‘Paul Richter’ and ‘Rose Copland’. The chemicals were applied as single soil drenches, 1 day after transferring plants to the glasshouse. These treatments did not affect flower or petal length nor delay flowering, but high concentrations resulted in increased floral bud blasting in early-season ‘Apeldoorn’ (with ancymidol only) and in early- and mid-season ‘Rose Copland’ (with UBI-P293 only). Both ancymidol and UBI-P293 effectively dwarfed plants of all 3 cultivars at each time of forcing, provided sufficiently high concentrations were selected. However, early- and mid-season ‘Apeldoorn’ plants were particularly responsive to these compounds, the highest doses of which resulted in reductions in stem length at flowering of 70–80% (for ancymidol) or 50–60% (for UBI-P293); similar doses achieved less dwarfing in ‘Paul Richter’ and ‘Rose Copland’ and in late-season ‘Apeldoorn’. In ‘Apeldoorn’ and ‘Paul Richter’, ancymidol reduced extension markedly in the first internode, with less effect distally; UBI-P293 was effective in reducing extension of all internodes. Compost drenches of 3 newer growth retardants were also evaluated. Mepiquat chloride had no effect on flowering except that floral bud blasting was increased in ‘Rose Copland’. The stem responses of ‘Paul Richter’ and ‘Rose Copland’ to mepiquat chloride were trivial, but in ‘Apeldoorn’ the highest dose tested (2500 mg a.i./pot) reduced overall stem length via an effect on first internode extension. A quaternary sulphonium carbamate (coded BTS 44 584) had no effects on flowering or stem extension at concentrations up to 2500 mg a.i./pot. An N -carbamoylimidazole (coded BTS 34 723), tested at 1–50 mg a.i./pot, had small effects on internode extension which depended on cultivar, but potentially useful dwarfing was not observed.