The effects of abscisic acid (ABA) form, concentration, and application timing on bud cold hardiness, phenology, and fruiting performance of Merlot grapevines (Vitis vinifera) were evaluated in a three-year field trial with site locations in British Columbia and Ontario, Canada, and in Washington and Idaho, United States. Solutions containing natural S-ABA (ABA_N) and a purported long-lived ABA analogue 8′-acetylene ABA (ABA_A) at differing concentrations were applied to the vine canopy at veraison or post-harvest. Postharvest foliar applications of ABA_N at concentrations greater than or equal to 5000 ppm tended to advance leaf abscission and increase autumn bud cold hardiness. Postharvest foliar applications of ABA_A at 1000 ppm tended to delay budbreak in the spring following application and increase spring bud cold hardiness. Fruit yield and basic composition were affected little by the ABA treatments. Bud hardiness was enhanced by ABA_N mostly in autumn and by ABA_A mostly in the spring, indicating that the suitability of ABA forms for reducing bud damage would depend on when injurious cold events are more likely.