Many growers in the Niagara Peninsula of Ontario are reluctant to use canopy division to address high vigor in their vineyards. An alternative approach of delayed shoot thinning was explored. Pinot noir and Cabernet franc vines in Ontario were subjected to six different shoot-thinning timings, based on the Eichhorn and Lorenz phenological stages 9 to 31, during the 2001 and 2002 seasons. An additional double-prune treatment (retaining four canes at dormant pruning and removing two at fruit set) was imposed on Cabernet franc. Minor differences between treatments were observed in 2001 and 2002 with respect to yield, periderm formation, and vine size. However, the study was carried out primarily to assess the impact of these treatments on berry, must, and wine composition and canopy microclimate. Early shoot-thinning treatments on Pinot noir resulted in increased titratable acidity (TA) and soluble solids in berries and must. Early shoot-thinning treatments on Cabernet franc generally resulted in higher soluble solids and color intensity in berries, lower TA in musts, and higher color intensity, total phenolics, and total anthocyanins in wines. The double-prune treatment was characterized by higher soluble solids, hue/tint, color intensity, and total phenolics overall. Canopy assessment showed that treatments imposed after bloom produced lower leaf layer numbers and better leaf and cluster exposure than did the control and early shoot thinning, especially in Cabernet franc. Early shoot-thinning treatments induced higher leaf areas in both cultivars compared to later treatments. Double pruning resulted in lower leaf layer numbers and better leaf exposure and had no impact on mean leaf area or photosynthetic photon flux density. Despite reductions in canopy density, delayed shoot thinning appeared to lead to concomitant delays in fruit maturity in terms of soluble solids, anthocyanins, and phenolics. We recommend that you consult the full text of this article, which was published in the American Journal of Enology and Viticulture 56: 343-356. To see the full text of this paper, follow the link on the right.