Although most roses need 6 or more hours of sunshine to thrive, some bloom with only 4 hours of sunlight and withstand the fungal diseases that flourish in shade. Growing even shade-tolerant roses in partial shade, however, can decrease the number of blooms. Varieties with glossy foliage may resist black spot and mildew better because fungal spores cannot fix themselves to the slick leaf surfaces. Make sure that partially shaded roses get enough warmth for their petals to open fully. Shade-tolerant roses usually have blooms with 5 to 20 petals. Few roses with more than 40 petals will bloom well in low light. 'Zephirine Drouhin' and Flower Carpet roses offer some degree of shade tolerance, but both bloom better and are more disease-resistant in full sun. Rosa glauca, an old-garden rose, also tolerates shade. It's grown more for its purple-tinged grayish leaves and red stems than for its 5-petaled pink flowers and handsome red hips, which will be sparse in shady conditions.