Technical Abstract: Broccoli (Brassica oleracea L. Italica Group) is considered a cool season vegetable requiring relatively cool conditions (e.g., <23°C) to induce and maintain vernalization and to allow normal floral and head development to proceed. In general, this requirement is a major limiting factor to production of broccoli in eastern states where prolonged cool seasons are not common. The goal of the current study was to compare performance of three experimental broccoli hybrids developed at the USDA, ARS, U.S. Vegetable Laboratory (USVL), Charleston, SC, that were selected for adaptation to hot summer conditions, to some important commercial broccoli hybrids (e.g., ‘Packman’, ‘Marathon’, ‘Gypsy’) in field trials conducted in high temperature environments (summer) in South Carolina and New York, as well as in more conventional growing environments (e.g., in fall) at those locations. All experimental and commercial hybrids produced marketable heads with high quality ratings in all fall field trials. The hybrids ‘Marathon’, ‘Greenbelt’, ‘Arcadia’, and ‘Patron’ failed to produce broccoli heads at all in SC summer trials. The remaining hybrids produced heads with similar mean head mass, stem diameter, and bead size in the South Carolina summer trials, but the three experimental hybrids produced marketable quality heads, while ‘Gypsy’ and ‘Packman’ did not. The primary flaws in ‘Gypsy’ and ‘Packman’ heads were increased yellow color, flattening of the dome, increased roughness, and non-uniformity of bead size. In the New York trials a subset of the hybrids tested in SC were evaluated and all of them produced heads. However, ‘Packman’ and ‘Marathon’ produced relatively poor quality heads when maturing in summer and better quality heads when maturing in fall. The experimental hybrids exhibited more consistent quality across different maturation times in the New York tests. Results of this research indicate that different broccoli hybrids respond very differently when grown in summer conditions at two eastern locations. Prominent hybrids like ‘Marathon’ and ‘Patron’ are not adapted to extreme summer conditions of SC because they will not be effectively vernalized, and will therefore not head. Others such as ‘Gypsy’ and ‘Packman’ will head, but the non-uniform arrest of bud development results in a rough-appearing curd in which flower buds are at various stages of development. The experimental USVL hybrids, which are all single crosses of inbreds selected for adaptation to hot summer conditions in SC, appear to represent a unique type of broccoli that does not require cool temperatures for development of inflorescences or high quality heads. Consistent performance of these latter hybrids across summer plantings in SC and NY indicate that these experimental hybrids may be generally adapted to a range of summer conditions and locations.