Aqueous solutions of 250 μM methyl jasmonate (MeJA) were sprayed on aerial plant surfaces 4 days before harvest at commercial maturity of five commercial broccoli (Brassica oleracea L. var. italica) hybrids, ‘Pirate’, ‘Expo’, ‘Imperial’, ‘Gypsy’, and ‘Green Magic’, and two kale cultivars, Red Winter (Brassica napus ssp. pabularia) and Dwarf Blue Curled Vates (Brassica oleracea L. var. acephala DC.) in replicated field trials over 2 years. While having no effect on broccoli floret concentrations, MeJA treatments significantly increased total phenolics in kale cultivars over two seasons by 27% and extract antioxidant activity by 31% using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. Partitioning experiment-wide trait variances indicated that the variability in broccoli floret concentrations of total phenolics (74%), quercetin (24%), kaempferol (34%), and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) diammonium salt (ABTS) (66%) and DPPH (62%) antioxidant activity was largely influenced by year-associated environmental factors. In broccoli, the differential accumulation of solar radiation among cultivars resulting from the variation in days to maturity was significantly correlated with total phenolics, ABTS, and DPPH antioxidant activity. Broccoli floret and kale total phenolic, quercetin, and kaempferol concentrations significantly correlated with DPPH and ABTS antioxidant activity. To summarize, total phenolic and flavonoid concentrations and their associated antioxidant activity in broccoli florets were unaffected by MeJA but varied significantly among cultivars and over growing seasons. Apical, compared with basal, leaves in kale were more responsive to MeJA-mediated increases in total phenolics and ABTS and DPPH antioxidant activity.