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Japanese spirea (Spiraea japonica L. f.), a popular landscape shrub, has shown the potential to become an invasive weed in both North America and Europe. Twenty commonly available S. japonica cultivars were evaluated for fertility using pollen and seed germination. Clones were grown in a randomized, replicated field plot, and additional seed samples were obtained from commercial nurseries and hand-pollinations in the greenhouse. Three sterile cultivars were identified: ‘Crispa’, ‘Dart's Red’, and ‘Neon Flash’. These cultivars demonstrated poor anther dehiscence and very low mean pollen germination, 2.7%, 3.0%, and 1.3%, respectively, which often produced abnormal pollen tubes. None of these three cultivars produced viable seed in the field plot, at commercial nurseries, or when hand-pollinated in the greenhouse, whereas seed germination from fertile clones ranged from 91.5% to 100%. The other 17 cultivars tested, which should be treated as entirely fertile for the purposes of invasive plant management, were ‘Albiflora’, ‘Anthony Waterer’, ‘Candlelight’, ‘Dakota Goldcharm’, var. alpina ‘Daphne’, ‘Flaming Mound’, ‘Flowering Choice’, ‘Froebelii’, ‘Golden Princess’, ‘Goldflame’, ‘Goldmound’, ‘Gumball’, ‘Lemon Princess’, ‘Little Princess’, ‘Magic Carpet’, ‘Norman’, and ‘Shibori’. Measurements of DNA content indicated that all tested clones are diploid; therefore, the observed sterility was not related to polyploidy. The identification of these three sterile cultivars can help reduce the use of fertile varieties in areas where Japanese spirea has shown the potential to become invasive.

A significant pest affecting commonly planted Betula spp. is the birch leafminer (Fenusa pusilla Lepeletier), an insect that can be present in large populations in the landscape and can greatly reduce the vigor and ornamental value of these trees. Twenty-two interspecific crosses were performed between leafminer resistant and susceptible Betula species in an attempt to create the novel combination of ornamental white bark and significant leafminer resistance. Of the nine successful crosses, two produced resistant offspring. Progeny of the diploid × hexaploid cross B. turkestanica Litvin (2x) × B. alleghaniensis Britt. (6x) displayed a broad range of resistance levels, likely the result of segregating alleles contributed by the hexaploid parent. All crosses involving highly resistant individuals of B. costata Trautv. (2x) yielded leafminer susceptible progeny. These results suggest that the larval antibiosis demonstrated by B. alleghaniensis and B. costata is inherited as a recessive trait, and exhibits a gene dosage effect as evidenced by the B. turkestanica × B. alleghaniensis offspring. While most progeny of the B. populifolia Marsh (2x) × B. maximowicziana Regal (2x) cross were susceptible, a single resistant offspring, which was found to be triploid (3x), displayed a mechanism of resistance similar to that of a hypersensitive response. No strong intersectional barriers to hybridization were observed and all interploidy crosses were successful. The chromosome numbers of B. costata (2n = 2x = 28) and B. turkestanica (2n = 2x = 28) are reported here for the first time. The results of this study indicate that the potential exists for the development of insect resistant, ornamental white-barked birch clones through the implementation of a planned, systematic breeding program.