Ageratum houstonianum ('Blue Horizon') produces 3- to 4-inch cushions of tiny blue flowers on 30-inch-tall plants. Free from insects, diseases, and pests, this cultivar is perfect for the annual border or as a cut flower. Plant 'Blue Horizon' in full sun and moderately fertile, well-drained soil. Various species of bees, wasps, and flies find the flower nectar nourishing.

This native of the Americas contains precocenes 1 and 2 (insect growth regulators), which interfere with juvenile hormone activity (precocious metamorphosis in insects). It also produces an oil that is toxic to the fungal plant path on Fusarium wilt. While scientists apparently have not studied the efficacy of planting Ageratum among vegetables and ornamentals prone to Fusarium infections, it can’t hurt.