Dahlia Blue Boy is a tuberous-rooted decorative dahlia forming an upright, bushy clump about 90–120 cm high and 50–100 cm wide. Dark stems and mid-green pinnate leaves support neat, rounded, fully double flowerheads 15–20 cm across, composed of flat to slightly incurved florets in light blue-purple, lavender, and lilac-pink tones.
Grow it in full sun in fertile, humus-rich, moist but well-drained clay, loam, or sandy soil. Enrich the bed with organic matter, pinch the growing tips, water freely in dry weather, and deadhead regularly from midsummer to the first frost. In zones 9–11 tubers may overwinter with suitable drainage and protection; in colder regions lift them after frost, dry them, and store frost-free for spring replanting.