Answer: You've listed most of the Meserve Hybrid Holly cultivars. They have similar parentage, but slightly different traits. Blue Boy has a shrubby habit, growing 10'-15' at maturity, and dark green foliage. Blue Girl is a female with red berries but otherwise resembles Blue Boy. Blue Princess is an improved Blue Girl with darker and more abundant fruit and darker bluish green foliage. Blue Stallion is a male with purplish stems and foliage without spines. If you have a choice, the Blue Princess is the closest match to your existing Blue Girl and Blue Boy. Since you already have a male holly, you don't need the Blue Stallion for fruit production.
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