While both providing flowers for a garden, make no doubt that a bottlebrush (Callistemon spp.) and a bottlebrush buckeye (Aesculus parviflora) are not the same. Any number of 25 different species of bottlebrushes exist, either as evergreen trees or shrubs. They received such similar names because their flowers are displayed in clusters or spikes that mimic the look of the cleaning utensil used to scour the insides of glasses and mugs.
Plants generically called "bottlebrushes" are native to Australia, where they grow in the openings of woodlands in bright light. The "bottlebrush buckeye" is a name assigned to a particular shrub native to the shaded parts of woodlands in the southeastern United States
While both bottlebrushes and bottlebrush buckeyes are both angiosperms (flowering plants), they are not closely related. Bottlebrush belongs to the eucalyptus or myrtle family, Myrtaceae while the bottlebrush buckeye is a member of the horse chestnut family, Hippocastanaceae.
Bottlebrushes have evergreen leaves that are lance-shaped and light to dark green on plants that range from 5 to 30 feet tall. Two species of bottlebrushes are often encountered in gardens: crimson or lemon-scented bottlebrush (Callistemon citrinus) and lemon bottlebrush (Callistemon pallidus). From late spring to autumn, these plants bear clusters of tiny flowers that are red, yellow, or hot pink on branch tips. Each blossom has many long whisker-like stamens that, when seen bunched with hundreds of others, create a cylinder-like flower head.
Bottlebrush buckeye is a deciduous shrub growing up to 10 feet tall and no more than 15 feet wide. Its green leaves are compound with five to seven leaflets that together make a hand-like leaf. In summer, the branch tips bear upright flower clusters called panicles that comprise white flowers with spidery stamens. In autumn, the leaves turn bright yellow and gold.
Bottlebrush plants native to Australia generally demonstrate limited tolerance and ability to survive prolonged winter subfreezing temperatures. Typically they are most commonly grown where winters are relatively frost-free or cold but mild and short--in USDA hardiness zones 8 and warmer. Bottlebrush buckeye survives much colder and longer winter dormancy periods, appropriate to grow in hardiness zones 5 through 9.
Grow any species of bottlebrush in a moist but well-draining soil that is neutral to acidic in pH. The soil need not be highly fertile but certainly should not be nutrient-lacking sand. The plants prosper in full-sun conditions, receiving at least 6 hours of direct sun rays daily year-round. They tolerate harsh pruning and will readily rejuvenate when pruned after flowering ends.
Bottlebrush buckeye grows best in deep, fertile soils rich in organic matter that are moist but well-draining. Avoid alkaline soils. These shrubs can be grown in full-sun exposures if summers are not too hot and the soil never becomes too dry, or in partially shaded locales such as under the branches of taller trees or buildings. Prune this shrub, if needed, only in late winter when dormant.