What is a bract? A bract is a part of a plant with which many people are quite unfamiliar. Did you know that all of the below images show bracts? The petal-like structures in the first six images are actually bracts, and in the last three which show conifer cones, the spiky/protruding parts are bracts.
According to my Shorter Oxford English Dictionary volume 1, ‘bract’ is a botanical term which describes a ‘leaf or scale, usually small, growing below the calyx of a plant’. In other places a bract is defined as a ‘modified leaf’.ref In angiosperms (flowering plants), it is part of an ‘inflorescence’ which is the entire flower head, including the stem, stalk, bract and the actual flower.ref In gymnosperms (conifers), bracts can be found on/in cones.
So a bract in reality is its own separate type of plant organ, and not a petal or a leaf. It plays its own role in supporting the plant’s development, which can be wide and varied depending on the species. Different studies have identified bracts to be responsible for protecting the flower from herbivoryref as well as the weatherref, for producing defensive compounds which kill insectsref, for flowering (or not)ref and for attracting pollinators.ref One study found that the presence of bracts is determined by the IFY gene, which either allows a bract to form, or recruits the cells which would have been used for the bract into the flower instead.ref
So what is the relevance for bonsai? Well, bracts are part and parcel of any flowers or cones that you have on your bonsai tree and they develop only once, as part of the inflorescence, or cone. This means that like flowers, fruit and cones, they cannot be reduced in size* in the same way that leaves can, because reducing leaf size is usually done by pruning or defoliation. If you remove the inflorescence or cone, you’ll need to wait until the plant regenerates a whole new reproductive organ and it likely won’t be reduced in size.
This needs to be considered if you want to bonsai a Cornus, a Davidia involucrata or a Bougainvillea. Varieties with naturally smaller bracts are preferable for bonsai, just like varieties with smaller fruit are better. Poinsettia are never going to make good bonsai, even though in their native Mexico they grow up to 3m tall.ref But a species with smaller bracts, such as a hop hornbeam (Ostrya carpinifolia), lime/linden and hornbeam, can be very pretty. In these species you need the tree to be at the reproductive phase in order to produce inflorescences (which include the bracts).
* This isn’t strictly true as there are a number of actions which can reduce the size of flowers & fruit, such as underwatering, and leaving as many fruit on the tree as possible, but that’s a topic for a different post.ref