Many redbud species have flowers that bloom directly on the trunk and branches. Both eastern redbud (Cercis canadensis) and Texas redbud (Cercis canadensis var. texensis) exhibit this growth habit. Being confronted with this phenomenon the first time can be a head-scratcher, but rest assured redbuds were designed this way…although the reasons why are uncertain.
The redbud is cauliflorous, meaning that it is normal behavior for flowers to appear on the trunk and stems. Cauliflory is derived from Latin, and means ‘stem flower’.
There are estimated to be over 100 species of plants that are cauliflorous. Theories as to why certain species of plants exhibit this behavior include the belief that pollination may be easier for some insects on the sturdier trunks. A large beetle required for pollinating a tree might have a hard time with flowers at tips of wispy branches, but what if the beetle could simply walk up the trunk to the flower?
In the case of redbud trees, the need to accommodate large pollinators is most probably not the case as the pollinators present in the redbud’s natural habitat (bees for example) are quite capable of pollinating the thousands of other species of plants present in the same landscape as the redbud. Trees with heavy fruits could benefit from the sturdiness of trunks and branches, but again, the flowers and eventual seedpods of Cercis canadensis are smallish and light.
Regardless of the reasons, cauliflory is exhibited in both young and mature redbuds, so you will be able to enjoy this unusual growth habit throughout the life of your tree in the garden.
Cauliflory is relatively rare in the plant world, but it is found among many different plant species from widely varying habitats. Certain fig trees, calabash trees, papaya, and notably cacao (from which chocolate is derived) are cauliflorous. A fantastic article (and partial source for this write-up) that goes into more detail about different cauliflorous species can be found at the Wisconsin Master Gardener Program.
In Louisville, we have many redbuds and quite a number bloom cauliflorously. But I had never seen anything like the mature specimen I ran into last April. I could barely embrace its trunk with my arms, it was at least 35-40′ tall, and it was COATED with a magnificent velvety furze of blossoms–a pink trunk down to the roots (with a few bare spots of course!) and limb and branches encrusted with pink. Absolutely amazing, and no wonder people new to the Americas sang of these trees.
Our family place in South Carolina had a giant redbud tree, planted by my grandfather circa 1898-1899. It had a massive trunk and was at least 35 feet tall, and bloomed riotously in early spring.