Key message Large aerial roots grow out from the branches of injured Dracaena draco trees. They integrate with the trunk or cause the branches to break off the tree and deform it. Abstract Dracaena draco, the dragon tree, is an iconic monocot of the Canary Islands with a tree-like growth habit and some distinctive features that are unique in the plant kingdom. We report about the massive aerial roots in this tree. They appear on trees that are injured or under environmental stress and affect growth form and the whole life of the plant. We analysed the growth of these roots and tested our findings in experiments on plants. Clusters of these roots emerge from the bases of the lowest branches and growing down they may reach the soil. Descending along the trunk, they cling tightly to the trunk, integrate with it and contribute to its radial growth. This may explain (1) why the trunk of a mature D. draco tree looks fibrous, as if made of many individual strands, and (2) how some trees reach enormous radial dimensions. Alternately, a large, 2-5 m high, multi-segmented branch with aerial roots at its base, may break off the tree and grow on its own, as a mammoth off-cut, perhaps the largest known in the plant kingdom. This detachment would cause a significant reduction in the size of the crown and deform its original, highly organized pattern of branching. In the extreme condition this may result in the destruction of the mother plant.

Key message Sunlight is a key environmental factor in growth, flowering and shaping of the Dracaena draco tree. Unidirectional light deforms the tree and may cause it to tilt. Abstract Dracaena draco, a tree-like monocot, lives in cycles of vegetative growth and flowering. The cycles, as well as the tree growth form, are under genetic control. What controls their length has been unknown before. We propose that it is sunlight. Our trees of the same origin, growing for 20 years in the garden in varying sunlight conditions, started to flower when 9-12, 16 and 18-19 years old, for those growing in full sun, part shade and shade, respectively. In full sun, they grow shorter trunks than those in shade, catching overhead sun. Their branches also had shorter or longer growth and flowering cycles depending on sunlight availability. D. draco tree exhibited strong phototropic response and its crown was organized by the direction of growing tips. In full and in overhead sun, it had a regular form but asymmetrical in unidirectional, oblique sunlight. An asymmetrical crown and the absence of reaction wood may cause the D. draco tree tilting and progressive loss of balance.