Hi @cludelessdiwhye,
Clumping bamboo has a compact, fibrous root system and does not send out long-running rhizomes like running bamboo. When it encounters a solid barrier such as a concrete or masonry retaining wall, the roots generally follow the path of least resistance rather than forcing their way through or causing structural damage. In practice, this means the clump will tend to develop more strongly in the open soil away from the wall rather than pushing into it.
That said, planting distance still matters. Even clumping bamboo needs room for the mature clump to expand, and planting too close to a retaining wall can result in congestion, reduced plant health, or pressure against fences over time. As a general guide, most clumping bamboo varieties benefit from being planted at least 600 to 1000 mm away from hard structures.
If leaning is an issue, selective pruning of older, heavier, or outward-leaning culms can reduce top weight and wind load, which can significantly improve stability and encourage more upright new growth. Removing a small number of mature culms at ground level also opens the clump, allowing light and air through and helping the plant balance itself rather than leaning toward open space.
To summarise, it is unlikely that your bamboo will damage the retaining wall structurally, but you should still plant it with a gap of around 600mm from the wall if possible. If this isn't possible given the circumstances of your garden, then you can reduce the lean by removing the culms that are leaning, which will encourage energy to be sent to the straighter growing culms, as well as allowing for new growth which can be trimmed to suit.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
