The answer to 'why is my retaining wall invasive plants' is that invasive plants can often take root and grow through or around retaining walls. This is a common problem caused by the wall's soil and moisture conditions, as well as the wall's structural design. Invasive plants like ivy, bamboo, or weeds can exploit cracks and gaps in the wall to establish themselves, eventually causing damage and destabilizing the wall.
To address this issue, you'll need to remove the invasive plants and take steps to prevent them from returning. This may involve repairing any cracks or gaps in the wall, applying a barrier or sealant, and carefully selecting non-invasive plants for the area around the wall.