The answer to 'why is my retaining wall weeds' is that weeds can grow on retaining walls for a few common reasons. Weeds may take root in the soil or cracks in the wall, or seeds may blow in and germinate. Inadequate drainage, poor soil preparation, or lack of maintenance can also contribute to weed growth on retaining walls.
To address this issue, you'll want to regularly inspect the wall and remove any weeds you find as soon as possible. Applying a pre-emergent herbicide can help prevent new weeds from sprouting. If the weeds are persistent, you may need to address underlying issues like poor drainage or deteriorating mortar. In some cases, it's best to call in a professional landscaper to properly maintain or repair the retaining wall.