Hi @Gertrude,
A warm welcome to the Bunnings Workshop community, it is fabulous to have you with us.
I'm not 100% certain what the issue you are experiencing is. Are you just looking to fill in around your Yucca for aesthetic purposes without damaging the trunk? Or is there an issue with water pooling around the trunk?
Do you have any photos of your Yucca that you can share?
As you have said, dirt should not be piled up against the trunk as it can retain moisture, which can eventually lead to rot.
Drainage gravel would be better for this purpose as it will not retain water in the same way.
It should be fine to use gravel assuming you are looking to use it for its aesthetics and ability to allow drainage.
If I have missed the point of your question, please let me know so I can rethink my answer and assist further.
Jacob
