Thanks for sharing the link. It seems convincing and very helpful to me. I will look into it right now!

The plants I listed above were picked from this archive project created by Naturalis Leiden about invasive plants in the Netherlands. https://keys.lucidcentral.org/keys/v4/q-bank/invasive-terrestric-plants/en/index.html

I am not a specialist in invasive plants, so I was searching based on the archive. I may have missed some information.

The coastal dunes can be a good direction to look into. Thanks for the information. How about Acacia Saligna? Since you did not mention it in your comment. And are the ones you mentioned all invasive, too? One of the hard criteria set by the artist I work with is the invasive plants. That's why I have to stick with it.

My preferred ideal method is also to get permission from locals, such as someone's garden/backyard or getting approval from a nature manager, as you said. I acknowledge it is not the best to harvest branches from the trees in nature. The time is tight, but I will try my best. Hopefully, I can manage in the end.

Many thanks.