Allan Brito found an amazing library of freely (donation-based) downloadble PBR vegetation textures.
In landscape architecture, you need a way to create vegetation using either textures or 3d models. For both scenarios, you assets library will be essential to achieve good results quickly.
That is a great reason to start building your asset library as quickly as possible because you never know when you might need to work in landscape architecture. It doesn't even have to be a landscape architecture only project. Just a garden for an external visualization might give you headaches to find quality assets.
Do you want to download some free PBR textures of leaves and vegetation? And add them to your library?