Building a natural garden arbor or arch like this one is obviously a result of careful planning, patience, and maintenance. The planning for natural elements like this begins at the type of plants you choose. How fast the plants will grow, how big they'll get, density, nutrient and water requirements, and even how long the plants will live are all things that need to be considered.
A natural arch or arbor made from plants will also need to consider the placement of the plants. Being a permanent structure, they need to be exactly where you'll want them from now on. And keep in mind, that when adding an element like this to your garden plans, it will have to be maintained and trimmed from now on.