Addendum : Viewing various pics from different arboreta perhaps what I can see in common apart from ubiquitous rocks ( though not always) is the choice of plants. It seems that they are restricted to those that haven't ( yet...) been domesticated heavily. Petunias grow wild in central Argentina. They could be included in a warm rock garden. However their highly hybridized relatives beautifully hanging in baskets somehow seem out of place... Just food for thought.
