We can always tell if a plant is in need of water based on its appearance. Specifically, we see the need in the color of the plant’s foliage. However, not all plants are created equally in this respect. Trees in particular do not show their need for water until they have either become stunted for the rest of their lives, or they slowly die off. A lot of this has to do with the storage capacity of a tree for water. Trees gather and conserve water for use during drought periods so that it can be used when it is not naturally, or manually receiving precipitation. Shrubs, turfgrass, perennials and annuals show their need for water much faster.
This leads to my hypothesis which is: An urban street tree has a greater success rate when it is planted with annuals, perennials or other small plant material than when it is located in isolation.