Because they are genetically programmed to complete their life cycle within a growing season, they very quickly develop the needed roots and foliage. Perennials are a little slower to establish and may require special attention for a month or two. Woody plants such as trees and shrubs are much slower growing and are usually larger to start with, so they will require extra attention throughout their first growing season. If they are planted late in the season, they may need some help early the next season too. If you are planting very large plants of any type, they will need even more care because they have much more plant material to support right away.
Bulbs are the exception.
After going through the initial period of time when you have babied them, plants will develop extra strength by going through slight periods of drought or other stress. But you certainly don't want to push them too far. In the wild, it isn't too crucial if a few trees out of a hundred die from some stress, but losing just one tree can be very important to the home landscape. Before a plant is introduced, you need to determine if the site is right. Of course, the most critical time is going to be right after it is planted.