The 3 year rule in gardening says perennials and shrubs need three growing seasons to hit their stride: 'first year they sleep, second year they creep, third year they leap.' Year one goes into root growth below ground, year two adds modest top growth, and year three brings the size and bloom the catalog promised.
Practically, that means not ripping out a slow plant after one disappointing summer, and not packing plants tight to make a new bed look full. Space them for their mature size and tolerate the gaps, or fill them with cheap annuals while you wait.
The rule is also why new landscapes photograph badly for the first year or two. If you want a preview of the grown-in look instead of the just-planted look, render your yard with AI landscape design.