The 3 year rule in gardening is a saying, which describes the growth stages of new perennials.
The 3 year rule in gardening describes the growth stages of new perennials as the first year they sleep, the second year they creep and the third year they leap.
The 3 year rule and principle in gardening sets realistic expectations for gardeners by way of indicating that a plant will focus n root establishment in it's first year.
Then the plant will show slow but steady growth in it's second year and then in it's third year, the plant will reach it's full and vigorous potential.
In year 1 of the 3 year rule in gardening, the plant focuses it's energy on developing a strong root system in order to anchor itself in the soil, with little visible growth occurring above ground.
In year 2 of the 3 year rule in gardening, the roots of the plant will continue to spread, and you will also begin to see more substantial top growth, with more leaves and also potentially some flowering.
And the plant might also be still less dense than it's mature form.
And in year 3 of the 3 year rule of gardening, the established root system of the plant is now fully developed and can now support vigorous new growth.
And you can also expect the plant by this time to reach it's full size and potential and filling out it's space.