I've heard you have to do supplemental watering, if necessary, their whole first growing season

At the garden I work at we give them 1 year, 2 if the season is very dry, then they are on their own
t

I personally hand water for the first year and slack off the next year if conditions are good for rain.

Barb

I water regularly for a few weeks and only if it doesn't rain. After the first year I only water during long periods of drought. Mostly I water very infrequently but when I do I water deeply. Partly this is because I have a lot of trees and don't want ot encourage their roots to the surface where they compete with smaller plants. Obviously, if something looks like it's thirsty I water it. -Ais.

i should've known! okay, 1 year it is....

The most complete answer is: it depends.

on what the plant is
what time of year you planted it
how big it was at the time (root ball size mainly)
what kind of soil - how water retentive
how much heat and precipitation you get

Rule of thumb for most plants is minimum 1 year's watering after planting. Some plants need longer just because their roots are slow growing, such as camellias. Others will need longer because they're in a dry climate or sandy soil or were tiny little things at planting time.

Here in rainy WA, I can plant many woody things from November through mid February and probably not have to water them that summer IF they're native or very well adapted, and they're in a bit of clay, and we have normal precipitation pattern and no heat waves AND the plant is normally expected to be drought tolerant once established. But spring planted stuff might require a second summer of water. Rhodies and camellias require a few years of watering before they're established.

We do have bone dry summers normally and soil that drains a little too well. In clay soils and summer rainfall areas you might be able to get away with little or no summer water for fall planted stuff its first summer but don't count on it.

Perennials will take hold sooner than shrubs and trees, and the bigger the plant the more supplemental watering it will need, because the more its roots will be disrupted. Certainly for all woody plants water as needed the entire first growing season. For small perennials, maybe only a few weeks, if you get them in before the weather gets hot. Observe the plants--if they wilt a bit during the hottest part of the day but recover quickly, they're fine. If they take a while to recover or remain limp, obviously they need water. If it's only a few outer leaves that are limp, that's probably OK too.