The Mistake

This poem is about a special plant in the garden. He describes the tree as "once had no identity" but now everyone knows about it. Even though the tree was "from Korea" the tree was sent, but now its found in Wales so "now we had something to boast about" now Wales can have something special to represent it and they were proud because they offered the leaf "proudly."
However "our pagan benediction" their religion will "charm away your cold" and acts as a remedy/ healer.
"Who in all of Great Britain processed such treasure" it makes them stand out from everywhere else. But then came a "summer of drought" following their boasting of this 'treasure' came bad things that became "tired of lies" as became untrue. Perhaps implying if you boast too much about certain things in live they will become untrue and that boast will be taken away.
In the last stanza the "parched tree asserted itself, sprouted ordinary walnuts" it carried on as normal, it offered false hope.

This poem may connect with either Larkin's 'First Sight' with the idea that nature can offer a sense of hope and that out of bad things often comes good however it could link to 'Faith Healing' with the idea of a religious healer- as the tree gave people a sense of hope and healing but in the end turned to be a mistake as all the boasting created the idea to become untrue.