Our neighbours have a 'Tree of Heaven' which has produced several
suckers on the roadside verge on the outside of their garden wall.
What is the best time and method to "rescue" some of these treelets ?

They are doomed anyway - as the council strip the verge of vegetation
to within an inch of the soil in June/July and to hell with the
wildlife that lives in it.

The parent tree is becoming a bit straggly, it hasn't been pruned for
at least three years - does it need regularly pruning to flourish or
will it survive without ?

Thanks

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