Trimming white pine tries

my white pine tries are now 9' high.I like to keep the tries at this size.

When is the right trimming time? Anything else I should know about.

Thanks for your help. John

 

Regular upright habit with single stem, or weeping?

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Regular upright habit with single stem.

 

Shear new growth in early summer.

 

It can be done as Ron suggests, but it won't look good. Better to remove it altogether, and replant with something else slower-growing.