[mdjlucan]

When do I cut the flower heads off from last year? Thanks

[Subversive_plot]

   Do you mean Hydrangea? 
You can cut Hydrangeas off as cut flowers when they are in peak color, or any time after that.

[mdjlucan]

I meant last year's flower heads

[mumofstig]

Still assuming you mean Hydrangea (not Hydronium!) I like to leave the old flowers on until after the last frosts, as they provide a bit of protection for the new growth.