In checking on the planting and caring of the Gayfeather, no where does it state what to do about the spend blooms. Do you cut the finished blooms off or leave them. Please advise.
Yes, you should dead head your Liatris perennial plant.
When the flowers fade you can use a garden shear to cut the spent flower down to the base of the plant.
This serves to tidy up the plant but can also stimulate additional flowering.
Fertilizing right after pruning with a 'bloom stimulator' fertilizer can actually give you another round of flowers.
Cut the application rate in half and water afterwards to avoid burning the roots.
Cut all spent flowers to the top of the foliage in the fall.