Hi, 

Perennials and shrubs/trees should be fine to start planting now. If they are fully leafed out or have emerged from the base (coming from a garden center), you can just watch from below freezing nights and cover them with a sheet or tarp. Many garden centers in Maryland get their stock from more southern states, or they have been grown in a greenhouse and not fully hardened off to our zone yet, so you will want to protect the new growth. 

Any frost now until warm weather fully gets here could damage the foliage or flower buds, but will not likely kill the roots or plants. It may just delay or prevent blooming. 

It is good, though, to plant now to help get roots growing and established before the heat of the summer, which will reduce transplant shock.

Do you have a list of plants you are planning on or specific questions about a certain species?

Emily