Please see the attached link for the last frost dates in your area. http://extension.umd.edu/learn/spring-frostfreeze-dates-maryland
You can usually begin setting out transplants of warm season crops like annual flowers, basil and tomato around Mothers Day or later. They will not start to put out a lot of growth until the soil warms. Be prepared to cover plants with a tarp or light blanket if frost is expected.