For timing, consider the area of your property where you wish to plant. Sunnier areas can accept annuals earlier than cooler shaded spots. Look at the perennials in your garden. Some hosta still look like asparagus poking out of the ground, while in otherspots they are in full circular bloom. Plant in the latter, sooner.
Zones 6a to b throughout April and May. Those tiny upstarts can't swim yet, so sitting in a pool of accumulated precipitation just isn't condusive to good growth.
Eastern Market (go after 2:00 pm for the best deals) or some of the local grower's greenhouse. Locally grown plants have a greater likelihood of thriving. Newer non-native favorites like Angelonia and Cleome should wait untilmid June.
Even if the plants are local, it's still best to "harden" them off. This takes only a couple days, and involves no more than setting them out during the daylight hours, and bringing them in at dusk. They can sit in the garage at night.
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