Generally your nurseries aren't going to ship until closer to the average last frost date in your area. Even for hardy plants, if they have been acclimated to & started growing in a greenhouse (rather than being subjected to 'normal' Spring fluctuations of temperature) the foliage may be more tender than normal & could suffer damage in a late cold snap. Although, cardinal flower & pickerel rush are pretty darn tough, so I wouldn't be as worried about them.