Wildflowers can grow very well without fertilizer, and many penstemons will perform well in some types of gardens without fertilizer. This does not mean that they should never be fertilized. Slow release fertilizer granules can be used in the spring when planting or cleaning up and diluted liquid fertilizer sprays may be applied before bloom and again later for long-blooming and reblooming varieties. Generally, the longer a plant blooms, the more often it should be fertilized, but never with heavy applications, and high nitrogen fertilizers should always be avoided.

Selecting, Growing and Caring for Penstemons
From Chapter 41

Lindgren, Dale and Wilde, Ellen. 2003. Growing Penstemons: Species, Cultivars and Hybrids: American Penstemon Society. 519 West Lancaster Avenue, Haverford, PA 19041-1413 : pp. 105-110