Answer: Some varieties of penstemon naturally have red tinged stems and so what you are seeing might be the norm. If the stems and leaves were originally green and are developing a red tinge, it usually indicates exposure to cold nighttime temperatures, feeding by spider mites, or a need for fertilizer. It's a little late in the growing season to feed so wait until spring and feed with a balanced fertilizer. Check the plants for spider mites by looking for webbing where the leaves meet the stem. If you discover these pests, you can treat with an insecticide. If the red tinge is from cold damage, expect to see more of it until the weather warms in the spring.
Best wishes with your penstemons!
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