Last year I planted 5 penstemons in a narrow strip between my house and a walkway. It put the plants in the afternoon sun. After only about a month in the ground I started to see brown spots on the leaves. Then the flowers dried up. The plants all started dying from the ground up. I pulled up the bad ones and cut the remaining two plants back for winter--they had been in rough shape so I fully expected them to not come back. This year, I went and got 2 more plants and the 2 from last year did come back but very slowly. The brand new plants started developing the same brown spots on the leaves within on a few weeks and the flowers have dried up. One of the older plants is dying from the bottom up. Is it possible I have a fungus in the soil that is killing them? Is there another possibility? Is there a remedy to save these plants or to prevent the next round of plants from the same fate? Thank you!
Multnomah County Oregon
Penstamons are wonderful plants that are native to mostly the Eastern US, but with some species native to Oregon (OSU Gardening with Oregon Native Plants http://corvallis.npsoregon.org/OSU%20Gardening%20with%20Oregon%20Native%20Plants.pdf). All penstamons require very well-drained soil and are drought-tolerant. Amending the soil with pumice or pea gravel can help improve soil drainage for them. They prefer less fertile soils. They also need space, which from your pictures you have given them. Space allows good air movement around the plant and keeps the leaves drier. Instead of our lush Willamette valley growth conditions, picture Bend OR growing conditions.
The spots on your plants are probably fungal, and result from moist conditions. Improve soil drainage and avoid wetting the leaves - use a watering can, drip irrigation or soaker hoses to water (infrequently) the plants, and that should improve conditions for them. This article has additional information, Penstamon Leaf Diseases https://www.gardenguides.com/99813-penstemon-leaf-diseases.html.
Penstamons come in such a wide range of colors and forms, they're worth creating a little microenvironment in your garden for them. They tend to be short-lived perennials (some only 2-3 years), so you might want to collect seed or cuttings to replace yours. This article has excellent information on growing them, How to Grow Penstamons https://www.unce.unr.edu/publications/files/ho/2013/fs1335.pdf