Rydberg's penstemon (Penstemon rydbergii) is native to all of the western United States, as far as eastern Colorado. This penstemon, sometimes called beardtongue, grows to 2 feet tall and sports lavender to purple flowers on a tall spike reminiscent of small snapdragons. It favors elevations between 4,000 and 10,000 feet, making it a good natural addition to high country gardens of the West. All members of the large Penstemon genus are relatively easy to grow in home gardens if you give them conditions similar to their native habitats and prune them at the end of their flowering season.
Cut old flower spikes down to their base in April or May when the plant begins to send out new growth. If you shake the spikes over newspaper or into a shoebox, seeds that remain will fall out, and then you can plant them by scattering them over an area in which you want this penstemon to naturalize.
Prune all old branches with your clippers at their base, taking care not to cut into the plant's main stalk.
Snip off every new branch in the fall about one-third of the way to its connection with the plant's main stem. This will help the plant to stay bushy instead of leggy and will leave sufficient foliage to protect it from winter frosts.
Weed around your penstemon in the fall by hand, pulling all unwanted plants. Do this gently so you do not damage the penstemon's root system. If you shake the soil off the weeds' roots around the base of your penstemon, it will serve as a form of mulch that can help to protect the plant from cold winter temperatures.
Fertilize your penstemon in spring after it begins to show new growth but before it forms flowers. If you use a low nitrogen, or "blossom booster" fertilizer, the number of flowers later in the summer will be greater.