Penstemmons should be dead headed. When all the flowers on an individual stem are finished remove half to two thirds of the stem. If you do this they should flower until well after the first mild frosts.

You can feed them too guest, as they go on flowering for some time into the autumn. They need a feed to push them into continuing to flower with decent sized flowers. They are a rewarding plant with a long repeat blooming period. Can get a bit sprawly too. Leave the them more or less intact over the winter to protect the base. Cut down in spring when growth starts, feed and off they go again.