How to grow: Penstemon are generally easy to grow as they require very little care and attention in your garden. The trick is to plant them in the right site to begin with. They are a long flowering, easily cultivated perennial sub-shrub, that is floriferous and resistant to drought. They are generally not bothered by pests and are not attacked by slugs and snails! They tolerate lime happily and will stand up on their own without any need of staking.
Site and Soil: They prefer an open sunny border, although some varieties will tolerate light shade, however the flower colour may fade in a shady position. They will tolerate dry conditions once established. Good drainage is essential so if you have a heavy soil, then add lots of grit when planting. If the soil is waterlogged in winter this will dramatically affect the ability of the plant to overwinter successfully. More Penstemon are killed by winter wet than cold temperatures.
If you are growing Penstemon in containers use a good quality multi-purpose potting compost with added John Innes. Mix some extra grit into the compost before potting.
Planting: The ideal time to plant Penstemon is in spring or summer although they can be planted whenever they are actively growing. Plant them about 18” apart – although this will vary slightly depending upon the ultimate spread of the plant. Once established they do not like to be moved, so choose a permanent site.
Pruning: The time to prune is in the spring once the danger of a sharp frost has passed. This is because Penstemon are technically sub-shrubs. Leaving the summer growth on the top of the plant in winter protects the crown and the new shoots from frost damage. Once the new shoots are 2-3” long, it is safe to cut them back. Cut back all the old stems back to a new shoot.
If you want to cut the top of the flowering shoots off to tidy the plants up in autumn this is fine, but leave the leafy growth at the bottom of the flower stem. It is a good idea to dead head them throughout the summer as this helps to promote new flowering shoots.
More information can be found on the BBC's coverage of the Hampton Court Palace Flower Show 2017