Perhaps a little like an open flowered antirrhinum to look at, but whereas the antirrhinum flowers in flushes, the penstemon flowers continuously until the first frosts and well overwinter in sheltered locations. RHS Perfect For Pollinators. Height 60cm (2'). HHA/HHP - Half hardy annual/ Half hardy perennial. Enjoying a resurgence among border plants for colour, reliability and long, constant flowering period.