Regulation of seed germination timing is an important component of pop
ulation persistence, especially for short-lived plants in seasonal env
ironments with unpredictable year-to-year weather variation. Seed germ
ination patterns were examined in the laboratory for seeds from 135 po
pulations belonging to 38 species and 13 sections of the genus Penstem
on (Scrophulariaceae), from habitats ranging from warm desert to alpin
e tundra. Seeds of most species were dormant at dispersal and required
a period of moist chilling to become germinable. Response to chilling
was related to probable chilling duration at the collection site. Pop
ulations from habitats with severe winters produced seeds with long ch
illing requirements, while those from habitats with mild winters produ
ced seeds with short chilling requirements. Populations from midelevat
ion habitats produced seeds with intermediate chilling requirements bu
t with a sizeable fraction whose dormancy was not broken by chilling o
f any length. Another pattern observed mostly in warm desert populatio
ns was little primary dormancy at autumn temperatures combined with in
duction of a fraction into secondary dormancy by short chilling. Speci
es with a wide habitat range included populations with contrasting ger
mination patterns. Parallel habitat-correlated patterns were observed
in different sections. Most species showed germination patterns that c
ombined predictive mechanisms with potential for carryover of a persis
tent seed bank. Results of common garden experiments suggested that ge
rmination differences had a strong genetic basis both among population
s and among plants in a population. Adaptive radiation in the genus Pe
nstemon has thus included the evolution of habitat-specific germinatio
n timing strategies in multiple lineages within the genus.