Seed germination rates of the invasive vine Humulus japonicus

Background/Question/Methods

Japanese hops (Humulus japonicus) is an aggressive invader of forest edge habitats, floodplains and riparian zones. Its rapid growth and dense cover allows it to outcompete native vegetation easily. This experiment was conducted to determine germination rates and potential after-ripening requirements required for a future study of the phenotypic plasticity in this spreading invasive. We modified proven techniques used to germinate American hops (H. lupulus) to develop a method for cleaning and stratification. Seeds were collected from multiple plants within multiple sites in forest edge and floodplain habitats in Madison County, IL, ca. 20 km northeast of St. Louis, MO. Germination trials were run at 2 months, 6 months, and 12 months post harvest. Sanitized seeds were placed into plates containing sterilized silica sand, and cold stratified at 4° C for 8 weeks. The plates were transferred to a light table with a 14/10 hour light/dark cycle and an average temperature of 23°C.  Germination (radicle emergence) was monitored daily until no new germination was observed for 5 consecutive days.

Results/Conclusions

Logistic regression was used to compare germination rates between trials.  Germination rate varied greatly (range 40-65% in trial 1, 83-92% in trial 2, 77-97% in trial 3) and significantly between trials (p < 0.001).  The first trial fell short of the 80% germination rate of domestic hops, while the second and third trials exceeded the expected rate (all p < 0.004).  There were no significant effects of seed source either at the level of the local population or individual parent.  The absence of these effects suggests a weak or nonexistent genetic component for plasticity in seed germination.  Differences in germination rates between trials may be caused by small differences in seed sanitation and stratification technique, but are more likely an effect of seed maturity before stratification.