Japanese hops (Humulus japonicus) is an introduced species from eastern Asia whose current distribution extends over most of the eastern U.S.A. H. japonicus is capable of rapid growth under warm, sunny conditions on moist to moderately dry soil and can form dense blankets of foliage that easily overtop native species. This experiment was designed to test for the potential of H japonicus to produce chemicals that may inhibit germination and growth in its competitors. A laboratory assay was designed using extracts obtained from soils, colonized with H. japonicus and without. Two soil samples were collected from three different sites. For each site, one sample was collected from a previously hops colonized area and the second from an adjacent area not colonized by H. japonicus. A protocol was developed to collect “available water” or water that can be extracted by plants. The soil water extracts were filtered through 0.22 µm, polyethersulfone, radio-sterilized filtration immediately prior use. Previously collected hops seeds were cleaned and stratified. Sterile Petri dishes were used with heat-sterilized silica sand per plate as the substrate. Each extract type was replicated 5 times with 10 seeds per plate and 8 mL of extract. DI water was used as the control. The plates were refrigerated at 4°C until the first evidence of radicle emergence. The plates were then transferred to a light table and incubated under cool white fluorescent light (125 µmol m2 S-l PAR) at ca. 23° C, with a 14:10 hour day: night cycle. Germination was monitored daily until no new germinants were recorded for 3 consecutive days.
Total germination was significantly higher in the plates that were treated with the hop soil extract than the soils without hop extract (p < .001). The hop soil extracts showed responses parallel to the DI controls, as opposed to the extracts from hop-free soil. The DI controls showed most germination occurring within 6-12 days after being moved to light. There was a significant difference in germination in the plates treated with soil extracts with hops as compared to the extracts where hops is absent of approximately 2-4 days (p < .001). The results of the soil extract assay supported the hypothesis that hops facilitates its own germination providing rational to further investigate in a field setting.