- Soil solarization is a pre-planting practice to reduce weed pressure by trapping solar energy using a transparent polyethylene film to heat soil. Soil solarization has been successfully used in regions with high solar radiation. The purpose of this research was to determine if soil solarization under Pacific Northwest conditions could be used to manage weeds in tree seedling nurseries. Field trials were conducted for two summers, 2016 and 2017, at three commercial nurseries located in Clackamas Co., OR, Yamhill Co., OR, and Thurston Co., WA. In one study, six-week solarization was compared to a non-solarized treatment. Another field study investigated the effect of initial soil moisture levels (“low”, “medium”, “high”, “very high”) and solarization durations (0, 3, 6, and 9 weeks) at the Clackamas Co. site. For both studies, weed control efficacy was assessed by (i) viability of seeds buried in packets at 5 and 10 cm depths, (ii) emergence counts of naturally-occurring weeds, and (iii) time required to hand weed treated plots. The weed species included in the seed packets were annual bluegrass (Poa annua), common purslane (Portulaca oleracea), Pennsylvania smartweed (Polygonum pensylvanicum), and redroot pigweed (Amaranthus retroflexus). Additionally, tubers of yellow nutsedge (Cyperus esculentus) were included at the Thurston Co. site. Warmer temperatures and less cloud coverage during the 2017 trial resulted in greater soil temperatures and thermal hours than in the 2016 trial. In all trials and at both depths, solarization was most effective for control of Pennsylvania smartweed and least effective for control of common purslane. While annual bluegrass had variable results in the seed packet study year to year, the emergence of naturally-occurring seedlings was reduced in solarized plots compared to non-solarized plots. Solarization did not affect the germination of yellow nutsedge tubers. In 2017, a 3-week solarization period was as effective in suppressing weeds as 6- or 9-weeks of solarization as long as the soil moisture level was medium or above, whereas, in the 2016 trial, a higher soil moisture level was required to achieve effective solarization with a 3-week period. This result suggests that longer solarization duration and adequate soil moisture becomes more critical under cooler or cloudier conditions. The time required for hand weeding was reduced 52 to 69% following solarization treatments depending on the year and the location, which translated to a reduction in labor cost for weed management. Thermal dose-response of weed seeds was studied under controlled conditions to evaluate the thermal sensitivity of the four species used in the seed packet study in field experiments. Seeds were imbibed and incubated at constant temperatures of 45, 50 and 55 ºC for varying times and assessed for seed viability. The estimated time required to kill 90% of seeds (LD90) varied depending on species and temperatures. The LD90 at 45 ºC ranged from 47 h for annual bluegrass to 3246 h for common purslane. The LD90 at 55 ºC was 1 h for annual bluegrass, 3 h for Pennsylvania smartweed, 5 h for redroot pigweed, and 41 h for common purslane. The results confirmed the thermal sensitivity of species observed in the field experiments. However, further study is necessary to understand the effect of fluctuating temperatures on seeds to be able to better predict the effect of soil solarization in the field. These studies suggest soil solarization can be a viable option as a weed management tool in Pacific Northwest tree seedling nurseries.