Phenotypic evaluation of 37 crimson clover (Trifolium incarnatum L.) accessions from the US National Plant Germplasm System. Focus of the trial was on traits important for cover crop performance, including fall emergence, winter survival, flowering time, biomass, nitrogen (N) content in aboveground biomass, and proportion of plant N from biological nitrogen fixation (BNF). Experiments were conducted at the Beltsville Agricultural Research Center (Maryland, USA) across three growing seasons (2012-2013, 2013-2014, 2014-2015). The field design was a randomized complete block design (RCBD) with four replications in each year, except for five accessions planted in 2015, which only had three replications due to limited seed availability. Each plot was a single row 0.6 m in length and 1.5 m between plots. Between 37 and 45 seeds were planted per plot, depending on seed availability in each year. Fall emergence was evaluated in late October of each year by counting the total number of plants in each plot. Winter survival was determined by counting total number of plants per plot in late April divided by the total number of plants counted in the fall. Flowering time was evaluated by recording percent flowering on a per-plot basis on a scale from 0% (no flower buds present) to 100% (all flowers dried up entire length of head). Flowering evaluations took place periodically between late April and early June. In 2013, evaluation took place on six dates: 23 Apr., 9 May, 15 May, 24 May, 30 May, and 4 June. In 2014, evaluation took place on five dates: 28 Apr., 6 May, 13 May, 19 May, and 27 May. In 2015, evaluation took place on eight dates: 25 Apr., 29 Apr., 4 May, 7 May, 11 May, 14 May, 18 May, and 21 May. Frequency of evaluations and total duration of evaluation period varied from year-to-year primarily due to the effects of year-to-year weather variation on the rate of growth and development. Once an accession was rated at 50% or greater for flowering, biomass was collected. All plants in the plot were pulled up with roots attached. Plants were counted and the roots were clipped. All plants within a plot were placed in the same brown paper bag and dried. Dry weight was recorded and plants were ground for laboratory evaluation of nitrogen content, proportion of nitrogen from BNF, and metagenomic analysis. The crimson clover biomass samples were separated into shoots and roots. Shoots were oven dried (60 °C) for approximately 72 h, weighed, and ground to pass a 1.0-mm screen. Tissue C and N concentrations and 15N natural abundance were determined for the shoot material of each accession using a Thermo Delta V Isotope Ratio Mass Spectrometer (Thermo Scientific, Waltham, MA) and Carlo Erba NC2500 Elemental Analyzer (Carlo Erba, Milan, Italy). Isotopic abundance data were expressed as δ15N in parts per thousand (‰), representing the abundance of plant tissue 15N relative to that of atmospheric N2.