Slickspot peppergrass (Lepidium papilliferum) is a reported "rare" ephemeral endemic plant listed as a species with "high" threat magnitude, and "imminent" immediacy of threat, under the Endangered Species Act. This plant is associated primarily with slickspots and other wet areas scattered throughout the Southwest Idaho high desert. The objective of this work was to evaluate the effects of three concentrations (10, 50 and 200 mg/L) of three humic acids isolated from the silt (HAs), vesicular (HAv) and clay (HAc) layers of an Idaho soil on the germination (11 days) and early growth (24 days) of slickspot peppergrass in controlled temperature, humidity and illumination conditions in a Fitotron chamber. All experiments were conducted in four replicates. Statistical analysis of data by ANOVA showed that: (a) sample HAs at any concentration increases seedling primary root length and promotes early plant growth, but has a concentration-differentiated effect on germination % and seedling primary shoot length; (b) HAv at any concentration exerts a positive effect on germination % and root elongation but a depressing effect on shoot elongation; and (c) sample HAc promotes germination % and root elongation at the highest concentrations with no or depressing effect at the lowesst concentration, whereas the effect on shoot elongation is concentration-dependent. The different effects of these HA on germination and early growth of slickspot peppergrass can be related to the differences in their C, H, N and O contents, C/N ratio, aliphatic, amide and carboxylic group composition, and fluorescent behaviour.