Our previous studies showed pure blue (B) spectra from LEDs can promote shade avoidance responses (SARs). However, it is unknown whether SARs vary among different peak wavelengths (λpeak) of B or how they compare with other photomorphologically-important wavebands, such as ultraviolet-A and far-red. To answer these questions, mustard (Brassica juncea, ‘Ruby Streaks’) and arugula (Brassica eruca, ‘Rocket’) seedlings, grown to the cotyledon unfolding stage under the following narrowband spectrum treatments: UVA (λpeak = 385 nm), B1 (λpeak = 404 nm), B2 (λpeak = 440 nm), B3 (λpeak = 455 nm), and FR (λpeak = 730 nm). Both red (R, λpeak = 660 nm) and dark (D) were used as control treatments. The spectrum treatments were provided at 50 μmol m−2 s−1 on a continuous basis. There were no differences among the B treatments except for reduced mustard fresh weight (FWt) in B3. Compared with R, the B treatments promoted hypocotyl elongation, reduced cotyledon size, and increased petiole length in arugula and B1 increased petiole length in mustard. Compared with the B treatments, UVA inhibited hypocotyl and petiole elongation, similar to or greater than R in both species. Compared with the other LED treatments, seedlings grown under FR generally had the lowest hypocotyl and petiole elongation and the smallest cotyledons. Compared to the LED treatments, D substantially promoted hypocotyl elongation and reduced cotyledon size, except compared to FR in arugula. Among the spectrum treatments, the three B treatments had the greatest SAR promotion effects in both species. Despite having the lowest phytochrome activity, FR inhibited SARs normally associated with high FR environments. Legacy parameters used to estimate SAR-promoting effects of spectrum treatments may not be appropriate for characterizing narrowband spectra from LEDs – new approaches must be developed.