In biological research on the ozone depletion issue, action spectra are typically used as biological spectral weighting functions (BSWF). There has been little testing, however, of the appropriateness of different functions under realistic field conditions. Here we quantitatively evaluate a new BSWF for plant growth response to UV radiation, and other action spectra potentially usable as BSWF, in two seasons of field experiments. We utilized supplemental UV-B radiation with various combinations of filtered solar UV-A radiation. Our new BSWF, which we call the UV plant growth weighting function, indicates responses in the UV-A waveband. In field tests it proved to be the most appropriate weighting function for the responses we measured in oat (Avena sativa L. cv. Otana) over the two field seasons in these studies. A 10-day field experiment with canola, Brassica rapa L. cv. Goldrush, also suggested substantial effects of both UV-B and UV-A radiation on growth. If this new UV plant growth BSWF proves to be appropriate for many plant species, it represents a lower radiation amplification factor (RAF) than several previously used weighting functions for plants. This means less change in biologically effective UV for each increment of ozone column change. However, the lower RAF also means that experiments utilizing supplemental UV treatments have supplied milder doses of additional UV than intended, since they were based on BSWF that overemphasized UV-B and ignored the UV-A region. While we await continued field testing of BSWF on diverse taxa, we recommend: (1) reporting weighted irradiance with our new BSWF, and other appropriate functions; (2) continued reporting of the generalized plant weighted UV for continuity and comparative purposes, since it has been so widely used.