We used, and present, robust methodologies to measure the shortwave (S) and longwave (L) spectral radiative properties (reflectivity (rho), transmissivity (tau) and absorptivity (alpha)) of nine plastic films and quantified their effect, when used as mulches, on soil heat flux density (G), net radiation (R-n, above the films) and soil temperature (T-s2cm, 2-cm depth; T-s10cm, 10-cm depth) in comparison to a control (bare soil). During the daytime, all treatments increased G when compared to the control to a maximum of similar to 45%, except for a plastic film that had a high shortwave rho(rho(s)) value (0.45). The films with the highest alpha(s) and rho(s) values had the highest and lowest daytime R-n, respectively. As a general rule, films with high tau(s) and alpha(L) values caused the greatest increases in G, T-s2cm and T-s10dm. In contrast, a comparison of three black plastic films with similarly high alpha(s) and different longwave properties showed that the film with the highest alpha(L) value achieved the lowest G and T-s2cm, which was unexpected. The daytime heat-sharing ratio (G/H, where H is sensible heat flux on the upward-facing side of the mulch) ranged in value from 0.80 +/- 0.47 (mulch with tau(s) = 0.79 alpha(L) = 0.39) to 0.26 +/- 0.13 (mulch with alpha(s) = 0.96 alpha(L) = 0.94). The lowest total daily downwelling shortwave radiation (S-d) required for total daily G to become positive was 9.4 MJ m(-2) compared to 17.6 MJ m(-2) for the control, which shows the potential of plastic films for growing-season extension.