Modern agricultural practices used in greenhouse production are designed to produce compact, attractive and marketable plants by using plant growth regulators. These substances in combination to micrometeorological parameters affect the growth of ornamental plants. A chemical growth regulator, paclobutrazol, controls shoot extension and internode length by inhibiting gibberellin biosynthesis. The purpose of this paper is twofold: to report on micrometeorological data and biometrical measurements of gardenia plants under different solar radiation regimes in a greenhouse; and to apply simple statistical models to a time series of the micrometeorological and biometrical data in order to evaluate the combined effect of micrometeorological parameters and the application of paclobutrazol on the gardenia plant size. The experimental plants were grown under two radiation levels of about 250 W/m2 in an unshaded main plot R1 and 90 W/m2 in the R2 main plot shaded by polyester and aluminum artificial cover (Model LS-16 Opal of Ludving Svenson Co., The Netherlands). The plants were pinched at the second internode from the tips. The triazole paclobutrazol was applied once as a soil drench at the concentrations of 0, 5, 10 and 20 mg/L on gardenia plants grown at both radiation levels four days after pinching. Results showed that during the first 20 days of growth period after the applications the size of gardenia plants grown at both radiation levels was affected by micrometeorological parameters while the action of paclobutrazol was not found to be important. However, from about the 21th to the 86th day the size of gardenia plants treated with 10 and 20 mg/L at the R1 level was found to be affected by paclobutrazol applications; the size of the plants treated with 0 and 5 mg/L at the R1 radiation level and of all plants grown at the R2 level was affected by both radiation and paclobutrazol action. Finally, the size of all plants grown at both radiation levels was found to be affected by the combination of the micrometeorological parameters and the action of paclobutrazol after the 86th day of the growth period.