I looked it up on Wikipedia:
> The powerful eruption of such an enormous volume of lava and ash injected significant quantities of aerosols and dust into the stratosphere. Sulfur dioxide oxidized in the atmosphere to produce a haze of sulfuric acid droplets, which gradually spread throughout the stratosphere over the year following the eruption. The injection of aerosols into the stratosphere is thought to have been the largest since the 1883 eruption of Krakatoa, with a total mass of SO
2 of about 17,000,000 t (19,000,000 short tons) being injected – the largest volume ever recorded by modern instruments (see chart and figure).
> Satellite measurements of ash and aerosol emissions from Mount Pinatubo
This very large stratospheric injection resulted in a volcanic winter, a reduction in the normal amount of sunlight reaching the Earth's surface by roughly 10% (see figure). This led to a decrease in Northern Hemisphere average temperatures of 0.5–0.6 °C (0.9–1.1 °F) and a global decrease of about 0.4 °C (0.7 °F).[21][22] At the same time, the temperature in the stratosphere rose to several degrees higher than normal, due to the absorption of radiation by the aerosol. The stratospheric cloud from the eruption persisted in the atmosphere for three years. The eruption, while not directly responsible, may have played a part in the formation of the 1993 Storm of the Century.[23]
> The eruption had a significant effect on ozone levels in the atmosphere, causing a large increase in the destruction rate of ozone. Ozone levels at middle latitudes reached their lowest recorded levels, while in the Southern Hemisphere winter of 1992, the ozone hole over Antarctica reached its largest ever size until then, with the fastest recorded ozone depletion rates. The eruption of Mount Hudson in Chile in August 1991 also contributed to southern hemisphere ozone destruction, with measurements showing a sharp decrease in ozone levels at the tropopause when the aerosol clouds from Pinatubo and Hudson arrived. Another noticeable effect of the dust in the atmosphere was the appearance of lunar eclipses. Normally even at mid-eclipse, the moon is still visible although much dimmed, whereas in the year following the Pinatubo eruption, the moon was hardly visible at all during eclipses, due to much greater absorption of sunlight by dust in the atmosphere. It has also been suggested that excess cloud condensation nuclei from the eruption were responsible for the "Great Flood of 1993" in the Midwestern United States.
So while no effect on plant growth is mentioned, it may have caused a major storm, a major flood, and enlarged the hole in the ozone layer. Sounds like a bad idea overall. Especially since that reduction in temperature was only temporary. For this approach to have any long-term impact, you'd have to keep injecting SO2 into the stratosphere every year.