To grow transgenic switchgrass in the greenhouse, air filters are required to block pollen escape. Therefore, information regarding pollen size is essential for choosing the correct filter sizes. In this study, no significant difference was found in average pollen size between transgenic and nontransgenic plants, and their pollen size ranges were very close. The diameter of viable pollen of Alamo varied from 42.5 to 52.0 μm, whereas some empty or aborted pollen were smaller in size. Filters with suffiiently small pore size (e.g.,