The morphology and chemical profile of the capitate glandular trichomes (CGTs) of Paragutzlaffia henryi (Acanthaceae) were investigated. Four new labdane diterpenoids named paraguhenryisins A-D (1-4), together with the known physacoztomatin (5), were localized to the CGTs using laser microdissection coupled with cryogenic H-1 NMR and HPLC analyses and were traced and isolated from the CGT extract of inflorescences. Their structures were determined by spectroscopic methods and single-crystal X-ray diffraction. Bioassays indicated significant inhibitory effect for these diterpenoids on Arabidopsis thaliana seed germination and seedling root elongation. The most potent inhibitor, paraguhenryisin C (3), was interestingly detected in both the rhizosphere soil and water rinsed inflorescences extract of P. henryi but not the roots, with average contents in the rhizosphere soil relevant to its phytotoxic EC50 values. These results suggested that phytotoxic labdane diterpenoids in the CGTs might be released into the environment as a defensive measure for P. henryi against other competitive plants.