Aims:　Variabilities　of　vegetation　and　soil　cause　uncertainty　to　the　factor　of　safety　(FoS)　of　unsaturated　vegetated　slopes,　yet　the　significance　of　these　variabilities　on　the　uncertainty　of　FoS　is　unclear.　This　study　aims　to　quantify　the　effect　of　the　uncertainties　of　root　reinforcement　and　soil　hydromechanical　properties　to　the　uncertainty　of　the　FoS.　Methods:　The　variance‐based　global　sensitivity　analysis　was　adopted　to　evaluate　how　the　variance　of　FoS　of　vegetated　slopes　can　be　apportioned　by　the　variabilities　of　soil　and　root　parameters.　A　copula　theory　was　applied　to　model　the　correlation　amongst　the　parameters.　Results:　For　slip　depths　shallower　than　0.30　m,　the　major　source　of　the　variance　of　the　FoS　included　the　parameters　that　define　root　reinforcement,　followed　by　the　parameters　of　soil　shear　strength.　The　variation　of　transpiration‐induced　soil　suction　had　limited　effect　on　the　FoS　variance　under　heavy　rainfall.　Taking　into　account　the　correlations　amongst　the　parameters　had　minor　influence　on　their　contribution　to　the　variance　of　the　FoS.　Conclusions:　We　observed　threshold　slip　depths,　where　the　relative　contribution　of　uncertainties　in　root　and　soil　parameters　on　the　FoS　uncertainty　underwent　a　transition.　Root　reinforcement　for　slips　as　deep　as　0.60　m　can　provide　reliable　slope　stabilisation　effects.　©　The　Author(s)　2025.