say it: TAHL-gras
tallgrass is a noun meaning "any of various grasses that are tall and that flourish with abundant moisture". It carries one recorded sense.
any of various grasses that are tall and that flourish with abundant moisture
Forms marked · are formed by the regular rule rather than recorded in the source data. English keeps a few hundred irregulars and the rest follow the pattern, so the rule is right far more often than not — but it is a rule, and the mark says so.
Definitions, synonyms and semantic relations for tallgrass come from Princeton University’s WordNet. The pronunciation comes from the CMU Pronouncing Dictionary. Commonness is measured against a corpus of everyday English. Full sources and licences.