To remember to use no hyphen and no space in semiannual, just remember that semi is a prefix, and therefore should not be separate from the base word. Also, hyphens between a prefix are only necessary in several situations: between a prefix and proper nouns: un-American.

How do you spell semiannual?

Also semiyearly . occurring, done, or published every half year or twice a year; biannual. lasting for half a year: a semiannual plant.

Does Semi have a hyphen?

semi- In general, no hyphen when used as a prefix, except to separate two i's: semiannual, semicolon, semiconductor, semiprivate, semitropical; semi-invalid. See Words Formed with Prefixes under General Style Preferences.