There are 21 13-letter words that start with 'hydro'.
Note: These 21 13-letter words that start with 'hydro' are found from the USA & Canada version of the tournament Scrabble word list.
Also see words start with hydrop, hydroc, hydrox, hydrol, hydrog, hydros, hydrom, hydrot, hydrob, hydrod, hydron, hydrof, hydroi, hydroq, hydroz, hydroe, hydrok, hydrou.
6-letter words start with 'hydro' (1)