13 letters

horseradishes ∙ horseshoeings ∙ horsewhippers ∙ horsewhipping ∙ horticultural ∙ horticultures ∙

14 letters

horizontalness ∙ horizontically hormonogenesis hormonopoiesis hormonopoietic hormonotherapy hornlessnesses ∙ horrendousness ∙ horriblenesses ∙ horrifications ∙ horripilations ∙ horsewomanship horticulturist ∙

15 letters

horizontalities ∙ hornblendophyre horologiography horrormongering horsemastership horticulturally ∙ horticulturists ∙

16 letters

horticulturalist

17 letters

horizontalization

Words with a ∙ are valid Scrabble words (SOWPODS word list)