"pertaining to the culture of gardens," 1768, from horticulture + -al (1).
Entries linking to horticultural
horticulture n.
1670s, "cultivation of a garden," coined from Latin hortus "garden" (from PIE root *gher- (1) "to grasp, enclose"), probably on model of agriculture. Famously punned upon by Dorothy Parker.
-al 1
suffix forming adjectives from nouns or other adjectives, "of, like, related to, pertaining to," Middle English -al, -el, from French or directly from Latin -alis (see -al (2)).