One of the senses of "rare" is "items in a row/matrix loosely separated from each other" (as opposed to dense, thick). The root HEK (which is behind hehta- and hequa among others) is glossed "aside, apart, separate". So perhaps an adjectival derivate of the root like *henka could mean "rare".

How about something from *winta- "scatter": *wintana/*wintie?

+Remy Corbin interesting