is there a single word to represent "once every nine months"? Since 9 months is a fraction larger than half a year, semi-annualy seems wrong.
Perhaps consider dodranial or perhaps dodrantal, formed from the Latin dodrans. Of dodrans, Wikipedia says:
The Latin word literally means nine-twelfths, and can refer to a time span of forty-five minutes or a length of nine inches.
Note, ignore the claim at languagehat/dodrans that dodrans means 2/3; a comment later in the page substantiates that dodrans is 3/4, while bes is 2/3. Also, phrontistery has an OED-based entry for dodrantal:
adj 1656-1883 of nine inches in length