What could cause sinus allergy symptoms every December?
According to Thomas Ogren’s book, Allergy Free Gardening , the genera Alnus, Baccharis, Casuarina, Festuca, Pennisetum, Juglans, Poa, Tamarix, Taxodium, Thujopsis, Xylosma, Zelkova, and palm trees all produce pollen during December.
(Source: Ogren, T.L., Allergy-Free Gardening: The revolutionary guide to healthy landscaping , 2000, pp.262-265)
Also check out Allergy Free Gardening website. There are a number of articles on low-allergy gardening listed.