I have a second shop where I sell ornaments - I sell them ALL year long. Granted, many early-in-the-year sales have wedding/birthday/1st communion themes, but they ARE glass all ornaments. :)
I do, however have a number of very Christmassy-themed ornaments that I typically only list during the holiday season (Sep-Dec). I let my ornament shop dwindle to about 250 items Jan-Aug...but in Aug/Sep, I have over 400 listings.
If all of your garlands are similar, you may consider having one listing with several variations for different garlands. Then, as the year progresses, you can split that original listing and create multiple listings (with only one design per listing). But that might not be worth the trouble involved. Listings don't cost all that much...
And, last thought - you can't sell what you don't have listed!