Good luck. Self-sydicating in the garden field is very tough, because many papers are only interested in a very local writer, or, more than likely, will use garden articles from a service or from cooperative extension which gives it to them free.
The most I ever did was three small papers at one time. With the amount of time, cost of duplicating, and mailing costs to get clips to many papers, I found that they lead you on, even accept your ideas, get a month's worth of articles, and then never publish them, and you get no pay. Maybe larger papers are better, especially with features rather than regular columns.
Again, good luck. My advice would be to expand your topic areas. Although I am not writing much any more but started with garden columns (and am still doing that), I expanded into general agriculture, foresty, and the bluestone industry, all of which are important in my local area. I did articles not only for small weekly papers, but for trade papers covering those industries, so selling an article twice or three times to different audiences was the norm.
Again, good luck.
Ann