Our botanic garden renews members year-round according to their anniversary
date.  Where I used to work at UCLA, we did it once a year.  I know that
Santa Barbara Botanic Garden does it year-round, too, but tries to get most
people to renew in March to reduce the load at other times.  Personally,
I like once a year best, but you still get new members at other times of the
year and have to decide how you will prorate their membership dues.
 
We might in the future switch to once a year.  Perhaps you could summarize the
responses to your request and make the summary available on the list?
 
Richard Chute
Rancho Santa Ana Botanic Garden