Dave,
I am not in a position to guess at what would be good dates in this moment - because setting good dates, in my opinion, is not about my opinion at all but really should be data driven. It's really not about what I think on some level. With that caveat, I could propose some good dates if I knew:
1) what an acceptable number (to you) is for inclusion. How many of the 10 guests do you need to have showing up at this year's parties?
2) what the guests' availabilities are (and that will vary from party to party, of course).
3) if are you 100pct committed to having breaks between party weeks or are there a few cases in which back-to-back parties would be acceptable?
To answer the second of the above, I can tell you what questions I'd need to ask the database to make good guesses. Or we could discuss a couple of options I have in my head. I'd be happy to make time to crunch numbers/data in a demographically/mathematically defensible way and come up with a list of proposed dates based on those conclusions and the other criteria in play.
If it is to me about maximizing participation (and it is), it could well be that based on the data my zone (5b) might be out in the cold in some categories. That would be fine if the numbers indicated it. I'd much prefer that the numbers tell me what to think (provided I'm bright enough to ask them the right questions) rather than the other way around.
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The one of the other issues seems to be the photo contests that accompany each week (and while that's very important to some, it may be very tangential to the weeks for others). And I think if that were spelled out more clearly, that may mitigate some of the participation concerns on that end (at least regarding the contests). So, to clarify:
1. Will there also be a year-end photo contest again (with the categories that don't appear in the celebration weeks)? (pollinators, tropicals, annuals, trees, landscapes, etc.)
2. Assuming there is, will the weeks' categories be included also (to account for August roses and May daffys, for example)?
3. If there is, will the winner of the week's contest be automatically included in the top-ten/final ballot, etc.? How might that be handled?
Knowing this stuff might give the plant weeks a little more perspective for the avocational photographers out there who get a kick out of the contests and want to participate.