Mutiny Update: they got away with it, and I'm furious. May I rant some more.
It seems that my organization took this garden way too much for granted. Our lease had lapsed. The property owner was indignant that she hasn't gotten a thank-you letter in years (she donates the property, we maintain it and insure it) and so she is honoring the new lease with the gardeners, even though they misled her into thinking she was renewing with the same lessee. We could fight her because she didn't give us notice but we've decided not to. Instead I have to take the high road and compose her a thank-you letter. Ewww.
It's a tiny garden that cost us money every year, but still, I hate it when cheaters prosper!
On a brighter note I have two new prospects for gardens elsewhere in the city, and no leases are going to lapse on my watch.