I think there's confusion about the parks--Magnolia Blossom Park in Willow Creek, Desert [forgot] Park in Oasis Springs, and the little one in Windenberg (Hare Square or something) are Park lots.
What the game refers to as "community gardens" are non-editable areas where our sims can go to harvest, water, and weed but not plant, fertilize, etc.
In Willow Creek
Community gardens can be found behind the BFF House and across the street from the Goth House.
In Oasis Spring
As
@reggikko pointed out in another thread, there's one across the street and north of Johnny Zest's trailer.
That's my understanding, but please correct me if I'm wrong.