"In the spring, we work in the garden."
Translation:Na jaře pracujeme na zahradě.
Well, you actually can say "v zahradě" though it kind of suggests you are there, writing a book, not tending to your flower bed. Even when writing a book you are more likely to say NA than V.
Now, if you went for a walk, it is more likely to promenade V zahradě, though it sounds like your garden is at least the size of the one in the Versailles.
It is just the way it is. "V zahradě" just doesn't sound right. I guess the logic behind it could be that you actually stand ON the ground, ON the grass, therefore you are "ON" the garden.