I'm going to try to explain what I'm trying to find out. I welcome any comments from other gardeners in Sims. If no one knows, then I'll have to create an extended test case.
In these crude drawings, the
dark black lines represent the grid we find on the ground in buy/build mode. Plants can be planted at either
1. the intersections of these lines,
2. the center of the square,
3. or the spaces in between.
This example is how I place my money trees. The red boxes are the trees, the green boxes represent adjacent grass, bold black squares are the lines seen in Buy mode. I notice that I only get money seed drops on the very outside edges of this area, not between the rows of trees.
So now I'm wondering whether dropped money seeds need an entire bold square of free space in order to drop, as portrayed below with red money trees placed one to a square, with a full square of grass in between.
If this is true, then my rate of drops should dramatically increase by changing the configuration, since all of the trees in the center of the grove are presently unable to drop seeds in my original way.