As a general rule, Revit does not let you scale families. If you stop and think about it, not being about to scale families does make sense. Most family objects, such as chairs and walls, would never simply be “scaled” up (or down) in the real world.
But in the landscape the rules are a little different. Plants, rocks, and other natural objects are more useful when scaleable. Planting families have this built-in, by default, with the Height parameter. But what about other families or nested planting families, like a rootball?
Even though families can’t be scaled with the scale tool, with the right framework and parameters they can still be made scaleable. Marcello Sgambelluri did an excellent job covering this topic in his AU 2013 class Size Does Matter: Learn How to Scale and Morph Families in Revit. He covers several different methods, but only two can be used within the traditional family editor: Scale by Spline and Scale by Planting Family.
Scale by Spline
The general concept of this method is built around the fact that splines in Revit will scale if either endpoint is moved. If you are unfamiliar with this, RevitCat has a nice explanation along with a useful spline-based tree application.
This is the method that I use to create a Planting rootball. The rootball is a simple revolve with a spline as part of the boundary sketch.
I also use some straight lines, which allows my rootball to flex non-proportionally (in both width and height).
Scale by Planting Family
Marcello credits this method to Revit Swat, but I accidentally discovered this myself when I created my first Planting rootball. It essentially involves a planting family nested into another planting family.
The nested family will act like a typical nested family in the Family Editor, but when loaded into a Project file, the nested family will scale with the Height of the host family. This is fine if you want to create tiny furniture (perfect for the Revit dollhouse), but kind of frustrating if you don’t want the family to scale.
To make the nested family not scale, it must be Shared. And just to compare, the plant on the left has a Shared rootball and the one on the right is not.