Patrick,
First, in a pinch you can always place a schedule in model space and then either white out a column with a 255,255,255 hatch or wipeout object, or then use 2 viewports in paper space to slice that column out.
I think what might be happening for you is that you might have another plant size field in the same field column in a different plant category (tree/shrub/shrub area/groundcover) that does use that column even once. The code in the software only knows these as "Field 1, Field 2, Field 3" and not what the actual contents are. Most likely, some plant in another category does have information in that field.
Try running a schedule with only the category with the Fuel Mod field checked to include in the plant schedule options. That will ignore the other categories. Even if the schedule you're running doesn't include any trees in that space, if that's checked, Land F/X will include that column.
If you don't think these aren't the case, send in the file in a ticket and I'll have a look to help troubleshoot further.
We're still traveling for ASLA, so I'll probably be able to review on Friday or Monday.
How to Submit a Technical Support Ticket
-Amanda