After placing plant labels or other callouts in a Work Area, you want to twist the Work Area around to keep the labels or callouts leveled horizontally.
You can use the AutoCAD Select Similar command to select all the labels, then change their Rotation in the Properties dialog box. But the leader lines will inevitably be at unusual angles.
For pipe labels:
Completing these steps in an irrigation design? When you set the new rotation, you can delete any pipe labels from the drawing. Size your lateral pipes, then size the mainline. The pipe labels should come in properly.
All other callouts:
You'll need to redo all other callouts (such as valve callouts)manually. The biggest challenge would be irrigation blocks with nested text in them. It might be best to leave these as is. Otherwise, you may need to block edit each block definition and rotate the nested block manually.