The monster plants are only a issue when they are above ground. After they eat a troop and go back into the ground the troops can move past. My own army has done this many times. Now if you click on map and forget your troops yes they will stand there in attack formation waiting for plant to come back up. Dont have them clicked to attck things on the way and you can move them past plants. Only if you also click them to move again after first attack.
I play orc and elves. My orcs have yet to have a issue of this amount with plants but I also dont attack a enemy wall with a handfull of archers or troops for that matter.
If enough troops are in the area the plants die really fast. Another suggestion if you do have one is a dragon fireball in the area of plant. It also work very good and has better accuracey then a treb cause yes lol they cant hit side of a barn.
Another suggestion is if you have a healing troop send them over plants they eat a person go away move past heal up troop is back lol. PLacement of these plants is crucial to if they work well or not.