Use the Displacement constraint.  You can Probe to get the Reaction Force for the Displacement.

Why are you solving in Transient Structural? You can solve the Displacement constraint problem in Static Structural.

Please reply with an image of the Force vs Time plot for the Displacement constraint.

If you insist on using a Force, use Transient Structural, and add a huge point mass at the top of the plant root to slow down the acceleration when the root lets go of the soil. You can subtract the effect of the point mass on the force by subtracting m*a where m is the point mass and a is the acceleration of the root.  But remove the  gravity load if you had it there in the first place.