Day Lily Determination

I've got several large patches of orange day lilies in the back yard and on the side of the house.  They've all come from the original small clump I planted when we moved into this house 24 years ago. The clump kept expanding, so every couple of years I'd dig up a piece and move it somewhere else.

I like the plants, because they're colorful, hardy, and bloom for a long time but they're also impossible to kill. A couple of weeks ago I noticed some green lily leaves growing among the rocks under the deck.  The rocks are piled several inches deep, and there's a weed barrier under the rocks.  Somehow, though, the lilies found a way.

As much as I admired their fortitude, the sprouts had to go.  One day when the ground was nice and soft after a rainfall I moved the rocks from around each sprout, then used a garden trowel to dig down and remove the small invaders.  Most of them came out easily, but two broke off just beneath ground level, leaving their tuberous roots behind.

As I was finishing the job it  started sprinkling again.  I know I should have continued to dig deeper and completely remove the complete roots, but I didn't feel like getting wet.  Instead just I tamped the soil back down and piled the rocks back on top.

Any bets on how long it will take until I have to repeat the job again?