Surprise lillies

When in doubt, there is always Round-up, or some similar plant killer containing the herbicide glyphosate. Since they are growing amongst your perennials, you will have to be careful that the spray doesn't hit any other plants. Spread plastic or a few layers of newspaper over all the plants you want to protect before you spray the lilies. Leave the protection on until the spray has completely dried. You should do this right away because the surprise lily leaves are already finished growing for this year. They will begin to fade away soon. It probably won't work very well to spray the flowers when they appear later this summer because they don't translocate nutrients back down to the bulb like the leaves are doing now. If you try the glyphosate treatment this year and still get flowers, then try to spray earlier in the spring next year when the leaves are just coming up and are fresh and new. Glyphosate works best on actively growing and photosyntheizing green tissue.

The other option is just to continue digging, maybe a little deeper and wider to be sure that you get all the bulbs and pieces of new bulblets (baby bulbs) that break off the older bulb. I am actually surprised that you are having trouble with them. I didn't know they were invasive--they just come back politely at the same spot in my garden each year. You are just especially talented at growing surprise lilies--congratulations!

Laura
Johnson Co. Extension Master Gardener