Try using a broad leaf killer as well as the round up. That's what finally killed some of the Texas Evening Primrose, but it took almost two weeks.

Keep digging. Keep it covered. Go to the weed forum and see what advice they can give you. Good Luck!
kay

get thestronger Roundup, called Round up for brush and weeds.The round up for weeds wont kill it, but the stronger one for Brush should do the trick..I used some to kilw

listeria and honey suckle here that's impossible to kill it.

jean

I've developed an unreasonable hatred of this plant, it taunts me every Spring.

Thanks for the tips. I'll try them both!

Hate to say but I like it! Such a sucker for sentimental plants from my childhood. But inland sea oats? Bought at a master gardener plant sale and I am convinced that EVERY master gardener plant sale I have been to is full of invasive plants from h&$/

Jenny- Buy a gallon of Round-up (Glyphosate) Herbicide, NOT the pre-mixed ready to apply. It is a non-restrictive Herbicide, so you do not need a license to purchase. It is much cheaper this way any how and can be used on most weeds, trees and brush. Mix a 4% Round-up, 2% sticker mix and then spray in late summer/early fall. This will take care of your problem lily, but remember that this is a non-selective mix and will kill anything it touches.

Hope this helps,
Gale

Thanks Gale, if I don't make any progress by late Summer I'll go looking for the Glyphosate.

Update:

I bought the concentrated Round Up and went to town with it. Saw some progress but the daylily continued to try and send up a few shoots so I put the plastic back down, more extra tile on top of it, and then left a bag of mulch on top of the area for extra weight. Waited 2 weeks, removed the bag but left the plastic and tiles. Added a few inches of mulch on top of it for aesthetic purposes.

I'm not seeing any signs of the ditch lily trying to push through all this but I pulled back a corner of the area and there look to still be live (though wilted and in bad shape) shoots underneath still!

Am I really going to need to leave it like this an entire year? The side of my house looks really odd with a 4ft gap between plants!

pretty indescriminate, but cheap... keep pouring full strength vinegar on it. I've found that more effective than round-up.

Indescriminate is fine, the plants that were originally next to it have been moved further away so I can poison at will.

Are you suggesting I remove the plastic/tiles/mulch and keep hitting any shoots with vinegar instead?