I have used the Bayer Season Long granules and also the kind you mix with water. I just put little palm full around the clumps also. I have heard that Merit (same as imidacloprid) only lasts a month so I add more about a month or so later. I use more around irises that are near bushes or evergreens. The moths like to hide and often lay eggs in the evergreens I was told by an iris farmer.
