Jim, Harking back to my old days of growing high alpines and bulbs in pots, there was a common misconception, still propagated now, that it is important to have drainage material at the base of the pot. This is nonsense, the only place that drainage needs to be is in the compost - and the same holds true for raised beds. I suspect your compost itself isn't well enough drained, 18" of raised bed should be way overkill to defeat capillary action, especially given evaporation from the surface and sides of your bed. Most of my raised dry sand beds here are 9" high, the maximum is 12" at the front. By the middle of a normal summer they are dry down at least 4", more than enough to keep a wide variety of bulbs happy. The Onco bed was only covered today with 8x4' sheets of twin wall PC, at a height of 12". I have some self sown Dicentra peregrina seedlings in one of these beds and their roots go down at least 2-3" - I had to water them yesterday because they were keeling over from drought. Best, J.