After seeing Molly's arrangement with the flue tiles, I investigated getting some myself, but I found that they are now rather expensive. Because there's a manufacturer near me, I asked about getting "seconds" that couldn't be used for chimneys, and they said they could not legally sell them to me because if they ever got used for chimneys (supposing that I was really a shoddy contractor and not a gardener), they could cause a fire and the company might be sued. The nice thing about using the flue tiles rather than clay drainpipes is that the former are square and can be stacked neatly together. However, you could use round ones and fill in the gaps with sand, in which seedlings would grow if you didn't catch all the seeds before they dropped. You should do this anyway to moderate moisture and temperature. Rodger is right, Molly's flue tile arrangement did have some summer shade. It was also on enough of a slope that it "stepped" up, making tending it a little easier. She told me she put the scraps of composition roofing on certain plants to restrict water in later spring, not to keep moisture in. Jane McGary Northwestern Oregon, USA