Douma does not know where the blue spider lily is. The flower is very rare, only blooms during the day for a short period, and is difficult to find because demons like Muzan, who are searching for it, cannot go out in daylight. Although there are fan theories suggesting Douma might have been growing the flower secretly in his garden or had some knowledge of it, there is no official confirmation that Douma actually knows its location or possesses the flower. The blue spider lily is a rare and mythical flower that even Muzan could not find, and only a few humans like Kie Kamado and Tanjiro Kamado knew the location where it bloomed.

In summary, Douma is not known to know where the blue spider lily is or have it, and such claims mostly stem from fan speculation rather than story canon.