A few years ago, when I was editing the NARGS journal Rock Garden Quarterly, I received an extremely detailed article about Erythronium seed dispersal, much of which we published. It should be available on the NARGS website. The article is in vol. 65, p. 265. The gist, for this current discussion, is that western American Erythronium species have seed dispersal by ants. As I recall, this process is called "myrmecophory." As Travis wrote, some Erythronium species in the wild, especially in the Pacific Northwest, occur as scattered populations of individual plants, presumably by seeding. I also observed this in colonies of Erythronium japonicum in Japan. In moist woodland in the American west and apparently in the UK, Erythronium revolutum is particularly happy to spread in this way. By contrast, commercial Erythronium hybrids such as 'Citronella' and 'Pagoda' have been selected for heavy offsetting of bulbs, a characteristic apparently derived from the narrow endemic Erythronium tuolumnense (a name I bet gets mangled in pronunciation; in the anglicized version of the Native American name Tuolumne, the stress accent is on the "o" and the first "e", and the "n" is elided).