We planted ours maybe 10 years ago, and they held their own. Then all at once they decided they were happy and started spreading like weeds. They are so pretty that we've been able to persuade even our usually flower-blind handyman not to mow that part of the lawn until they've gone to seed... If you can live with the tall grass for the few extra weeks, it is worth it for the spring fritillaria show! Some white, some dark, some in between colors...
