Day lilies are a nearly bulletproof plant and that is one reason why Jim loves using them in the garden. There are very few things one needs to worry about from the standpoint of keeping them happy and healthy. But we should address fertility and also, when we divide them. Here Jim divides all of the day lilies after three to five years. It is easier to pull the divisions apart then. Regarding fertility with day lilies - Jim thinks it is best to fertilize them probably in March, then two and a half months later with a slow release fertilizer, then two and a half months after that. Also, it is better to divide your day lilies in September because they are then going to be well rooted for the winter, which is especially critical if there happens to be a hard winter freeze. Eric agrees, he feels we do need to divide them periodically because they get really, really tight which will diminish the flower power. And, importantly it is a great way to start with a few day lilies and end up with thousands of them in time. So the plant is going to be happier. Another point about dividing in the fall is it's easier to locate where our plants are instead of waiting until they are completely dormant. What a great plus when we can share day lilies with friends. That is always nice.