I have an established rock garden that includes a 4x4 patch of perennial forget-me-not. It is blooming a beautiful sky blue now. I didn't get a chance to divide the plant before it flowered this spring. Can I divide it this fall (late August), or should I wait until next spring?
Arapahoe County Colorado
The rule of thumb is:
Spring and early-summer blooming perennials are normally divided in the fall.
Mid- to late-summer blooming perennials are divided in the spring.
Iris and daylilies (rhizomes) are divided right after flowering.
So, your Myosotis sylvatica (Forget-Me-Not) should be divided in the fall—before the ground freezes or when the foliage dies back (mid-September to mid-October).
CSU information on dividing perennials: