There are two times that dividing perennials works the best.
- The Very Early Spring
- The Very Early Fall.
The very early spring when plants are “just” breaking through the ground is the absolute best time to divide a perennial. If in doubt, do it in the spring. The video below (even with its poor quality) shows you the best time to do this.
If you know where the plant is, you don’t even have to wait until it starts growing. Pop it out of the ground, whack off the chunks and replant. The plant won’t even know it’s been done. 🙂
This is sometimes hard for gardeners to understand but there is a small difference in success between early fall and mid-fall when it comes to being successful with perennial division.
In our nursery, we found it was better to dig and divide in September (USDA 4) than in October. Plants had a higher success rate the following spring when we did a September division. In the nursery business, this is critical as a few percentage points between “living” and “dead” is the profit margin.
So – will your plants survive if you divide them in October. Probably. But you’ll have better success in September.
I Hear These Questions All The Tiime
But my plants look great in September. Some are still blooming. Do I really have to do it then? See above. You’ll get a better success rate in September but you can delay. Your call. I do all mine in September – particularly the expensive ones. 🙂
I’m in Zone 5 (not 4) Do I still have to do it in September.
My take on that is it’s a variable that can be modified – by about a week or so. So if you’re in a Zone 5, then you have a solid 4-week window except your window goes from the beginning of the second week in September until the beginning of the second week in October.
Zone 6? Beginning of third week in September until the beginning of third week in October.
You get to push it back a week for every zone.
I Always Do Mine In October and It Always Works
Fine. I never argue with success. 🙂 I only point out we got those few extra percentage points of success with a September division rather than a later one.
Are There Plants You Should Only Divide In Spring Or Fall
Well, there are some plants that do better. For example:
- Grasses: Divide in spring only. They want to be divided when they’re about to start growing, not when they’re about to go to sleep.
- Dianthus: Spring only for me.
- Peonies: Can be either but I’ve had better luck with a fall division than a spring one.