September is a great month for doing a little renovating in your perennial garden by dividing and/or transplanting. The heat of summer has hopefully passed, the chances of rain have returned and there is still plenty of time for plants to recover from being moved before the ground freezes.
The rule of thumb for deciding which perennials to transplant or divide is based on bloom time. Late summer and fall bloomers are suited for moving in the spring while spring and early summer flowering perennials can be transplanted in fall.
There are several signs that can tell you it’s time to divide a perennial when all the growth appears on the outer edges, it doesn’t bloom as well as it used to or the blooms are smaller than usual. All these indicators are symptoms of overcrowded roots.
Transplanting can be motivated by the desire to change the look of your garden or if you’ve discovered that the perennial needs a different growing environment.
Whether transplanting or dividing you should give the plants about 6 weeks before the first hard freeze occurs in your garden so they can be settled in to their new home and ready for winter.
Start by digging around the entire clump with a garden fork or sharp shooter (narrow shovel) and lifting the plant, soil and all, from the hole. Then gently break as much of the soil away as you can. If you are dividing the plant, once it is out of the ground, separate the crowns by cutting them with a sharp knife or shovel blade. You don’t have to be gentle, but try to preserve as many of the roots as possible.
Keep newly dug and/or divided plants covered and protected from wind and sun while you get their new homes ready. If you can’t transplant them the same day, place them in the shade, spray the root ball with water and cover them with wet newspapers. They’ll be okay for a few days, but I recommend getting them in the ground ASAP.
Prepare the new planting spot or revive the old one by turning the soil at least 8-inches deep. Remove rocks, roots and debris. Add plenty of compost and some aged manure.
Dig a hole that is 1.5 times as deep and wide as the plant’s roots. Build a firm mound of soil in the middle of the hole. Spread the roots over the mound so that the crown sits at or just below the soil line. Gently back fill the hole and pull the soil up around the crown just as you would a container grown plant.
Water the plant and keep it consistently moist until a hard freeze. Don’t bother with fertilizer as it will only encourage top growth, which takes energy away from the roots.
Once the ground freezes, apply a 3-inch layer of mulch and you are done. Next spring your perennials will emerge with a new lease on life.
Plants to Divide in Fall
- Asiatic Lily
- Oriental Lily
- Bleeding Heart
- Siberian Iris
- Japanese Iris