The cooler weather of the past couple of weeks has heralded the arrival of the fall gardening season and with it one of the best times to divide perennials.
Perennials usually need dividing for three basic reasons. First, they have outgrown their spot in the garden and are threatening to take over the place. Second, overgrown perennials usually quit producing as many flowers and look ratty by the end of the summer. Lastly, you simply want more of a particularly nice perennial in another spot.
In our colder climate, dividing should be done in early September so that the plants have six to eight weeks to develop a new root system. If they don’t have some roots to hold themselves in the ground they are prone to frost heaving over the winter. In my deer-infested garden, plants that don’t have a good root system usually get pulled up by the deer looking for a winter snack.
Usually it is best to divide spring- and summer-blooming perennials in the fall and wait until spring to divide fall-blooming plants. The actual process of dividing is not hard, as long as a few important steps are followed.
Wait until the forecast is for a few days of cooler weather; cloudy, rainy days are best but uncommon for us this time of year. Water the plants well the day before so they are well hydrated and easy to dig. Prepare new beds in advance by working in compost and watering well. Add compost to old beds. If you have to keep plants out of the ground for a few days, store them in a temporary bed or place them in a shady location and cover the root balls with moist mulch.
The actual division of a perennial depends on its root structure. Plants with running root systems can simply have clumps of roots and stems pulled apart and replanted. Those that grow from a clump or a crown are best divided by cutting the crown into pieces each with some growing buds attached. Tough clumps may need to be sawed apart. Rhizomes like iris need to be divided by pulling the rhizomes apart and replanting large healthy pieces with a fan of leaves attached. The leaves can be cut down by two-thirds to keep them neat.
Fall is the only time of year to move peonies and poppies. Peonies need to be dug after the first frost and replanted at the same level they were at. Dormant poppy roots can be dug from a clump and replanted at the same level.
Your new plantings will need regular watering until the end of October and later if it stays dry. Plants, especially newly planted ones, that go into winter dry are less likely to survive the harsh weather. Mulch new plants this first winter with pine needles, compost or shredded leaves to reduce the potential of frost heaving over winter. Don’t fertilize the new plants.