The days and nights are getting colder, the leaves starting to change to more brilliant shades of red and gold. It’s time to start cutting back the gardens and preparing them for a long sleep under the snow.
Summer-blooming perennials that have gotten too large for their homes over the summer should be dug up and divided this time of year. The cooler weather makes for a much more plant-friendly transition when transplanting, as new roots can grow in the still warm soil, and moisture stays around much longer. It is best to divide spring and summer blooming perennials in the fall, so energy can go to root and leaf growth. Fall-flowering perennials can be divided in the spring before they have gotten too big, when there is still plenty of time to set good roots and flower buds for the coming season.
Most perennials should be divided every three to five years. Signs that perennials need dividing are flowers that are smaller than normal, centers of plants that are hollow and dead-looking, or when lower foliage is sparse and unhealthy looking. Plants that are growing happily should be left alone unless you just want more of a certain plant.
Ideally, water your perennials to be divided a few days beforehand, and prepare the area that you plan on putting the new divisions. Watering thoroughly will hydrate the plant and ease the division. Cut back the foliage to around 6 inches from the ground; this will make lifting the plant and separating it much, much easier.
To dig your perennials, take a sharp, flat spade, and dig straight down about 8 inches out from the plant. Cut around all sides, and pry upwards from underneath to lift the entire clump to be divided. If the plant is in more serious need of division and has gotten enormous, it may need to be cut into pieces with your shovel before it can be lifted. Shake away much of the soil from the root mass to make it easier to move and see. This also helps loosen tangled root balls and make the actual division go more smoothly.
Different plants have different types of roots, all of which need to be treated differently. A good rule of thumb is to keep one to two growing points on fleshy or tuberous roots and rhizomes, such as hosta and iris, and three to five shoots on plants that have more fibrous, spreading root systems, like lamb’s ears or asters. Daylilies should be divided to clumps of three fans to encourage flowering the next year. Fall is the time to divide peonies, which should have at least two to three eyespots on each division.
The most important thing to remember when dividing perennials is to never, ever let the divisions dry out. The new plants should be put in the ground as soon as possible after digging, to the same depth that they were originally. A shovel or two of compost or cow manure mixed in with the soil is beneficial to root growth and general plant health. Water well after planting, and spread a layer of mulch over the newly planted bed. This helps insulate new, tender roots against freeze-thaw cycles that are so common in the early winter months, and has the added benefit of already being done in the spring!