80 Basic Perennial Garden PlantinG & Maintenance
division only with suffi cient moisture. In the Midwest, September is a good
month for autumn divisions. Divisions should be done about 1 month before
killing frost.
Most plants will not divide well in summer; an exception would be bearded
iris, which is dormant in the summer and traditionally is divided in August. The
tough nuts like hosta and daylily and a few other rugged perennials will take
summer division. A hosta may respond by flopping, but by the next season it will
regain its normal habit. When dividing in the summer and autumn it is best to
cut the plants back by half to two-thirds to reduce transpiration (water loss by
evaporation from leaves and stems). Cutting the plants back before division also
makes it easier to see where to divide, although many gardeners cut back the
plants after division. With summer divisions, extra care is needed to keep plants
moist and shaded if hot and dry weather is expected.
Perennials with tough woody roots or taproots, such as species of Actaea,
Aruncus, Asclepias, and Echinops, should be divided in early spring (before top
growth emerges) or very early in the autumn. This also applies to plants in the
genus Filipendula, except for Filipendula vulgaris, which divides easily. These
so-calleddiffi cult-to-divide genera don’t always live up to their reputations. I
have had fairly good luck with dividing young plants of Echinops ritro ‘Taplow
Blue’ in early spring when growth was approximately 3 in. high. I have not been
so lucky with older plants, though I know of gardeners who have.
Ideally, spring divisions are made when the foliage is 2–3 in. high. In most
cases the entire plant can be lifted from the ground using a spade or spading fork
inserted into the ground about 1 ft. from the outside of the clump, depending on
the species. Smaller-growing perennials don’t have large root systems, so
normally can be divided easily with a sharp nonserrated knife or sharp spade. If
using a spade, work from the outside of the clump inward. Remember that the
dead, woody center is tough and often hard to pierce, even with the strongest
spade. Pulling apart clumps by hand is suffi cient for genera with loose, spread-
ing crowns and numerous shoots, such as Monarda, Stachys, and Symphyotrichum.
If you are doing small divisions, you may need to wash the soil off the roots to seeA sign that division is needed:
when a hole or dead space
develops in the center of the
plant.