Propagating
your own perennials by dividing them is
one of the easiest ways to get “something for nothing”. It is also necessary for some plants to be divided every 3 to 5 years to keep them healthy. If you understand what’s
happening to the root system of the plant,
you can multiply your plants with a high
chance of success. The most important factor
for success is timing and this means knowing
a few basics about how perennials develop
in your area.
Dividing
a plant always leads to some damage of
the tiny feeder roots. It’s important
to divide a plant when it will rapidly
be able to grow new roots to support the
new division. This means that the ideal
time is when there is no competing flower
development. The first rule of thumb, then,
is to divide plants during the season when they are not blooming.
In general, divide spring bloomers in the
fall and fall bloomers in the spring. Plants
that bloom in mid-summer can usually be
divided in both spring and fall.
The second rule of thumb is to divide is when the plant does not have a lot of actively growing foliage.
Few or small leaves mean that there won’t
be heavy demand on the roots for water
just when the roots are least able to supply
it. Usually, the best time to divide plants
is in early spring when new growth is just
starting, or in early fall when the plant
is preparing to lose its leaves.
The third rule of thumb is to divide when there is plenty of time for the plant to develop new roots, before harsh conditions stress the plant. Plants need a couple of months before the onset of winter or the heat and drought of mid-summer. Again this means in very early spring and in early fall. Both times will be followed by a long period of moderate soil and air temperatures which encourage root growth.
Dig up the
plant and gently shake off as much soil
as possible from the roots. If the plant
has a lot of foliage, cut the leaves back
by at least half to reduce the moisture
demands which would be too much from the
diminished root system. Examine the roots
and determine if pieces can be separated
by gently pulling them apart with your
fingers. If not, cut through the plant
with a sharp knife, making sure that each
new clump includes both roots and buds
or growing shoots. This may seem like rough
treatment, but the plant will recover quickly
if the dividing is done at the optimum
time. Replant as soon as possible at the
same depth as the plant was before. If
there is some delay, make sure the roots
stay moist. Water deeply and immediately
and then be prepared to water more frequently
than usual until the new roots can develop,
for 1-2 months. The ideal time to divide
and replant is on a cool, cloudy day to
reduce moisture loss through the leaves.
If this is not possible, shade the plants
for a day or two with an umbrella or other
sun barrier. The size of the division depends
on your purpose. For most garden settings,
replant chunks about 3-4” across
so the plant will fill its new planting
space fairly soon.
The exceptions: Some
plants should be divided at specific times
which depend on their unusual growth cycles.
For example, spring flowering bulbs which
have become too crowded should be dug up
and replanted at wider spacing when their
foliage is yellowing and starting to fade—sometime in June. Irises prefer division in July. Peonies do better with division in August or early September. And some plants should just be left alone. Examples are the shrubby ones like lavender and blue mist shrub (Caryopteris), as well as some surprising ones such as columbine and baby’s
breath.
For a detailed, plant-by-plant examination of dividing and other care, I suggest a great book, The Well-Tended Perennial Garden by Tracy DiSabato-Aust.
© Andrea
Green 2006
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