From the Garden: Dividing Perennials - When, How, and What Then?

For the gar­den­er, the crisp autum­nal air sig­ni­fies that the hard work is done. There’s final­ly time to walk the gar­dens, inspect­ing new plant addi­tions and not­ing next season’s projects. One project that always seems to be on that list is divid­ing peren­ni­als. It is a task that is easy to put off, per­haps because we are not quite sure when or how to do it.

Signs of a plant need­ing divi­sion are when it flow­ers less and the blos­soms are small­er, when its foliage is sparse and unhealthy, or when the cen­ter of the plant is unpro­duc­tive. Three major rea­sons to divide are to main­tain the size of a plant, to reju­ve­nate it, and to increase the num­ber of spec­i­mens. By divid­ing rapid­ly spread­ing peren­ni­als, they are kept at a man­age­able size before they out-com­pete oth­er plants. Divid­ing an old­er plant that has crowd­ed itself will reju­ve­nate it, result­ing in vig­or­ous growth. Divid­ing peren­ni­als is also an excel­lent way to cov­er more gar­den space at a min­i­mal cost.

Gen­er­al­ly it’s best to divide spring and sum­mer bloomers in the fall, and fall bloomers in the spring. This sched­ule allows the plant to be in ​“stor­age mode” rather than in ​“flow­er­ing” mode, so it can put its ener­gy into root and leaf devel­op­ment as it gets estab­lished. Fall divi­sions should take place 4 – 6 weeks before the ground freezes and be mulched to pro­tect the roots and pre­vent heav­ing. Spring divi­sions ide­al­ly take place just as the plant begins to grow. Divide ear­ly enough to allow time for roots to get estab­lished before hot weath­er. If tem­per­a­tures increase quick­ly, apply­ing a lay­er of mulch will help to keep the roots cool. Be aware that peren­ni­als divid­ed in the spring usu­al­ly bloom a lit­tle later.

Before divid­ing, give plants a good soak­ing the day before and choose a cool and cloudy day. It’s best to cut the stems and foliage back to about 6” to pre­vent loss of mois­ture. To reduce root dam­age use a spade and dig straight down about 5” away on all four sides before lift­ing the plant up. Once out split the plant into divi­sions by spade, fork, knife — or even ax! Some­times hos­ing the root mass can help you see what you are doing, but it does stress the root sys­tem. Each divi­sion should include 3 – 5 sec­tions, be they stems, fans, eyes, or rhi­zomes. As a rule, divid­ing every three to five years will help keep most plants hap­py and healthy.

Any extra divi­sions are espe­cial­ly wel­come at the Arboretum’s spring and fall plant sales. They also make great gifts to fel­low gar­den­ers. As Luther Bur­bank said, ​“Flow­ers always make peo­ple bet­ter, hap­pi­er, and more help­ful; they are sun­shine, food and med­i­cine for the soul.”