Do I need to divide the perennials in my garden?

FAQ
Question: 

Do I need to divide the perennials in my garden?

Answer: 

Some perennials, such as garden peonies, can be left undisturbed for 50 or more years.  Others need to be divided periodically to contain their spread or to rejuvenate declining plants.  Perennials, such as gooseneck loosestrife (Lysimachia clethroides), obedient plant (Physostegia virginiana), and bee balm (Monarda didyma) are vigorous growers.  They grow so rapidly that they begin to crowd out neighboring plants within a few years.  Gooseneck loosestrife and other vigorous growers should be divided every 2 to 3 years.  After dividing the perennials, replant some of the divisions, give others to relatives or neighbors, and discard remaining plants in the compost pile.  In contrast, the performance of some perennials, such as bearded iris (Iris hybrids), begins to decline after several years. If not divided every 3 to 5 years, bearded irises usually don’t bloom well because of overcrowding.  Declining perennials should be divided to rejuvenate the plants.  

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