As a garden enthusiast, I understand the need to cut, prune, divide and otherwise clean-up plants in my garden and yard.  However, I have not found a reliable and easy system to know the best timing for these maintenance efforts.  I have purchased gardening calendars and gardening books to help create a maintenance schedule.  I even started a catalog for my individual plants, noting the recommended maintenance tasks and timing. I still found myself searching the internet every time I wanted to understand maintenance timing for a specific plant.  There must be a better way to retain garden maintenance knowledge! This idea led me to review many garden maintenance articles and develop a simple rule set based on common plant characteristics that can be used to guide appropriate timing for plant cutting, pruning and dividing in northern climates.  Here are seven rules to help everyone remember when to cut, prune and divide plants.

Rule 1: Dead, significantly diseased or severely insect-infected plant parts should be cut off from perennials and shrubs as soon as the problems are apparent.

Rule 2: Herbaceous perennials (stems die back to the ground each year) should be left intact to overwinter and then be cut down to the ground in the spring prior to blooming. This practice gives insects and animals food and shelter during the winter months.  In addition, leaving the plants intact improves their winter survival chances.

Exception: In the autumn, cut down to the ground perennials that have a history of disease or are especially prone to insect infestations.  Cutting these plants back in the autumn reduces the chances of pests surviving through winter. Common plants that meet this exception are Bee balm, Rudbeckia, Iris, Phlox, Peonies and Asparagus (acronym BRIPPA).

Rule 3: Division of perennial flowers should be completed in the spring for fall-blooming plants and in autumn for spring-blooming or summer-blooming plants.

Rule 4: Spring-flowering shrubs (bloom before June, e.g. lilacs, forsythia, mock orange, azaleas and rhododendron) should be trimmed immediately after the blooming period ends.  These shrubs grow blooms on one-year-old wood. Trimming after the bloom period keeps the blooms intact for the current year and promotes new growth for blooms in the following year.

Exception: A heavy prune to regenerate an overgrown spring-blooming plant should be completed in the winter.  This will reduce the spring blooms but will ultimately promote a healthier plant.

Rule 5: Summer-flowering shrubs (bloom after June, e.g., Ninebark, panicle Hydrangeas) should be trimmed during late winter to early spring. These shrubs grow blooms on “new wood” (this season’s growth) and should therefore be trimmed prior to the time the new branches begin to grow to preserve the current year’s blooms. Do not prune shrubs late summer through autumn since this will promote new growth that would be susceptible to winter damage.

Rule 6: Deciduous trees, including fruit trees, should be pruned late winter. Winter trimming allows a full view of the tree’s shape and allows the wounds to heal before warm-weather pests begin to flourish.  

Refinements: Oak trees should not be pruned after February to avoid the spread of oak wilt disease.  River birch trees should be pruned early in the winter after the leaves have fallen.

Rule 7: Evergreen shrubs and trees should not be pruned since branch removal may damage the integrity of the tree and new growth will not occur when pruning back to older growth. New evergreen buds (but not branches) may be trimmed in early spring to help shape the evergreens and promote density.

I’ve created a short rhyme to help me remember the appropriate timing for cutting, dividing and pruning plants:

Garden Pruning Calendar

Herbaceous perennials peak through the snow

But in the spring, cut to the ground or mow.

Divide perennials in autumn if they bloom in the spring

But divide in the spring if autumn blooms cling.

Trim spring-blooming shrubs after flowers fade

But wait until winter if there is a fall flower parade.

Prune trees in the winter if they drop their leaves

But evergreen plants should be left to do as they please.

Cutting and pruning schedules for specific plants can be found in the citations below which also provide additional guidance for how to cut and prune plants.

Citations with individual plant lists

“How to Divide and Transplant Perennials”, Iowa State University http://yardandgarden.extension.iastate.edu/how-to/how-divide-and-transplant-perennials

“Pruning Flowering Shrubs”, Rutgers University http://njaes.rutgers.edu/fs1221/

“43 Perennials to Cut Back in the Spring”, The Spruce http://www.thespruce.com/pruning-perennials-in-spring-1402713

“Pruning Guide”, Bachman’s http://bachmans.com/information/resource-hub/pruning-guide

“Pruning trees and shrubs”, University of Minnesota http://extension.umn.edu/planting-and-growing-guides/pruning-trees-and-shrubs

General citations

“How and when to divide perennials”, University of Minnesota http://extension.umn.edu/planting-and-growing-guides/dividing-perennials

“Proper Time to Prune Trees and Shrubs”, Iowa State University http://yardandgarden.extension.iastate.edu/how-to/proper-time-prune-trees-and-shrubs

“Pruning Deciduous Shrubs”, University of Wisconsin http://hort.extension.wisc.edu/articles/pruning-deciduous-shrubs/

“Cut Back Perennials Now or Later?”, Purdue University http://www.purdue.edu/hla/sites/yardandgarden/cut-back-perennials-now-or-later-2/

“How and When to Prune Flowering Shrubs”, Penn State http://extension.psu.edu/how-and-when-to-prune-flowering-shrubs

“The Nuances of Pruning Spring-flowering Shrubs”, North Dakota State University http://www.ag.ndsu.edu/news/columns/dakota-gardener/dakota-gardener-the-nuances-of-pruning-spring-flowering-shrubs

“Dormant Season Tree and Shrub Pruning”, University of Illinois http://extension.illinois.edu/sites/default/files/dormant_season_pruning_infosheet.pdf

“Pruning Shrubs and Hedges in the Home Garden”, University of Maryland http://extension.umd.edu/resource/pruning-shrubs-and-hedges-home-garden/

“Leaving Perennials Standing: Pros and Cons”, Northern Gardener, Minnesota State Horticultural Society http://northerngardener.org/leaving-perennials-standing-pros-and-cons/

Photo Credit: www.forestryimages.org (Creative Commons)(1), Les Hutchinson, flickr.com(2) flickr.com(3)