Why Knot?


Image result for images of black knot disease Mature Black Knot

In recent years the spring season in Calgary has been windy and wet, two prime contributors to the outbreak of black knot disease in the city and surrounding areas.  Owners and potential purchasers alike are in a fever pitch about the unattractive fungal overgrowth affecting their landscapes.  First let me give you the good news.  Black knot is not fatal in most cases.  Like the flu, if the individual is weak, and symptoms are left unchecked, death can occur, but rarely is it the case.  What is most distressing about black knot is how easily, and widespread it can occur, and how truly ugly in appearance it is.

You know what I am talking about, that “poop-on-a-stick” you see all over the park and neighborhood trees.  What you are seeing is called a gall, and by the time it is black and highly visible it is 2-3 years old and ready to spread ascospores to susceptible trees within the prunus species.  Caused by Apiosporina morbosa, or Dibotryon morbosa (same fungus, different names), it is an airborne fungus that requires wind to spread, and moisture (rain) to stick to the tender new growth on the tips of branches. This new growth is the only part of the tree that can be infected each year.  This is good news, as it provides a year to year and a half window of opportunity to prune, and therefore control the spread of black knot.  Galls begin to form the first year and become visible in the fall or early spring of the following year.  They are small, and green, to light brown in appearance.  This is the time to prune as the spores are not mature and viable until the gall is 2-3 years old.

Image result for images of black knot disease Immature Black Knot

When pruning the immature gall, cut 8″ below the knot, and disinfect your pruners between EACH CUT with a 1:10 solution of bleach and H2O.  If you are pruning a mature gall, some arborists recommend 12″ below as the fungus may have extended through the branch itself.  Be sure to burn or bury the branches immediately after pruning.  Spores can live in a pruned branch for up to 4 months.

Image result for images of black knot diseaseBlack Knot on trunk

In some cases black knot will appear on the trunk or other critical juncture on the tree.  In this instance, hire a certified arborist to remove the damaged tissue.


The best way to control black knot is to prune diseased branch tips as soon as evidence of the disease it noticeable.  Do not prune unaffected branches, or you will lose all your fruit bearing capacity for the year!  Be sure to check your Nanking Cherry,  Sandcherry, and flowering plum shrubs along with Mayday, Schubert Chokecherry, Amur Cherry and Sour Cherry and Plum trees for immature, green galls.

The winter here has been so mild and warm, creating a perfect window to eradicate black knot before the spring season.  I encourage you to prune on the next double digit day.  The spores that carry black knot fungus are spread throughout the blooming period and up to two weeks after. Best to rid your trees of black knot before it can spread.  Good luck!


Snow Damaged Trees and Shrubs

What a week of crazy weather we have had here in Calgary! This morning minus 4, next week, 22 above. Welcome to gardening beneath the arch, or as it is most commonly referred to as The Chinook Zone. One thing we can always predict, is the unpredictability of the weather here in Cowtown and surrounding regions. This freak snowstorm has wrought havoc with electrical lines and caused considerable damage to many trees and shrubs. I just wanted to take a minute and talk about what you can and should do regarding any damage to your landscape. I have learned on the job just how attached owners become to their trees. I can relate as I have lost a 20 year old tree in this mess myself.

So, what to do? First take a careful inventory of damages. If you live in an area where the power lines are above ground, please make sure that the area is safe and no lines are down. Elms, poplars, and evergreens typically experience the worst damage. If there are loose and dangling branches, exercise caution walking below. Resist temptation to just tear down any dangling branches to avoid further injury to the tree. If the tree is young and you have the tools, prune judiciously, bearing in mind what the tree will look like in a couple of years. Plan your cuts as much a possible to help the tree grow back into a pleasing shape. This could take a couple of years, depending upon the damage. If your damages are extensive; broken leaders, branches that very long and heavy, requiring more than one cut, please call an arborist to assess the viability of the tree and to execute the pruning. You are more likely to save the tree with the help of a professional who can brace and cable branches if necessary.

Down n Out
Down n Out

Stand back from your tree or shrub and gently loosen snow with a broom. You don’t want a branch to fall and knock you out in the process! Check evergreens carefully, as their damage is not as evident, the damaged branch remains green and you would not notice it until spring. Damaged trees and shrubs are more vulnerable to disease and therefore it is wise to take care of pruning as soon as possible. In some cases, arborists are able to save branches and leader trunks if the repair occurs before the wood dries out.
Do not use pruning pastes after cuts- they can actually impede the healing of wound. If you are uncertain whether to paste, check with an arborist.


Most woody perennial shrubs can bear a severe pruning if necessary, but one third of the current growth is safe for all, and can spur some rejuvenation, which is actually good for older shrubs.

Next week, when the temperatures return to normal, you will find most of your landscape will as well. Don’t be too quick to dispose of trees and shrubs that are damaged. Work with them and you will find most will come back next year none the worse for wear. In the meanwhile, visit YouTube and check out some of the great pruning video tutorials, sharpen and oil your pruners, and start planning next weekend’s BBQ!
Calgary is offering free disposal of branches at its landfills. Thank You City of Calgary.