Spirea looks like it has been in a fire

Asked April 26, 2018, 10:20 PM EDT

Townhouse complex with multiple beds of spirea shrubs...landscape crew cut the plants back on a 98 degree day...over the winter many plant stems turned black...they now appear as if they have been in a fire..is this a disease rather than abuse by care? The plants were very healthy until they were cut back drastically on the hot day.

Hennepin County Minnesota

3 Responses

We can't be sure what has affected the plants without seeing them or sharp photos. However, generally speaking, spirea tolerate pruning well and it's unlikely that the plants would turn black as a consequence.

Pruning in late summer or early fall can be detrimental because it encourages late season growth that is subject to winter damage. However, in that case the damage would not be apparent until the following growing season.

Spirea plants may be infested by aphids that excrete honeydew. When that happens, a fungus called sooty mold sometimes develops on the honeydew giving the plants a blackened or scorched appearance.
https://www.extension.umn.edu/garden/yard-garden/trees-shrubs/sooty-mold/

Spirea plants are also susceptible to a bacterial diseases called fire blight that blackens stems and foliage. Go here to learn about fire blight:
https://hort.uwex.edu/articles/fire-blight/


Attached are 3 photos showing the plants...they are still some plants that look ok, but many look like this.
Plants have broken off branches.
I tried cutting some of the black branches out of what appears to be a healthy plant, and the black sticks are still green inside.

Because the stems have been cut back and are dead or dormant, the photos don't provide the information needed to assess the spireas' health.

Stems (if any) that were blackened by fire blight last summer will be dead. Dry and brittle stems are clearly dead. Stems (if any) blackened by sooty mold may be alive. Stems with green tissue under the bark may be alive but the amount and vigor of new growth this spring will be primary indicators of the shrubs' health. Watchful waiting is needed to determine whether the black appearance of the stems is of any significance.

If new growth develops, inspect it frequently to rule out the presence of aphids. If aphids appear, take measures to control them.
http://extension.illinois.edu/hortanswers/detailProblem.cfm?PathogenID=102