Cedar Trees leaves turning brown/yellow after Sept pruning

(Question)

The trees are 18 to 20 feet tall, about 10 -12 years old
they have sun all day, the only changes is weather and we did prune them back in the 1st week of sept (1 mth ago).
Is this normal? 6 large emerald cedars and 3 large cedars.
We have had rain in Mississauga in Sept and October so I believe they are getting lots of water. They have been healthy up until we see this leaf discoloration starting. Please review the picture. I have posted more pictures at this google link in the Google cloud…

https://docs.google.com/document/d/1TSDhN9yUOYNHWxk2PFejOcSKHQa44pAIvtFkTFdiJIY/edit

 

(Answer)

There are a number of possibilities for your ‘Emerald’ eastern white cedar (Thuja occidentalis) problem.

The first is the most hopeful: It’s hard to see accurately from the pictures, but could your pruning have revealed the die-back of older/inner foliage that is a natural part of aging for any non-deciduous conifer around this time of year? If these brown bits are really older/inner needles that are now on the surface due to pruning, your cedar might be just doing what comes naturally. The weather extremes we experienced in Summer 2012 might also have resulted in a higher proportion of “natural” browning due to drought and heat stress at the height of summer.

On the other hand, a common cause of foliar browning in ‘Emerald’ cedars is due to root rot or root damage. September 2012 was wetter than normal, after a dry, hot summer. If your garden is on clay soil and drainage is slow, the heavier rains this September might have caused problems at the root level.

Other root-related issues include compaction of soil or mechanical damage. Thuja roots are relatively shallow/close to the surface, and at 10-12 years, your cedars’ root systems would be quite extensive – growing much further from the tree than you’d expect. So any new digging, for example, even if not immediately beside the trees might have an impact.

Finally, you should check closely to see if you detect any insects or diseases. Sucking insects such as mites or scale could contribute to the browning you see. Here is a useful fact sheet from the University of Guelph on pests and diseases of Thuja occidentalis.

http://www.uoguelph.ca/pdc/Factsheets/PDFs/006CedarPestsDiseases.PDF

Please let us know if any of these points help you make sense of your problem, or if you have further questions.