Fast-Yellowing Cedars

(Question)

We live in a semi-arid Zone 4 area in BC with clay soil. Several of us have city lots divided by 20+ year old, 7 foot high cedar hedges, well-irrigated. The first cedar hedge has gone completely yellow in the last two months, and the second hedge (see attached picture) is starting to go. This can’t be a water issue, as all of us monitor our yards carefully. What is it? Can we save this hedge or the others which are still green? Thanks.

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(Answer)

Thank you for contacting the Toronto Master Gardeners with your inquiry.

We receive a number of questions from homeowners on the yellowing of cedar hedges.

Below is some general information on the subject of yellowing and browning of cedars from one of our earlier posts.  It provides some guidance on watering, pruning and soil amendments that you may find helpful:

  • Cedars thrive on moisture and nutrient-rich soil.  Moisture deficiency after they are planted and before the ground is frozen can lead to browning of the bottom of trees.  If watering isn’t sufficient to encourage strong root growth in the last season or the roots in the root ball were not separated (teased out) before planting, roots may not be able to penetrate into the surrounding soil.  Watering deeply and thoroughly (at least once or twice a week after planting) is key so that the roots can take hold and spread.  This can be done easily by using a soaker hose along the base of the cedars for several hours.  Watering should continue well into the autumn until the ground is frozen to maintain adequate moisture through the winter.  The cold winter winds can desiccate the foliage – once the ground is frozen, the tree cannot take up moisture to replace what is lost from the needles/leaves.
  • What type of soil do you have?  Soil type is very important.  If you have clay soil, roots may have difficulty growing into it, so adding organic matter may be helpful in adjusting the soil structure.  You may also want to check the root ball – if it is still intact or if you find the roots are girdled (growing around each other), you may need to find a way to tease the roots apart to encourage outward growth of new roots.  Make sure the soil around the root ball will allow the roots to penetrate into it so try not to excessively compact the soil during planting.
  • Good quality topsoil and mulch should be used when cedars are planted.  Compost, triple mix or manure are good choices to add along with topsoil when planting – not only do they contain more organic material, they provide more usable nutrients for the roots.  They can easily be added as topdressing on a annual basis to help improve soil structure and allow the soil to retain moisture.  The use of mulch is an excellent way to maintain moisture around the shallow root system of cedars – it also gradually adds nutrients to the soil as it decomposes and will also be beneficial in keeping weeds at bay.
  • Fertilizing the trees after they were planted is a good idea.   Although excess fertilizer can cause root burn and prevent the roots from growing, so it is vital to follow the instructions on the container carefully.  It is also important to use a fertilizer with a higher middle (P – Phosphorus) number such as 5-15-5 in order to encourage root growth.

You mention that you monitor your watering regularly so lack of water doesn’t seem to be the issue. Another potenial cause of foliar browning in cedars could be due to root rot especially in clay soils where the drainage is slow. Root rot due to poor drainage could support the pathogen  Armillaria fungus (also known as honey fungus).  Symptoms to look for are the sudden death of the upper parts of the plant during periods of hot dry weather (which we experienced here in Toronto), indicating the failure of the root system’s ability to absorb water. Another tell tale sign is the appearance of white fungal growth at the base of the tree and under the bark of the trunk.

Other root-related issues include compaction of soil or mechanical damage. Thuja roots are relatively shallow/close to the surface, and at 20 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.

Lastly, you may wish to contact the Master Gardeners Association of BC to find a chapter near your area. They may have insight and questions from other homeowners in your area experiencing a similar problem.

Good Luck!