Yellow Cedars After Fertilisation

Question:

We planted a cedar hedge last spring and everything seemed to be going well. Since it rained regularly, I didn’t have to water often. Recently, my neighbor put some cedar fertilizer and did some light pruning. Since then, the cedars have been turning orange and yellow. What should I do?

Marie Gauvreau

Answer :

Editor’s note: The plants under discussion in this article are arborvitaes (Thuja occidentalis), not cedars. The arborvitae is a North American conifer commonly used in hedges throughout the temperate world. Cedars (Cedrus spp.), mostly Asiatic and much less hardy, are very different and, being tall trees, are rarely used as hedges.

You didn’t include a photo and it’s hard to tell what’s going on without seeing the plants. However, chances are your neighbor simply applied too much fertilizer to plants that are still fragile from a recent planting. Remember that chemical fertilizers can be toxic to plants: they should only be applied sparingly, according to the manufacturer’s instructions. You should never exceed the recommended dose and even then, it is wiser to always reduce it by half.

Cedar Fertilizer?

Besides, “cedar” fertilizer is actually more or less a rip-off. Often it is very, very high in nitrogen: 30-10-10 or some other staggeringly high number. Yes, a huge boost in nitrogen (the first number) can cause arborvitaes (well-established ones, not newly planted young ones) to green up, but often causes winter dieback afterwards. Note that fertilizing in late summer encourages cedars to make new shoots that may not have time to harden off before winter.

So, whatever you win by using such fertilizer is largely lost due to the winter damage they cause! Better to simply apply an all-purpose organic slow-release fertilizer. Such a fertilizer will likely not bear mineral numbers higher than 8 or 9. And use it in the spring or early summer, certainly not at the end of the summer or in the fall.

Too Much Fertilizer!

What to do in the case of too much fertilizer? Water thoroughly to dissolve the fertilizer. With luck, the contaminated water will drain out of the root zone. As for the arborvitaes, you can only wait until early next summer to see if there has been any improvement. If there are new green shoots, eureka! They are recovering. You can then prune to remove dead parts.