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Question from Stephanie Sorenson (received 7/28/20) -
I planted 13 arborvitaes in early May. They were all doing really well up until a month ago. Now I think a couple of them have died or are really sick. They are watered twice a week right now for an hour at a time, due to the heat. They do sit on the north side of the yard. Our house was built 3 1/2 years ago. The trees on the ends seem to be doing really well, however the closer you get to the center the trees appear to be dying. We've had some one come look at them, however he is more of a tree trimming expert. He says the ones that look dead still have some bend to the branch, which indicates there is still water in the tree. He has been treating them with Osmocote and Super Thrive. I'm just not sure what is happening to these trees. Is there any hope?
Thanks,
Stephanie Sorenson
Answer from Dr. Mike Kuhns, USU Extension Forester and Professor (sent 7/28/20) -
Stephanie,
I like arborvitaes, but they are not very heat or drought tolerant. I think that it is likely that lack of water is the problem. The reason I can tell this is because I know that they need a lot of water, the weather now is very hot and dry, and combine that with something I saw in one of your photos — some brown tubing that I assume is how you are watering, meaning that you are using drip irrigation or something similar. Watering twice a week sounds pretty good, and for an hour each time sounds good. But it may not be nearly enough depending how much water is applied and where it is applied. For example, if you have one drip emitter for each tree at or near the tree’s base putting out 1-1/2 gallons per hour, that means that each tree is getting 3 gallons per week. With no other source of water, that seems to me not nearly enough.
So what you need to do is figure out how much water you are giving each tree and where it is going. You should be watering enough that the soil is moist and stays moist at least a foot deep and covers the entire soil area under the tree’s canopy. And since these trees really need wider coverage than that because of their narrow, upright form, you should probably apply water to at least twice the width of the canopy. Also, you need to irrigate more than enough for the tree’s’ needs now because they will need more water and access to more soil as they grow.
To give you an idea of what you are dealing with, a large tree like an arborvitae can use more than 100 gallons a day, and a very large tree can use 200 or 300 gallons a day. Drought hardy trees may like to have so much water available to them, but they can get by on quite a bit less. However, trees that do poorly when it gets hot and dry, like arborvitaes, are much more susceptible to attack by insects and diseases if they get stressed at all.
You can check how deeply and how wide your trees are being watered by shoving a steel probe, like a long screwdriver or other narrow steel rod into the soil all over our landscape. Such a probe will readily penetrate moist soil and will stop when it contacts dry soil.
So check your irrigation amounts and coverage. Also check that your trees were planted right by reviewing our tree planting factsheet.
Good Luck