Dog Urine: Tree Killer Or Problem Solver?

Will dog urine kill trees? There are two things in life that dogs make sure we all know.  They are loyal, affectionate companions and they like to pee on things that stick out of the ground like fire hydrants and trees.  If you have a dog, then you know they like to make their mark. Everywhere. Every ten feet so it sometimes seems. And if you have trees around, you know that those pooches like to moisten their bases.  But will this kill the tree? Let’s take a deeper look into the science of urine chemistry to find out.

Urine Chemistry 101:  What is it in Dog Pee that hurts trees?

Chemistry is the science behind understanding dog urine.
Chemistry is the science behind understanding dog urine.

I’ve heard that the primary ingredient in urine is ammonia.  And if you have a cat, you can testify to this. So, we know that dog urine is high in nitrogen.  And we see the signs of this in spots where dogs naturally like to leave their mark. Those yellow spots on the grass, those are from nitrogen burn.  Nitrogen is one of the primary components of fertilizer. But too much nitrogen and plants like grass start to suffer. Ever heard that expression about too much of a good thing being bad?

    “Dog urine contains a variety of nitrogen compounds. Too much nitrogen will burn the grass and create yellow patches.”

[Source: Joe Schwarcz Ph.D. of McGill University]

In our quest to find out if dog urine will kill trees, we need to investigate the following.

  1. What is dog urine made of?
  2. What is it in dog urine that hurts trees?
  3. How much of ______ is safe for trees? (Answer will depend on what we find out in question 2.)

Dog Urine Is Comprised Of The Following:

  • Ammonia
  • Calcium
  • Carbohydrates
  • Chloride
  • Creatinine
  • Enzymes
  • Fatty acids
  • Hormones
  • Magnesium
  • Potassium
  • Sodium
  • Urea
  • Uric acid

[Source: Bio Pro]

But How Much of Dog Urine Is Nitrogen?

There was a study done in 1910 that I found interesting in the Journal of Biological Chemistry:

We can extrapolate some numbers from this chart that tells us a better idea of amounts.  For example, if we take the average amount of urine deposited by the dog each day, we see that the average amount is 862 mL of urine per day.  Of this, 2% or 17.41 mL is basically Nitrogen. This is one dog of average size. So if you live on a corner lot and there is a tree that the community dogs like to pee on, it could be getting substantial amounts of nitrogen.

How much Dog Urine Will Kill A Tree?

So we’ve all seen those numbers on the bags of fertilizer right?  18-5-5  These numbers tell us the nitrogen, phosphorus and potassium levels of that particular fertilizer.  In this example, it would be 18% nitrogen, 5% phosphorus, and 5% potassium. Different kinds of plants like different levels of each basic ingredient of the fertilizer.  A typical flower fertilizer will be something like 15-30-15. A good lawn greening fertilizer would be more along the lines of 25-6-4, which is high in nitrogen. Dog pee is actually really good for your lawn, as long is it is spread out and not concentrated in one area of course.  But what about trees?

Tree Urine Fertilization

Trees, in general, prefer a fertilizer that would be more along the lines of 12-30% nitrogen and 3-12% for both phosphorous and potassium.  So again, a general fertilizer like a 20-5-5 would be great for trees. But how much will hurt them if they like nitrogen?

According to the Morton Arboretum’s article on Fertilizing Trees, 3lbs of nitrogen per 1000 square feet of land, is the recommended annual amount of fertilizer for trees in general.

One pound of water = 453 mL.

Therefore, 3 lbs of water = 1359 mL

One average dog can urinate about 17 mL of nitrogen per day, which we can derive from the chart above.

So that would be 1.359 mL of nitrogen, per square foot, annually applied to meet the recommended guidelines for fertilizing trees.

That’s about 3.7 mL of nitrogen per day to meet the recommended fertilization.

If we assume the dog pees 3 x per day then one large pee could contain about 5 mL of nitrogen.  If a dog pees on a tree, it is depositing potentially 3-4 times the recommended amount of nitrogen to fertilize a tree.

So we can make a simple rule-based upon these extrapolations, which you will find in the Conclusion below.

Conclusion

A tree will benefit from 1 dog pee per day and any more than 2 dog pees per day on a tree could be harmful to the health of the tree.

What To Do To Help Your Tree – 4 Tips To Save Your Tree From Pee

So maybe you’ve got a lot of neighbors who have dogs that love your trees. Well don’t fret, there are a few things you can do to help out your trees and grass.

  1. Frequent Watering – Watering your lawn and/or trees will help to dilute nitrogen (there’s that urine chemistry talk again) in the soil due to localized dog pee. Although it doesn’t solve the issue, it can definitely help to lessen the damage to your plants.
  2. Plant Barrier Flowers – There are many flowers that dogs will try to avoid like Marigolds. Planting a barrier of flowers that dogs dislike can keep them away from your trees. Check out my article on plants dogs hate here.
  3. Spread Mulch – Mulch is usually made from softwood chips. These softwood chips use up nitrogen to break down. This is sometimes an issue for gardeners when they spread dyed mulch on a bed as a ground cover and it reduces the nitrogen content of the soil. We can use this hindrance to our advantage and spread mulch around the base of the tree. This will help to soak up and consume excess nitrogen left by our furry friends.
  4. Put Up A Fence – Lastly, you can put up a physical barrier like a fence. Keep in mind the roots of a tree often extend as far underground at the crown extends above ground, if not more so. With that in mind, maintaining a fence at a distance equal to the distance the crown extends horizontally from the base of the tree is recommended.

Image by Gianni Crestani from Pixabay

Did you find this article interesting? If so, let me know, in the comments below.

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© 2019, Jeremy Shantz. All rights reserved.

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