A FORAY INTO THE ROOTS OF WHY WE NEED TO SUPPLEMENTAL WATER OUR SHRUBS AND TREES

Updated: Oct 11


Those points in your cut celery and the long strings that get caught in your teeth are the vascular tissues.


The majority of tree decline I am seeing the past few years is a combination of tree roots buried too deep (under mulch or soil) and insufficient watering. Focusing on water - in decades prior, we could go the whole winter without watering our trees. There was plenty of winter precipitation in the form of rain and snow and the trees moved into winter without any water deficit. Now, the water deficit happens year-round even in places where increased floods are happening.

What?!? How can this be? Well, increased flooding in a climate-crisis-world occurs in concert with increased drought occurrence. The extremes are what become the normal events.

This graph may help to explain what is happening.



In a normal distribution (the world as it once was) we are only going to experience the extreme event around 2% of the time, say 2 out of 100 summers will have an extreme drought event(s). But now the extreme event is happening annually and multiple times a year. This means the tails are fatter and so the probability is now much higher than 2%, it is in the 95% probability range. We have a curve more like the blue line (a bubble shape) kind of distribution (platykurtotic for the extreme nerds out there). It’s actually more of a one-tailed curve but for simplicities sake, I’m showing a two-tailed.

All of which means increased flooding and droughts in the Midwest and Southeast. Anything that was going to happen with infrequent regularity (say once every 100 years) is going to happen with a heightened frequency (once every 5 to 10 years). For example, the fires in the Southwest. The Southwest always had exceptionally hot summers, fires, and drought in the higher elevations, and now the Southwest has more of all of these. The Southeastern US always had periods of drought and flooding and now those extreme events are the norm.

Like every living organism trees have evolved in a particular climate. For the Southwestern deserts, trees and plants have adapted to high UV and high heat by growing relatively smaller leaves and sometimes sporting green trunks that can photosynthesize. Expose those plants to extremely low water (rain or snow) and they will dry and with the drying they are more susceptible to disease and pests. Now you’ve got a stand of very dry trees sick (lower resin production) and ready to burn hot and fast if hit by lightning.

But the Southwest isn’t the only place trees are suffering – it is planet-wide. In the Midwest and Southeastern US increased flooding alternating with sever drought is not the norm. To complicate this further we have hotter night-time temps. I for one, have a restless sleep if I am too hot. The same is true for plants. Hotter temps mean their metabolism doesn’t get a break. Every living organism as a limited amount of energy and resources (food, water, habitat, vitamins for cell repair, etc.). If your body isn’t getting enough rest it shifts the limited resources away from immune function to basic functioning. So, brain function declines and we become more prone to poor reasoning and disease. Likewise, a tree that can’t rest as much has fewer resources to apportion to repair and growth. We can cool the tree and provide supplemental resources by watering year-round and provide the extra resources needed for them to survive the new climate. This reduced metabolic rest has led to increased infection rates and trees dying from fungal infections that used to be simply low-level, chronic infections that might age a tree faster but not kill it. I suspect this is the root cause of the Sycamore anthracnose deaths we’ve seen this summer and the massive loss of Oaks to Oak Decline diseases. Supplemental watering throughout the year in addition to systemic treatments, are crucial.

Thus, I have been providing my clients with new water regime suggestions. Watering a tree during the growing season when there are six days without rain (sputtering rain doesn’t count) for 15 to 30 minutes is recommended. During the winter or dormant season for deciduous trees, I recommend watering if more than 10 days without precipitation and IF temps are above freezing. Watering for 10 – 30 minutes should be sufficient.

My recommendation is a very generalized one and not tested by science but based on some knowledge of tree physiology and a lifetime of experience. It reflects the fact that tree water requirements are highly variable. A sugar maple is going to need more water than a hackberry and both need less water than a black (water) tupelo. Know your tree – is it a lowland plant that has evolved in sites relatively moister? If so, water it longer. The age of your tree will matter as well, young growing trees need more water than older mature trees. How is the health of the tree? A chronically infected sycamore is going to (sycamore anthracnose) is going to need more water. Oaks suffering oak decline will need more water and both tree species, even if treated will need supplemental water so they can repair damaged tissues while the treatment knocks back or out, the pathogen.

As for fertilization – I’m extremely cautious with the application of synthetic NPKs because they disturb the important relationship between trees/shrubs and their beneficial microbes. Plants and these microbes have been helping each other out since plants took to land and the way we fertilize our lawns and trees has disrupted this relationship and shifted the plants microbiome that is less healthy or helpful. People tend to focus on the mycorrhizal fungi which are excellent in helping plants access nitrogen and phosphorous resources and played a pivotal role in plants moving to land (and all that means for their morphology). However, there are many microbes that help plants immune function and losing those is also an issue with our current soil fertilizing paradigms in the hort business. Deep root fertilization is not going to help your Oak suffering decline, nor any tree succumbing to an anthracnose disease. It might give some color for a season but it’s a band aid on an open wound. Plants are complex organisms situated in a complex world and their care should reflect the highly variable context within which they exist. If I’m treating a tree in an arboretum versus one in a highly fertilized golf course landscape – the treatment will be quite different, reflecting the different soil and water management provided.

Getting back to watering change - before any changes in watering regime will do any good – make sure your trees roots are not girdled and if they are, invest in the surgery. It is a one-time investment that lasts a lifetime. And saving a tree is always less expensive than removing one. You can’t replace the value a healthy, mature tree adds to a property or the sustainability and health of a community.



Note: I give an annual webinar on this topic called “Volcano Mulching – STOP!” which you can find through EventBrite by searching for the business name.


Graphs and picture from Wikipedia (whom I donate to annually, btw).

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More INFO and DIY videos via youtube include:

1.  https://youtu.be/8BM1_9tkyso - Protecting Roots from Construction Damage

2.  https://youtu.be/lZGEstTCczI - Micronutrient Fertilization is Part of Total Plant Recovery

3.  Smart pest management means when, what, and how to apply insecticides to save pollinators!

https://youtu.be/iw1NXEdw0F0

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