Trees are woody perennial plants, capable of synthesizing their own food and resourcing up to 90 different elements. And they’ve been doing it for more than 350 million years.
So, what do trees need from us? The answer is, nothing…that is, until we take them out of their habitat and introduce them to an urban setting.
It’s estimated that one in a billion seeds survives to become a tree on the forest floor. Think of that next time you’re walking around the Bighorns and see a sapling. It’s one in a billion.
In town, we plant trees everywhere they don’t want to be and demand each one survive. Urban trees live in servitude.
Trees shade our roadways, pull water from floodplains, stabilize soil and reduce erosion. Trees stimulate business, increase property values, replenish oxygen; they increase survival rates of cancer patients in treatment centers that prioritize tree planting and maintenance.
And my personal favorite, trees are a place to play!
So, here’s a better question: What do urban trees need from us?
We want our trees to experience an optimal environment. But first we need to understand what ‘optimal’ means and what trees endure in the urban environment. Let’s think about a tree’s metabolic function.
Water accumulates and moves around the soil, bonding with elements and is pulled in by tree roots. Dry air surrounding the canopy pulls water from the leaf. Water leaving the leaf pulls up water from the roots like a chain. Carbon dioxide meets this element rich water at the leaf’s surface and assists in synthesizing the tree’s food.
We can sum this up by saying that trees require sunlight, water and elements. And we tend to operate with a basic understanding of these needs.
We plant trees in sunny areas. We water our trees. We fertilize to supply our tree with elements it needs.
We prove our love of trees through these investments, believing it’s imperative to the survival of these most ancient of living organisms.
But can we go too far? Is all this necessary? Have we become our tree’s helicopter parent?
Let’s take a closer look.
A tree is about 80 percent water (H2O) by mass, 19 percent of its material needs are filled by carbon dioxide (CO2), and the remaining 1 percent is made up of elements — only 19 of which are essential to a tree. All but two of these 19 elements — boron (B) and molybdenum (Mo) — are readily available in the universe.
Urban soil is made up of clay that is easily compacted and lacks porous space, making it hard for element rich water to move around the soil. We know that trees need an abundant supply of water, but we need to be sure that water is available to the tree. Supply and availability, are two different things.
When we apply water to clay, it eliminates pore space. This is why potters use clay as a medium, because it’s quite easy to eliminate voids by applying water. Porous pots make weak pots. For the same reason, in urban development, we build on clay because it ensures that structures are on firm foundation.
There are two ways to make water unavailable to trees.
The first is to supply the tree with no water. The second is to flood the soil with water.
Ideally, trees receive a steady supply of available water, especially as day temperatures rise. The reality is that clay soil can make a surplus of water a negative thing.
So, we need to recondition ourselves to monitor a tree’s available water instead of its supply.
To do this, you’ll need a soil moisture meter, an inexpensive tool that can be purchased at your local nursery or online. The meter is inserted into the top 6 inches of soil and reads within seconds, whether the soil is dry, moist, or wet. Water when we are nearing the intersection of moist to dry.
It can’t be any easier to do this and most people enjoy finding out they were investing more than necessary on watering their trees, ultimately maintaining an overly saturated soil in the past.
Now let’s talk about those elements that make up 1 percent of a tree’s material needs. We rarely see a low element supply causing mortality in trees. In fact, I’ve never removed a tree because an element was lacking in supply.
For a tree to exist in the first place, the essential elements must be present. Simply put, if your tree is tall or has been around for a while, we’re very likely not looking at a tree that is lacking in element supply.
Iron chlorosis — a tree’s inability to resource iron — is the most common deficiency we see. Iron is used for producing chlorophyll, which is essential to photosynthesis. A lack in available iron can inhibit a tree’s ability to make food. This can be an acute or chronic issue, and can lead to early mortality.
However, when we do soil samples of trees exhibiting iron chlorosis, we find plenty of iron in supply. Remember, supply and availability are two completely different things. If soil is too alkaline, iron becomes a solid and is unavailable to bonding with water, therefore pH can be the factor. Iron chlorosis is also exacerbated by compact soils that lack oxygen. In these instances, we can inject iron into the soil and see no response. We may even see a negative response as void soil space is packed with more stuff, impeding the percolation of water.
Every spring, as trees are waking up, we take limited void space in our soil and pack it full of elements via fertilization, without ever checking the tree’s supply.
Fertilizers are mostly nitrogen based. Nitrogen is used exclusively for putting on new growth. Before you apply nitrogen fertilizer, check to see if your tree grew last year. If your tree put on new growth, nitrogen supply isn’t lacking.
If your tree didn’t put on growth or didn’t put on much, it’s more likely that it was allocating energy to other life processes and less likely that there’s a lack in supply.
A sick or wounded tree pulls in little nitrogen; it’s focus is on resourcing elements to meet different objectives. Trees are far more in tune with their needs than we are.
Give nitrogen to a tree with iron chlorosis and you can stress it to the point of mortality. Nitrogen increases leaf mass, increasing iron demands, putting compounded strain on iron availability.
Fertilizer is not preventive maintenance and shouldn’t be administered as routine. Fertilizer is a treatment. Treatment without diagnosis is malpractice.
Before we consider fertilizing, an element supply deficiency must be identified through lab analysis of soil and foliar samples throughout a growing season.
Only after a supply deficiency is recognized should that specific element be applied and at the proper amount.
In conclusion, there’s an 80 percent chance that responses we see in trees is due to water availability. There’s a 99 percent chance that it doesn’t need fertilizer.
So this season, let’s reconsider how we approach tree care and get back to the basics — check before you water and lay off of routine fertilization.
Nicholas Patterson is an ISA Certified Arborist and is ISA Tree Risk Assessment Qualified. This content was sponsored by Inner Tree.