Should I add fertilizer to my trees and shrubs? 5 things to know about garden fertilizers

Picture of Paul Cappiello Ph.D.

Paul Cappiello Ph.D.

Yew Dell Botanical Gardens Executive Director

Supersize me!

That’s right, folks. We’re in the land and time of overdoing pretty much  everything. Don’t believe me? Some time back, my wife and I went shopping for a new sofa and the smallest thing we could find was literally named the U.S.S. Nimitz! And the number 1 coffee chain in the US sells a very popular drink that
is so large, its contents actually exceed the capacity of the human stomach! I think my new business venture might be a line of strip-mall-based stomach stretching salons where after your procedure you can take home a paint-bucket-sized mocha-locha-latte-rama-lama-ding-dong . . . free refills, of course.

The same thing seems to have been happening in the garden world. Not content to have our trees and shrubs grow along at anything less than hyper-space speed, each year we dump millions of tons of fertilizer on their roots in an attempt to grow a 300-foot-tall redwood on a quarter-acre lot in the time it takes to watch a Pilates class on Zoom.

So, let’s get back to basics and talk a bit about trees and fertilizer . . .

First off . . . and I know this will hurt a bit . . . trees don’t need us all that much. Indeed, the earliest trees appeared on earth about 360 million years ago. The earliest humany-looking inhabitants of the earth showed up about 4 million years ago. If my math is correct, that means that trees did just fine without us for somewhere around 356 million years!

Of course way back then, trees didn’t have to contend with overhead power lines, underground sewer lines and semi-trucks tearing branches from their trunks. Without our “help” trees simply cruised along, happily gathering what nutrients they needed from the soil around them.

Where Do Soil Nutrients Come From?
There are basically three naturally-occurring nutrient sources to consider. Some nutrients come from the rock below the soil – what we refer to as the parent material. As weather and other forces break down the rock into small mineral bits, some of the elements locked up in the rock become available as they dissolve into the water that surrounds them.

Soil nutrients are also deposited by wind and water as storms blow dust and debris and flooding rivers and streams leave their banks and spread across the land. Still other soil nutrients come from soil microbes breaking down organic matter (leaves, sticks, branches, beetles, worms and even us!) – our natural composters.

The above are all long-established and still functioning processes and what they have in common is that they either are very slow (parent material weathering happens over millennia) or produce nutrients in forms that don’t leach from the soil very quickly. The end result is that nutrients are made available at a rate closer to what trees need to grow along quite happily.

Where Does All My Fertilizer Go?
What happens when we stick our super-sized, horticultural-minded noses into the mix? A lot . . .

When we dump a bag-full of granular fertilizer on the garden the first thing that happens is that most of those nutrients end up in the local stream, creek, river, lake or groundwater system. Most applied nutrients, in fact, never even have a chance of being absorbed by a root because they are simply never close enough to a root to get themselves absorbed. They just sail right through the soil without ever having a chance.

In the case of soluble fertilizer application, it’s even worse. Much of that soluble blue liquid we dump on the garden ends up running down the driveway and into the storm sewer, wasting money and polluting our water system.

Some studies have estimated that more than 95% of all soluble fertilizer nutrients miss their targets!

Do I Actually Need to Fertilize My Trees and Shrubs?
Yes and no . . . is the best answer to this one.

In most cases, our landscape trees and shrubs get plenty of nutrition without any help from us. Indeed, heavy fertilization actually causes plants to push much more growth that is both softer and more susceptible to insect and disease attack. At Yew Dell Botanical Gardens we rarely fertilize our woody plants. We also rarely spray for any insect or disease problems. Coincidence? I don’t think so.

But if you’ve just moved into the Whispering Pines subdivision (where the only thing left of the Pines is the Whisper) your newly planted trees are likely sitting in a big old pile of graded and regraded subsoil with a level of nutrition about on par with a depression-era box of marbles, it’s not a bad idea to feed a bit.

What If My Tree Looks Sick?
In some cases, if you find you have a tree with a specific nutrient deficiency, you may be able to correct the problem with fertilization but before you go dumping bags of fertilizer on the roots, it’s best to get a soil test and maybe even a tissue analysis to confirm the source of the problem. Your Cooperative Extension Service can help with diagnosis and testing.

But if your tree is seriously sick, sometimes the last thing you want to do is dump a bunch of fertilizer on it to bring it back from the brink. One thing to keep in mind is that for every bit of nitrogen (one of the most important plant nutrients) that a plant assimilates, it requires more than 20 units of carbon to complete the process. What that means is that if you overdo the fertilizer on a tree that is suffering, you can actually cause it to cannibalize existing tree tissues to allow it to make use of the applied nutrients. This is a delicate balance that usually benefits from having a pro look over the situation.

What’s The Best Way to Fertilize Trees & Shrubs?
Like anything . . . in moderation. Compost-based nutrients (such as leaf compost or even compost teas) strike the best balance of offering low levels of nutrients that hang around in the soil longer, feed both roots and soil microbes and resist leaching away and missing their targets. Top dressing your garden beds with well-processed compost, applying compost teas to the lawn and even mulching your leaves when mowing all work to make low level nutrition available to your trees.

So put down that fertilizer spreader and go get yourself a cup of coffee!

This article was originally submitted to the Courier Journal on February 7, 2023.

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