Trees are generally admired for their surface beauty, but their health and vigor springs from what’s underground. That’s according to Dr. Kelby Fite, Director of Research for Bartlett Tree Research Lab in Charlotte, North Carolina, who spoke to Maryland’s master gardeners last week. His lecture entitled ‘Managing the Landscape Below Ground’ provided a wealth of information about how to improve the life of the trees in our landscape. According to Fite, it all starts with the soil.
“We are all guilty of fouling up the soil,” he said. “But, it’s far easier to preserve soils than to remediate after they’re damaged.”
Before reaching for the fertilizer, Fite advises digging a little deeper into the source of the problem. Following are five common reasons that trees fail and what to do about them.
When it comes to tree health, nothing hurts a tree more than compacted soil. Often brought on by construction, soil compaction impacts trees at their roots, depriving them of essential water, oxygen and nutrients. It also makes it harder for a tree to anchor itself in the soil.
Compacted soil makes tree anchorage difficult
Soil compaction occurs when soil particles are compressed by external factors like mechanical or human traffic, resulting in reduced pore volume (meaning there is less space for air and water.) Heavily compacted soils have a reduced rate of water infiltration and drainage and are often hydrophobic (meaning the water runs off). For a tree, this leads to poor growth, higher water needs and increased susceptibility to pests and disease.
Fertile, aerated soil
Compacted clay soil
Think of soil like a sponge, with large and small particles.
“What happens when we squeeze a sponge?” Fite asked. “We are increasing the amount of solid matter.”
It makes sense. Tree roots are extensive and with the exception of the taproot, located for the most part just 8 to 10 inches below the soil. They need space to grow. With compaction, however, many trees will never develop a taproot, instead establishing a network of weakened lateral roots that are unable to keep the tree vertical. And at the soil surface, the fine feeder roots (which under ideal conditions can extend 4 to 7 times the drip line of the tree) are compromised, too. This reduces a tree’s ability to absorb water, oxygen and nutrients and ultimately leads to its decline.
SOLUTION: Build away from a tree’s drip line and keep mowers and other machinery off the soil. For existing trees under stress, add a mulch circle. (More on that below.)
Raking up leaves deprives trees of important nutrients
This is a tough one. I’m not sure I’m ready to shred leaves and leave them all over my lawn. But think of it, we spend time and money to rake up one of our plants’ most important sources of nutrients, push them to the curb, then go to the store and buy them back again as mulch or soil additives.
“It’s not a great business model,” said Fite.
According to Fite, we are removing one pound of nitrogen per 1000 feet just by removing the leaves from our properties. This is a big loss for trees as well as other plants. And this doesn’t even take in to account the fossil fuels burned to operate leaf vacuums.
Our soils are low in organic matter because we are not allowing leaves to decompose like they do in the forest. When left alone, leaves return nitrogen and other organic matter to the soil. They also protect the soil surface, adding to its water holding capacity. And as they break down, they provide porosity and aeration, allowing more water to infiltrate the soil.
Shredded leaf mulch
SOLUTION: Shred your leaves with a mower and make leaf compost or leaf mold mulch (not the same thing.) For some great info on how to do both, click here for thespruce.com’s excellent article Rake leaves and make compost, mulch.
Modified soils have higher pH which is bad for trees
In a perfect soil world, soils have a slightly acidic to neutral pH (a pH of 5.5 to 6.5). Urban and suburban soils, however, often contain debris left behind from construction. This generally results in a higher soil pH due to leeching from masonry walls and foundations.
Leeching from masonry leads to soils with higher pH
Trees growing in these locations may show signs of nutrient deficiency (like yellowing of leaves) and may be slower to establish. And irrigation water typically has higher pH, too. This means it gradually increases the pH of adjacent soils over time. This can result in a dramatic difference between the nutrient ability of your native soil and that affected by construction materials.
SOLUTION: Do a soil test to determine the pH and get the turf out from under your trees. Lawn thrives on alkaline soil (that’s why we add lime.) But trees like a lower pH. All developed soils tend to have a higher pH and modified temperature, putting more heat stress on plants.
A soil test report from University of Delaware (my go-to destination for soil reports)
Lack of mulch
Mulch improves soil structure while providing protection to a tree’s roots. It supplies organic matter to the soil that reduces compaction. And it moderates soil temperatures, conserves moisture and eliminates competition from grass. Yet, many landscape trees are planted out in the lawn with no protection.
Fite recommends mulching out to the drip line of a tree if you can. It will help prevent ‘mower blight (or your mower banging into the trees) and also protect against stripping from string trimmers. Mowers, by the way, also contribute to soil compaction, especially when they’re operated in wet conditions.
Mowers contribute to soil compaction and can damage a tree
SOLUTION: Create a mulch circle around your trees. It will help protect them from mower damage and as the mulch decomposes, it will aerate the soil and provide nutrients.
Fertilizing when it won’t make any difference
According to Fite, fertilizer doesn’t fix everything. Pull a soil sample from around your tree first to determine whether or not to fertilize. Most soil labs will run diagnostics on your sample and send you back a prescription for what to add to your soil to improve its composition.
Before fertilizing, ask yourself what is the objective? If construction has occurred and interrupted a tree’s root zone, no amount of fertilizer is going to fix the problem.
And remember, although native soil usually contains lots of organic matter, during construction this layer is often buried under layers of sand or debris. Or sometimes, it’s stripped off entirely. No amount of fertilizer is going to bring life back to this kind of soil.
SOLUTION: Do a soil test to find out what your soil is made of before reaching for the fertilizer. Fix the soil first, then use fertilizer to adjust accordingly.
OTHER THINGS YOU CAN DO TO HELP YOUR TREES
Fite offered a couple additional suggestions for how to get your trees back in shape (assuming you haven’t destroyed most of the root system by building too close):
Invigorate the roots
Many times the best thing you can do for your trees is to invigorate their roots. Bartlett uses an air spade, a tool that uses an air compressor to generate a high velocity jet of air to dislodge the soil. The rush of air breaks up and ‘tills’ the soil without removing it to a depth of about 8’ (the depth of the feeder roots). This method of improving the soil leaves root systems intact.
Air spade in use at Western Illinois University
The air spade was originally developed by the military to clear land mines. Nowadays, it is used by landscapers and arborists to invigorate roots. After the soil is excavated, add soil, compost and mulch to settle the soil back down. Then water the tree.
Look into Biochar
Mulch breaks down because the microbes eat it for carbon, resulting in it having to be replaced every year. Charcoal, on the other hand, is stable, meaning it lasts for centuries. Biochar is a fine-grained, highly porous charcoal that is added to soil to help it retain water and nutrients. It acts like a dry sponge, but on its own contains no nutrients, so blend with compost for best results.
Biochar acts as a dry sponge, but contains no nutrients
“Put the two together and magic happens,” said Fite.
Buyer beware, though, biochar is not regulated, so buy from a reputable source and read the label carefully.
Kelby Fite, Ph.D. is a VP and Director at Bartlett Tree Research Lab.