International Society of Arboriculture
All trees are
sensitive to root disturbance. Examples include construction, landscaping,
sprinkler installation, and grade changes. The effects of these changes on
pre-existing trees can be quite devastating and can take five to ten years to
become fully visible. To understand how trees are affected by root disturbance
it is important to understand the structure of a tree’s root system. Ninety
percent of the root system is located in the first 12 to 18 inches of soil. The
roots extend radials from the trunk one to two times the height of the tree. During
construction, the root system is cut to install foundations, sidewalks,
driveways, utilities, pools, landscape beds, and irrigation systems. The closer
to the tree the construction occurs, the more destructive it is.
THERE ARE 2 MAIN
TYPES OF TREE ROOTS:
These roots anchor the tree and keep it from falling over. The
structural roots begin at the base of the tree called the root flare. They grow
mostly horizontally in the soil and taper in diameter as they move away from
the tree. The cumulative mass of the root system keeps the tree upright, not
just the tap root. The tap root can dissipate over time and is replaced with a
series of sinker roots (smaller tap roots) through the entire root zone.
conclusion, the closer to the trunk roots are cut, the higher the chances the
tree will be unstable and fall over. A good rule of thumb is to stay
approximately 6” to 12” from the trunk for every inch in diameter the tree is
at DBH (diameter at breast height or 4.5’ above grade) For example: A 16” Live
Oak requires a construction free distance of 8’ to 16’ from the trunk. Your
certified arborist can assist you in deciding the critical distance depending
on your individual situation.
damage from construction
are the small fibrous roots that absorb water and minerals. The more of these
roots that are
destroyed, the more the tree’s ability to feed
itself is impacted. Cutting roots is not the only way these roots are killed.
Damage also occurs through compaction of the soil from heavy equipment
repeatedly driving over the root zone or supplies being stored under the tree.
Compaction of the soil reduces the pore space between soil particles,
eliminating the oxygen in the soil which causes root death. Signs of feeder
root damage are small pale colored leaves, leaves turning brown on the edges or
shedding early, and the tips of the
limbs dying over time. In general, it is
recommended not to remove more than 20-30% of the tree above and below ground
at a given time.
Tree species react differently to construction changes, but
all trees take several years to acclimate and recover. The most common damage
following construction is from irrigation installation and over-watering.
Sprinkler installation can cause just as much damage as initial construction
due to the amount of trenching in the root zone. When laying out sprinkler
lines, limit the trenching across the root zone under the trees. Radial
trenching can aid in this process.
After all of the construction is completed
and the irrigation is installed, it is critical to not over water the existing
trees. Most of our native trees are adapted to dry conditions and are adapted
to receiving approximately 30” of rain a year. When an increase in water
occurs, the soil can stay saturated, reducing the amount of oxygen. Roots begin
to rot in this anaerobic condition, and trees can decline or die.
stay saturated longer than sandier soils. It is recommended to limit watering
to 1” of water a week during the growing season including rainfall. This allows
the soil to be moistened and then dry out, mimicking our region’s natural
rainfall pattern. Remember slope, drainage, rainfall, and sun exposure will
vary the frequency and duration of the sprinkler system schedule. It is also
helpful to match the water.
When it comes to fertilizing trees,
you'll find no shortage of options and decisions. You have choices of how and
how much fertilizer to apply, what its nutrient content should be and much
more. Examining all these options, for which plenty of differing opinions
exist, could fill a book. So to provide a brief yet useful step-by-step guide
to tree fertilization, this article will focus on the most common method:
liquid injection. Liquid injection involves placing fertilizer as a
liquid suspension or solution into the soil around the tree. One advantage of
liquid injection is that by applying fertilizer below the soil surface, you can
make certain that nutrients, such as phosphorus, are more available to tree
roots by placing them below the thick turf zone.
Most managers' choice if you are like most grounds
managers, once you've decided on performing liquid injection, you will use a
fertilizer that is primarily slow-release but also includes some fast-release
For a general rule of thumb in determining how much nitrogen to
apply, use the rate suggested by the forthcoming American National Standards
Institute (ANSI) A300 guidelines: 2 to 6 pounds (0.9 to 2.7
kg) of actual nitrogen per 1,000 square feet. The ANSI guidelines also
suggest injection spacing and depth levels: 18 to 36 inches apart and 4 to 12
inches deep. Of course, it goes without saying that
you'll also need the proper equipment to perform liquid-fertilizer injections,
which basically means a truck with a pressurized pumping system.
Determine how much fertilizer to inject before you
can begin the injection process, you must calibrate the probe. That is, you
must determine the amount of fertilizer to deliver per injection point. To do
so, you need to consider the following: *Injection spacing *Analysis of the
fertilizer (specifically the amount of nitrogen) *Mixing rate of raw fertilizer
per 100 gallons (378.5417
Ltr) (described on the fertilizer container)
*Rate of nitrogen per 1,000 square feet *Probe flow rate (how many gallons per
minute flow through the probe). You can determine the probe flow rate by
observing how fast the probe fills a 5-gallon (18.9 ltr) bucket and dividing the number of minutes, or
fraction thereof, into 5.
Knowing this information, let's consider an example
to get started. Imagine you have the following factors: *3-foot injection
spacing *26-3-13 fertilizer (26 percent nitrogen) *Mixing rate of 20 pounds (9 kg) (of fertilizer per 100 gallons
*Desired nitrogen rate of 2 pounds (0.9
kg) actual nitrogen per 1,000 square feet *A 9- x 9-foot area to fertilize. Now run the math: A 9- x 9-foot area is 81 square
feet. With a 3-foot spacing, the area will contain nine injection sites.
You want 2 pounds (0.9 kg) of nitrogen for each 1,000 square feet.
that 20 (9
kg) pounds of 26-3-13 fertilizer per 100 gallons (378.5412 ltr) equals about 5 pounds of nitrogen per
100 gallons (378.5412
ltr) (26 percent x 20 pounds (9 kg) = 5 pounds (18.9 ltr), with rounding). But you want to use only 2 pounds (0.9 kg) of nitrogen per 1,000 square feet. So
to apply 2 pounds (0.9kg) per 1,000 square feet, you need 40 gallons (151.4 ltr) of fertilizer.
Now that you understand how much fertilizer you
need for 1,000 square feet, you can determine how much you need for 81 square
feet. Figure the ratios again: 40 gallons / 1,000 square feet = X gallons / 81
square feet 151.4 ltr/ 1,000 square feet = X ltr/ 81 square
To solve for the number of gallons (kg) per 81
square feet: 3,240 (40 x 81) / 1,000 = 3.25 gallons of
fertilizer 12,080 (151.4 x 80) / 1,000 = 12.08 ltr of
fertilizer Let's say it took the probe 90 seconds (1.5
minutes) to fill a 5-gallon (18.9
ltr) bucket. 5 gallons / 1.5 minutes = 3.3 gallons per minute 18.9 ltr / 1.5 minutes = 12.4 ltr per minute If you need 3.25 gallons (12.3 ltr) of fertilizer and the probe delivers
3.3 gallons (12.4 ltr) per minute, you need roughly 1 minute of flow divided between the nine injection holes
in the area to be fertilized.
To determine how long each injection should last
per hole: 60 seconds (the 1 minute of flow needed over the
entire area) / 9 holes = 6.5 seconds per hole. Thus, leave the probe in each injection hole for
roughly 6.5 seconds to provide 2 pounds (0.9kg) of nitrogen per 1,000 square feet.
(Bear in mind that these are rough guidelines.
Factors such as soil density, the number of probes you are using and the
pressure of the system all affect these values. If these variables change, you
may have to re-calibrate the probe. Unlike injecting medication into humans,
varying the injection amounts by as much as 25 percent is okay.)
Prepare the injection site Before you begin the
injections, you must assess and prepare the area around the tree. Check the
Are there irrigation heads? If so, try to determine where the
irrigation lines are so you do not damage them. Check for gas lines, too. If
you are at a residence and you see a gas grill without a propane cylinder, a
gas line may be running underground from the house to the grill, so you'll need
to watch for it too. Are flower beds planted near the tree?
If so, you will need to both step carefully and watch that your hose does not
drag through the flowers.
Be aware of property lines as well.
Unless you have obtained written consent from a neighbor, you can only
fertilize the area of the tree that is on your clients' property. (In which
case, you should fertilize at the pre-determined rate, not at an increased rate
to compensate for partial fertilizing.)
Establish the injection grid Most applicators base
the injection grid on an imaginary box on the ground that encompasses the
canopy edges. The injections will fall within the box at your pre-determined
injection spacing. Because the grid is something you'll likely want to
simply "eyeball," it is helpful to find a visual reference point
beyond the tree's dripline.
To choose one, stand at the edge of the canopy.
Pick a point that looks like it is at the edge of the canopy on the other side
of your box.
Then pick another point on a side of a building, or perhaps across
the street, that is beyond the original point. By sighting both points as you move
through your grid, you will be able to systematically and thoroughly cover the
area to be fertilized. This technique is particularly helpful if the ground is
uneven or if the area is not square.
Begin the injections Now that you have oriented
yourself in the grid box, you are ready to begin the injections:
*Stand at your
starting point at the edge of the canopy.
*Insert the probe into the soil.
*Inject the fertilizer for the time calibrated. (In the above example, that
would be roughly 6 to 7 seconds.)
*Remove the probe.
*Walk 3 feet (again based
on the above example) to your next grid point, keeping an eye on your visual
*Insert the probe into the soil and repeat the process.
until you have reached the other side of the canopy. Step over 3 feet to begin
the next row.
*Insert the probe and repeat the process until you have covered the
entire grid area beneath the canopy.
Bear in mind that trees have many roots
growing close to the trunk. So it is best to stay a foot or two away from it so
that you can avoid driving the probe into the large roots close to the soil
surface. You don't always have to use a grid
pattern to determine the injection area.
For example, if you need to fertilize
a conifer or other tree under whose canopy you cannot reach, the injection
pattern should be circular, using about three rings. Perform the first ring of
injections by reaching inside the canopy. The second ring should be at the edge
of the canopy.
The third injection ring should be about a foot or two outside
the canopy. Inspect your injections Whether you use the box or
circular pattern, uniformity is important. About 2 to 4 weeks after you have
fertilized the tree, tufts of greener grass should appear at each injection
Your client will be able to see if your injections varied from the grid.
If they have, your work will have an unprofessional look to it. Be sure to return to the site after you
have fertilized a tree. Look for the tufts of grass to see whether you adhered
to the grid. And observe the tree.
Is it doing well? Does it look better or
worse than before you fertilized it? As you observe, remember that
fertilization is not a cure-all. Many other factors-such as soil moisture,
amount of organic material, soil compaction, root damage and so on-may actually
affect a tree more than fertilization.
Tree fertilization using the liquid-injection
method takes some practice. Calibrating the probe may seem complicated at
first, but it does eventually become easier. Likewise, following a grid pattern
by sight does become more efficient over time. The benefits to the tree are
well worth the learning curve.
Dr. Rex Bastian is director of technical services for The Care of Trees
with headquarters in Wheeling, Ill., and offices serving metropolitan
Washington, D.C., and New York City.
is one of
the most beneficial practices a homeowner can use for bett er tree health. Mulches are materials placed over the
soil surface to maintain moisture and improve soil conditions.
Mulching is one
of the most beneficial acts a homeowner can do for the health of a tree.
However, improper mulching materials and practices may have little, or even
negative, impact on the trees in your landscape.
• Helps reduce soil moisture loss
• Helps control weed germination and
• Insulates soil, protecting roots
from extreme summer and winter temperatures
• Can improve soil biology,
aeration, structure (aggregation of soil particles), and drainage over time
• Can improve soil fertility as
certain mulch types decompose
• Inhibits certain plant diseases
• Reduces the likelihood of tree
damage from “weed whackers” or the dreaded “lawn mower blight”
• Gives planting beds a uniform,
well-cared-for lookTrees growing in a natural forest
environment have their roots anchored in a rich, well-aerated soil full of
essential nutrients and soil microorganisms.
The soil is blanketed by leaves,
organic materials, and living organisms that replenish and recycle nutrients.
This environment is optimal for root growth and mineral uptake. Urban landscapes
and new developments, however, are typically harsher environments with poor
quality soils, reduced organic matter, and large fluctuations in soil
temperature and moisture.
Applying a 2- to 4-inch (5- to 10-cm) layer of
organic mulch can mimic a more natural environment and improve plant health.
are available in many forms.
The two major types of mulch are inorganic and
organic. Inorganic mulches include various types of stone, lava rock,
pulverized rubber , geotextile fabrics, and other materials. Inorganic mulches
do not decompose and do not need to be replenished often.
On the other hand,
they do not improve soil structure, add organic materials, or provide
For these reasons, most horticulturists and arborists prefer organic
mulches.Organic mulches include wood chips,
pine needles, hardwood and softwood bark, cocoa hulls, leaves, compost mixes,
and a variety of other products usually derived from plants. Organic mulches
decompose in the landscape at different rates depending on the material,
climate, and soil microorganisms present.
Those that decompose faster must be
replenished more often. Because the decomposition process improves soil quality
and fertility, many arborists and other landscape professionals consider that
characteristic a positive one, despite the added maintenance.As beneficial as mulch is, too much
can be harmful.
The generally recommended mulching depth is 2 to 4 inches (5 to
10 cm). Unfortunately, many landscapes are falling victim to a plague of
overmulching. “Mulch volcanoes” are excessive piles of mulch materials applied
around the base of trees.
While organic mulches must be replenished over time,
buildup can occur if reapplication outpaces decomposition or if new material is
added simply to refresh color.
Deep mulch can be effective in suppressing weeds
and reducing maintenance, but it often causes additional problems.
• On wet soils, deep mulch can lead
to excess moisture in the root zone, which can stress the plant and cause root
• Piling mulch against the trunk or
stems of plants can stress stem tissues and may lead to the development of
insect and disease problems or stem girdling roots.
mulches, especially those containing fresh grass clippings, can affect soil pH
and may eventually lead to nutrient deficiencies or toxic buildups.
piled high against the trunks of young trees may create habitats for rodents
that chew the bark and can girdle the trees.
blankets of fine mulch can become matted and may reduce the penetration of
water and air.
Anaerobic “sour” mulch may give off pungent odors, and the alcohols and organic
acids that build up may be toxic to young plants. The choice of mulch and the method of
application can be important to the health of landscape plants. The following
are some guidelines to use when applying mulch:
Determine whether soil drainage is adequate and if there are plants that may be
affected by the choice of mulch. Most commonly available mulches work well in
most landscapes. Some plants may benefit from the use of slightly acidifying
mulch, such as pine bark.
• For well-drained sites, apply a 2-
to 4-inch (5- to 10-cm) layer of mulch (less if poorly drained). Coarse mulches
can be applied slightly deeper without harm. Place mulch out to the edge of a
tree’s crown or beyond. Remember, if a tree had a say in the matter, its entire
root system (which usually extends well beyond the drip line) would be mulched.
• If mulch is already present, check
the depth. If sufficient mulch is present, break up any matted layers and
refresh the appearance with a rake. Some landscape maintenance companies spray
mulch with a water-soluble, vegetable-based dye to add color to faded material.
If mulch is piled against the stems or tree trunks, pull it back several
inches/centimeters so that the base of the trunk is exposed. Composted wood
chips can make good mulch, especially when they include some bark and leaves.
Fresh wood chips also may be used around established trees and shrubs. Avoid
using fine, non-composted wood chips, as soil nitrogen may be taken up by the
roots as the wood chips decompose.