Fertilizing Landscape Trees and Shrubs. Tutorial, permaculture

Fertilizing Landscape Trees and ...

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Fertilizing Landscape Trees and Shrubs. Tutorial, permaculture

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Virginia
Cooperative
Extension
Horticulture
REVISED 1999
PUBLICATION 430-018
Fertilizing Landscape Trees and Shrubs
by Bonnie Appleton and Kathy Kauffman*
Maintenance programs should be developed for trees
and shrubs in both residential and commercial landscapes.
A good maintenance program includes monitoring and
controlling insect and disease problems, suppressing weed
competition, and making timely applications of water,
mulch, and fertilizer.
Tree and shrub fertilization is especially important in
urban and suburban areas of Virginia where soils have
been altered due to construction. These urban soils tend to
be heavily compacted, poorly aerated, poorly drained, and
low in organic matter. Even where soils have not been af-
fected, fertilization may be needed as part of a mainte-
nance program to increase plant vigor or to improve root
or top growth.
Determining the Need to Fertilize
Visual inspection of trees and shrubs is often the best
overall factor to use in making fertilization decisions.
Look for:
• Poor or chlorotic leaf color (pale green to yellow)
• Reduced leaf size and retention
• Premature fall coloration and leaf drop
• Reduced twig and branch growth and retention
• Overall reduced plant growth and vigor
In addition to observing signs of possible nutrient defi-
ciencies on plants, soil and foliar analysis can be used to
help determine or confirm whether supplemental fertiliza-
tion is needed.
Fertilizer Objectives
How and when to fertilize landscape trees and shrubs
depends on:
• Maintenance objectives (stimulate new vs. maintain ex-
isting growth)
• Tree and shrub ages (generally more for younger and less
for older plants)
• Plant stress levels
Fertilizer Selection
A variety of fertilizer types exist:
• Complete (N-P-K) vs. incomplete (one or more select
nutrients)
• Organic vs. inorganic
• Fast release vs. slow release
• Dry (granulated, pelletized, spikes, pulverized, encapsu-
lated) vs. liquid
To help determine the type of fertilizer to apply, con-
sider the following: type of plant, time of year, desired rate
of plant reaction, application methods and equipment cost,
Chemical fertilizers, analysis, speed of reaction and effect on soil pH.
#’s of each
Speed of
fertilizer to use
Reaction and
to get roughly
Fertilizer
Analysis
Leaching
Soil Reaction
1#N/1000ft2
Ammonium nitrate
33-0-0
Rapid
Acidic
3
Ammonium sulfate
20-0-0
Rapid
Very acidic
5
Urea
46-0-0
Rapid
Slightly acidic
2
Ureaformaldehyde
38-0-0
Slow
Slightly acidic
2 1/2
Di-ammonium phosphate
18-46-0
Rapid
Acidic
5 1/2
Calcium nitrate
15-0-0
Rapid
Alkaline
6 1/2
Potassium nitrate
13-0-44
Rapid
Neutral
7 1/2
10-10-10
10-10-10
Rapid
Varies with N source
10
Osmocote
18-6-12
Slow
Acidic
5 1/2
* Extension Horticulturist; Graduate Student, Hampton Roads AREC
VIRGINIA POLYTECHNIC INSTITUTE
AND STATE UNIVERSITY
Virginia Cooperative Extension programs and employment are open to all, regardless of race, color, religion, sex, age, veteran status,
national origin, disability, or political affiliation. An equal opportunity/affirmative action employer. Issued in furtherance of
Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department
of Agriculture cooperating. J. David Barrett, Interim Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg;
Lorenza W. Lyons, Administrator, 1890 Extension Program, Virginia State, Petersburg.
VT/028/0699/2M/994095/430018
VIRGINIA STATE UNIVERSITY
proximity to water sources, effect of soil type and pH, type
of deficiency, and results of a soil test or other sampling
method.
Most landscape plants benefit from a slow release ni-
trogen fertilizer that may be organic or inorganic. Keep in
mind that nitrogen is readily leached (washed through soil)
but phosphorus and potassium are not, meaning they re-
quire less frequent application. In Virginia soils, secondary
and minor nutrient deficiencies are rare, with the exception
of iron.
uptake is faster in moist warm soils)
• Plant vigor (plants under stress are less able to take up
available nutrients due to damaged or reduced root
systems)
Fertilizer Rates
Fertilizer use rates should be based on plant type, with
younger trees and shrubs generally receiving higher rates
(of N) than mature plants. Fertilizer rates for trees are no
longer based on trunk size or caliper, but on root system
spread, calculated by doubling the area of crown coverage
(crown coverage = 3.14 x radius
2
).
In general, use one to six pounds of actual N/1000ft
2
of
root zone. Evergreen shrubs and trees need less (1-3#)
while deciduous trees and shrubs commonly need more (3-
6#). Reduce the rate when plants are growing in restrict-
ed areas (sidewalk cuts, parking lot islands) or where roots
of multiple plants overlap.
If applied fertilizer will go over a turf area, do not ex-
ceed 1 1/2# N/1000ft
2
for any one application to avoid
overstimulating or burning the grass. Use split applica-
tions a few months apart if higher rates are needed. If a
soil test shows that P or K is needed, apply at the rate of 1-
2# and 1 1/2# N/1000ft
2
, respectively. If a complete fertil-
izer is used, the ratio of N-P-K should be 3-1-2 or 3-1-1.
Application Methods
Fertilizers can be applied either directly or indirectly to
plants. When turf is fertilized, tree and shrub roots that ex-
tend into the turf area absorb some of the fertilizer, and are
therefore indirectly fertilized. Turf fertilization rates
should be supplemented only if trees and shrubs are show-
ing symptoms of nutrient deficiency.
Direct application of fertilizer may involve incorpora-
tion into the backfill soil or placement in the planting hole
at planting time. However, the most common form of di-
rect fertilizer application, broadcasting, is generally the
most effective, especially relative to cost. Simply broad-
casting the desired fertilizer over the soil atop the tree and
shrub roots and watering it in is usually adequate. Com-
pacted soil should first be aerated or raked. The chart
below describes other direct application methods.
Application Timing
Fertilizer should be applied when plants need it, when
it will be most effective, and when plants can readily take
it up. Late summer and early fall fertilization may stimu-
late new growth that is not winter hardy, and summer
drought may interfere with nutrient uptake, but spring, fall,
and winter applications are acceptable. A split application
may be beneficial, applying half the yearly rate in early
spring and the rest in the fall as or after plants go dormant.
If water is unavailable, do not fertilize at all - plants will
be unable to absorb the nutrients.
(During a dry season,
fertigation - application of fertilizer through an irrigation sys-
tem - can be beneficial.)
Fertilizer Placement
Fertilizer should not be concentrated around the stem
or trunk of a tree or shrub, but should be applied over as
much of the plant’s root zone as possible. For trees and
shrubs, fertilizer should be applied over an area twice as
large as the crown spread or dripline. Since most land-
scape plant roots grow in the top foot of soil, surface or
shallow, but not deep application, is recommended.
Factors Affecting Fertilizer Uptake
Numerous factors affect how easily and well trees and
shrubs absorb fertilizers. The most important uptake fac-
tors are:
• Fertilizer form (inorganic, fast release, or liquid forms are
absorbed faster than organic, slow-release, or dry forms)
• Soil type (clay particles and organic matter adsorb or
bind more nutrients than sand, so fertilizer application
needs to be more frequent in sandy soils, but with lower
rates each time due to leaching potential)
• Soil moisture content and soil temperature (nutrient
Summary
Tree and shrub fertilization is only one part of total
plant maintenance. Fertilization may not benefit a plant if
it is under stress from poor soil aeration or drainage, satu-
rated soil, insufficient light or space, or excessive pest
problems. All factors influencing plant growth should be
kept at optimum levels to ensure plant vigor.
Application Method Advantages
Disadvantages
Subsurface
Aerates soil Convenient
Special fertilizer and drilling or soil injection
equipment needed
Foliar sprays
Relieves symptoms of
Temporary benefits; doesn’t address underlying
micronutrient deficiencies
soil problem
Injection and implantation
Relieves deficiency symptoms
Temporary benefits; wound creates entry for
insects/diseases
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