FAQ - Fertilizing Trees and Shrubs
Landscape plants, like all living
things, need nutrients to survive. Many of the elements that
are essential for a healthy landscape are already in the
soil, but fertilization is often needed to supplement these
nutrients. When nutrients are added, care must be taken to
apply only what the plants will use. Too much fertilizer can
damage plants and can also impair water quality.
You do not, however, have to
choose between having a healthy, attractive landscape and
protecting water quality. Knowing how to determine proper
nutrient applications for your landscape is the key. Soil
properties, types of nutrients, plant needs, fertilizer
types, application methods, and application timing need to
be considered. Here is some basic information to help you
make decisions that will enhance your landscape and are
Fig 1: Tree left,
fertilized 3 consecutive years. Tree right, no
the fertilizer needs of trees and shrubs doesn't have to be
a mystery. Just like people, plants have basic nutritional
needs. And just like people, they need more of some
nutrients than they do of others. Plant nutrients can be
groups into macro-nutrients (those they need a lot of) and
micro-nutrients (those they
need in small amounts).
Nutrients) Every package of fertilizer should give
its nutritional value. Usually it is indicated by three
numbers such as 36-6-6. Those numbers represent the
phosphorous and potassium. 36-6-6 means that package
contains 36% nitrogen, 6% phosphorous and 6% potassium by
weight. Most fertilizers also contain some of the
micro-nutrients. They may be specifically identified or the
label may just indicate that they are included. Fortunately,
almost all the micro-nutrients plants need are already
available in the soil.
When you walk over to the
fertilizer section of the garden center you will notice
there are lots and lots of choices, some of them with the
same nutritional analysis. How do you decide which one to
use? If you can, take a few minutes to learn a little about
synthetic vs. organic fertilizer and water soluble vs. dry
formulas. They are all good products that will meet the
needs of your plants, but you also want the fertilizer you
choose to fit your needs. Some are easier to use than
others, some are less expensive and some last longer than
trees and shrubs need to be fertilized?
Trees and shrubs that are young and actively growing
will perform much better (see fig. 1) if they are
fertilized. On the other hand, they won't die if they have
to rely on their own resources. Research as to whether or
not mature plants should be fertilized is less conclusive.
We do know for sure that mature trees need fertilizer and
should be applied at moderately amounts and more often, as
mature trees grow more slowly. High application amounts will
cause rapid lush growth, the trees energy is diverted from
defense to rapid growth and that rings a loud dinner bell
there any circumstances in which trees and shrubs should not
It is best to avoid fertilizing when trees and shrubs
are newly planted (their first growing season) and when they
are not healthy (unless a trained arborist or horticulturist
has looked at the tree and diagnosed a specific nutritional
deficiency). However, a miracle grow type fertilizer can be
applied about once a month along with regular watering.
Don't the trees and shrubs get enough fertilizer from what
is put down on the lawn?
Actually, they do make use of some of that
fertilizer. That can be both a good thing and a bad thing.
Too often trees and shrubs are damaged when they absorb lawn
fertilizers that contain herbicides. When you are growing
flowering trees or shrubs, lawn fertilizers often supply too
much nitrogen and not enough phosphorus. The result may be
decorative flowering trees such as Bradford pears, lilacs or
crabapples that produce lots of luscious green growth and
not many blooms. Until a tree is relatively mature with an
extensive, far reaching root system, it is best not to rely
on your lawn fertilizers to provide all their nutritional
Why do trees and shrubs have special
fertilizing needs when plants in nature get along without
In nature, plants rely on
sunlight, rain and the nutrients in the soil. Nature limits
the types and numbers of plants by the relative amounts of
these basics available. In our landscapes, we grow many
plants together with differing requirements, often in a
variety of soil types, surrounded by competing grass. If
they are to thrive, we have to supplement what Nature can't
What makes fertilizing trees and shrubs any different than
fertilizing any other plants?
In our climate, all plants are either herbaceous or
woody. Woody plants have parts (such as trunks, branches,
twigs, or evergreen needles or leaves) that live over from
season to season. That gives them a big head start each
year. Herbaceous plants do not have that advantage. They
start from the soil line in spring and die back to the
ground in fall. That means they have to expend a tremendous
amount of energy during the growing season to produce all
those stems and leaves and to help them out, we need to make
sure they have the fertilizer they need.
there a difference between the fertilizer needs of woody
plants and other plants?
Most importantly, woody plants shouldn't be heavily
fertilized in the growing season. Since the natural response
to fertilizing is growth, slow and natural growth is good,
rapid lush growth is ringing a loud dinner bell for insects
and predators. The tree diverts its energy from defense to
growth. To avoid this problem, we do not recommend
fertilizing heavy applications, the rule of thumb is lighter
and more often applications for trees.
all trees and shrubs have the same fertilizer needs?
Most trees and shrubs are grown for their foliage and
structure but there are also some varieties grown for their
beautiful flowers. Plants that are grown especially for
their flowers need extra phosphorus. This is the middle
number in the analysis (10-20-10). Phosphorus encourages
blooming as well as strong roots and disease resistance.
Trees and shrubs not grown for their flowers need less
phosphorus and more nitrogen. Nitrogen is the first number
in the analysis (10-20-10) and encourages leaf growth.
(see Nitrogen article)
there different types of fertilizer for woodies?
Fertilizers come in several forms. Fertilizers such
as granulated 10-10-10 are loose, dry products, The same
fertilizer is often compressed to form stakes or tablets to
be driven into the ground. There are also liquid forms that
can be applies as a foliar spray, deep root injected into
the soil, drenched or poured directly on the ground. I am
not a strong advocate of the tree spikes, they can
(by chance driven into and
damage a major root causing burning) also they are expensive
and they are good money makers for the seller and
How do I know what kind to use?
The fertilizer needs to match both your needs and the needs
of the plant. Dry fertilizer is inexpensive and the best
method of application. Spreading it on the surface, one and
a half times the drip edge provides better distribution and
the roots can absorb the nutrients as they leach into the
soils. ( ISA 's research confirmed, surface
application is the best and most effective method).
Cation Exchange) If you are drilling it in, it will
take more of your time and labor and can prove hazardous
should you drill into and damage a root. It may have a
tendency to cause burning high into the canopy or cause root
rot. Spikes are quick and easy to use, but they are a little
more expensive and their nutrients aren't as evenly spread
around the tree. Also spikes can cause similar damage as
drilling applications. Deep Root feeders are easy, but it
takes quite a bit of time to move them from site to site to
get good coverage. However, their nutrition is directed
where it is needed, at the tree roots. Ninety five percent
of all tree roots are with in the top twelve or sixteen
inches of the surface. There isn't just one right answer.
What would be a good dry fertilizer for trees and shrubs?
A quality product Garden Food 10-20-10 works well
with decorative flowering trees and shrubs. With
non-flowering woodies, a basic 13-13-13 works well. However
in our area our soils are high alkaline 7.8 to 8.3
Soil pH) and there is usually ample phosphorus and
potassium. Because our soils are high alkaline and usually
high in potassium and phosphorous, this combination acts as
an iron and mineral blocker
Chlorosis). As a general rule a high nitrogen 21-0-0
with 24% sulfur is sufficient. The sulfur reacts with soil
moisture and lowers the pH and then the phosphorous,
potassium and other minerals become more soluble. It would
be best to contact your local Nurseryman, County Extension
Agent or an Arborist for a suitable recommendation.
should dry fertilizers be applied?
You have three options. It can be spread with a lawn
fertilizer spreader but you have to be careful not to
over-fertilize any surrounding plants. Remember light
applications and more often are better that one heavy
application per year. The next best option is to deep root
injection into the ground about 2-3 feet apart and 6-8
inches deep, on a grid system covering the area about one
and a half to twice the drip edge area of the tree. Foliar
applications should be applied two or three times a year for
best results. However you may have to hire a tree service or
arborist for these types of application
there a good organic alternative for trees and shrubs?
Both Synchronicity and Milorganite are great balanced
organic fertilizers that work with trees, shrubs, and lawns.
Organic fertilizers are naturally slow-release.
I use a product such as Miracle-Gro?
Water soluble fertilizer such as Miracle-Gro or Excel Gro
can be used as a supplement the first few seasons, but isn't
very effective with trees and shrubs once they are
well-established. It washes through the soil too quickly.
Are the root feeders a good idea?
To be effective, you need to understand a few things
about root feeders. First of all, it is essential that they
are not pushed too far into the ground. In fact, six to
eight inches is ideal. Because the probe is 2-3 feet long,
it is tempting to push it further into the ground. But all
you want to do is push it far enough to bypass the surface
root systems of the surrounding turf or other ornamental
plants. Research has shown that no matter how big that plant
is, almost 95% of all the roots of trees and shrubs that
actually absorb the water are within the top 12 inches of
the soil. Trees and shrubs have roots that go deeper, but
they are thicker roots whose job it is to stabilize the
plants, not absorb water and nutrients. It is important to
remember that the fertilizers in root feeders are water
soluble and are used up quickly and for these reasons it is
important use organic fertilizers.
Organic fertilizers are naturally slow release. There are
slow-release fertilizers that can be injected into the soil
by professional arborists that do work well.
How often should woody plants be
During the first year after they are planted, they should
just be watered or given a very weak solution, such as
Start-Up. Once they are established, trees and shrubs will
need to be fertilized moderately and regularly throughout
the year. Dry applications should be light and often up to 3
or 4 times a year.Your tree roots continue to grow during
the winter months and fertilization is important before
dormancy and again in early spring.
What time of year should trees and shrubs be fertilized?
In our area, they can be fertilized , early to late
winter for root growth, early spring just prior to bud
break, late spring to early summer following spring rains,
as a nutrient replacement and again early fall to provide
supplemental nutrients before trees go dormant. During
these periods, the roots are actively growing.
Are there any trees and shrubs that
don't like to be fertilized?
There are varieties of Native trees and shrubs that would do
well with very little fertilizer if they were growing in
their natural settings. But when we plant them in our
landscapes, they can become stressed by excessive
watering, compacted soil and competition from grass,
fertilizers may help them grow but we must make every
attempt to simulate their natural setting and growing
little Tree Humor)
will I know if the plants are getting enough fertilizer or
the right fertilizer?
If your plants aren't growing or blooming as much as
you think they should, they may need a boost of fertilizer.
With some plants, the leaves will be a lighter green than
normal if they need fertilizer. But several other factors
can influence plant growth or color, so it is a good idea to
bring a sample in and ask a horticulturist before assuming
fertilizer is the answer.
there a danger of over-fertilizing?
Too much at one time can be hard on any plant,
especially if the soil is dry. Follow the directions on the
package for application rates and that shouldn't be a
problem. With all plants, fertilizing too often may result
in excessive growth that is weak and susceptible to disease
problems and insect attack.It may also cause leaf
deformation or burning. Over-fertilized plants are more
easily stressed by lack of water, excess water, insects or
diseases. For trees and shrubs, too much fertilizing may
weaken their root system and make them less winter-hardy.
Are there any other tips for
fertilizing trees and shrubs?
Just be sure to follow all the directions on the package and
try to spread the fertilizer out evenly. In dry weather is
best to, water the plants the day before fertilizing. Also,
water in dry fertilizer after it is applied. This helps
activate it right away and keeps it in place. If we are in a
drought situation and water restrictions are in force, just
hold off all fertilizing until conditions are more
favorable. Keep in mind that fertilizers will leach through
sandy soils more quickly than heavy, clay soils.
fertilizing trees and shrubs are there any other important
tips for plant care?
The application of "Protium
Hydroxide" is one of the single most important things
you can apply to your trees and shrubs. Use it liberally,
quantatively and frequently. It is not available in your
garden supply store or local nursery. However, is available
in your local grocery store and better still at the end of
your water hose. It is H2O
Typical Established Tree or Shrub Root
Most of the feeder roots (95%) capable of taking up
fertilizers are within the top 12-18 inches of the soil
On average, feeder roots extend out as far from the trunk as
the tree is tall.
Apply fertilizer ring around the tree starting just inside
the dripline and extending several feet out beyond the
Roots close to the trunk are heavy conduits for the finer
roots located at or beyond the dripline where the rain
falls. They do not absorb fertilizers.
Root System for a New Tree or Shrub
Feeder root systems develop slowly extending into the
surrounding soil as the plant grows. Be sure to apply the
fertilizer to the feeder roots.
Be sure to follow directions on the fertilizer package for
application rates. Tender young roots are easily burned if
the fertilizer is applied too heavily.
Contact: cell: 830.257.8871
Jim Rediker - Nurseryman - Arborist - TDA Certified
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