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Fertilizing Trees and Shrubs
Meeting 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). Every
package of fertilizer should give its nutritional value. Usually it is
indicated by three numbers such as 10-20-10. Those numbers represent the
macro-nutrients nitrogen, phosphorous and potassium. 10-20-10 means that
package contains 10% nitrogen, 20% phosphorous and 10% 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 others.
Do
all trees and shrubs need to be fertilized?
Trees and shrubs that are young and actively growing will perform much
better 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 a lot less fertilizer because they are
growing much more slowly.
Are
there any circumstances in which trees and shrubs should not be
fertilized?
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).
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 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 needs.
Why
do trees and shrubs have special fertilizing needs when plants in nature
get along without it?
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 provide.
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.
Is
there a difference between the fertilizer needs of woody plants and
other plants?
Most importantly, woody plants shouldn't be fertilized late in the
growing season. Since the natural response to fertilizing is growth, it
is important that any new growth have plenty of time to harden.
Do
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.
Are
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 injected into the soil or poured on.
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. If you are spreading it on the
surface, dry fertilizers are easy to apply. If you are drilling it in,
it will take more of your time and labor. 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. Root feeders are easy, but it takes quite
a bit of time to move them from site to site to get good coverage and
their nutrition doesn't last as long. There isn't just one right answer.
What
would be a good dry fertilizer for trees and shrubs?
A product such as 10-20-10 works well with flowering trees and shrubs.
With non-flowering woodies, a basic 10-10-10 works well.
How
should dry fertilizers be applied?
You have two options. It can be spread with a lawn fertilizer spreader
but you have to be careful not to over-fertilize any surrounding plants.
The better option is to drill or punch holes into the ground about 2-3
feet apart and 6-8 inches deep on a grid system covering the area to be
fertilized. Divide the total amount of fertilizer needed between the
holes. Cover the holes with a handful of soil or sand.
Is
there a good organic alternative for trees and shrubs?
Both Synchronicity and Milorganite are great balanced organic
fertilizers that work with trees and shrubs. Organic fertilizers are
naturally slow-release.
Can
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 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 all of
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 also important to remember
that the fertilizers used in root feeders are water soluble and are used
up quickly. Nutritionally, they aren't much different than using a
product such as Miracle-Gro.
What
about slow-release fertilizers?
Organic fertilizers are naturally slow release. Products such as
Osmocote are not well suited for trees and shrubs because they would
still be fertilizing late in the growing season which isn't advisable in
our climate. 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 fertilized?
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 yearly.
What
time of year should trees and shrubs be fertilized?
In cold northern climates they can be fertilized either in the spring as
soon as they show signs of new growth or in fall as soon as they have
gone dormant. During both of these periods, the roots are actively
growing. Avoid fertilizing during the last 6 weeks of the growing
season.
Are
there any trees and shrubs that don't like to be fertilized?
There are varieties of 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 where they are stressed by
compacted soil and competition from grass, fertilizers not only help
them grow but keep them stronger.
How
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.
Is
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 problems.
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, 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. Keep
in mind that fertilizers will leach through sandy soils more quickly
than heavy, clay soils.
Typical
Established Tree or Shrub Root System
Most of the feeder roots capable of taking up fertilizers are within the
top 12-18 inches of the soil surface.
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 dripline.
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.
Typical
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.
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