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.
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.
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.
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.
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.