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Karen Rockoff 
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                     Karen Rockoff  Arborist  - TDA Certified

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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 environmentally sound.

Fig 1: Tree left, fertilized 3 consecutive years. Tree right, no fertilizer 

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). (see Essential 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 macro-nutrients (NPK) nitrogen, 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 others.   

Do all 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 for insects.    

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). 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 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't 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 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.   

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. (see Nitrogen article)

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 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 dangerous,
(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 manufacturer.  

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).  
(see 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 (See 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 (see Iron 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.   

How 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

Is 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.

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 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.  

What about slow-release 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 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 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 conditions
(see A little Tree Humor)

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 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.
Besides 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 System

Most of the feeder roots (95%) 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.

Karen Rockoff 
Contact:  Cell: 830.955.0304
                     Karen Rockoff  Arborist  - TDA Certified


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