Fertilize woody ornamentals the right way

Another article on fertilization.What's the big deal, anyway? Anyone with a bag of 10-10-10 and a spreader can apply fertilizer, right? Well, yes and no. To do it correctly-minimizing fertilizer input and maximizing efficiency while maintaining healthy, attractive ornamentals-requires an understanding of nutrients, factors affecting nutrient absorption, fertilizer application rates and timing, and a host of other factors.

Fertilizer is of no value unless plants absorb it. Of course, most of the nutrients that plants absorb enter by the roots, so soil factors such as cation exchange capacity (CEC), pH and soil texture affect absorption. In addition, the method, timing, frequency and rate of fertilizer application, as well as the form (granular vs. liquid) and release rate of the fertilizer affect nutrient absorption by the roots. Let's consider some of these factors that affect nutrient absorption.

Application methods The five common ways to apply fertilizer are: surface application, soil injection (dry or liquid), foliar spray, trunk injection and incorporation. Each technique has its advantages and disadvantages. Surface application is the most economical but requires that the soil is not compacted and has a minimal slope to avoid fertilizer loss through runoff. This technique requires rain or irrigation water to move the fertilizer into the soil. Thus, compact or steeply sloped soils may not be suitable for surface application. For this reason, soil injection may be worth the additional cost of time and equipment on compacted or sloping sites. In addition, soil injection places immobile nutrients such as phosphorus (P) directly into the soil profile. For nitrogen (N), research has shown that surface application is just as effective as soil injection because N is mobile in most soils. Foliar sprays and trunk injections are effective with micronutrients (boron, copper, iron, manganese, molybdenum and zinc) but are only a partial solution. It is normally not possible to get enough N, P or potassium (K) into the plant with these techniques. Soil incorporation probably is the best technique for long-term P fertility, but it is limited to the time during soil preparation when you are planting the ornamental.

Designing a fertility program You can achieve a well-designed fertility program with three simple steps: 1. Maintain your soil pH between 5.5 and 6.5 for most ornamental plants. Exceptions exist, such as azaleas and rhododendrons, that perform best when the soil pH is 4.5 to 5.5. 2. Soil test to determine your P and K needs. 3. Manage N by choosing the proper N source (water-soluble or controlled-release) and applying the proper rate at the correct time.

Let's use these three steps as a framework with which to examine fertilizing ornamental plants.

* Soil pH. Soil pH is like a door. Soil pH between 5.5 and 6.5 opens the "door" and plant roots will have access to nutrients in the soil. If soil pH is outside of this range, availability of specific nutrients may be limited regardless of how much you apply (see figure, page Contractor 26). For example, P availability decreases dramatically below a pH of 5.5 and above a pH of 7.5. In addition, the availability of some micronutrients such as manganese and iron decreases with increasing pH. The effect of soil pH on micronutrient availability is the major reason that plants such as azaleas require a lower pH.

You should check soil pH, if not yearly, then every 2 to 3 years, because pH is not static. In the eastern United States, soil pH naturally decreases over time and using a fertilizer containing ammonium or urea will accelerate the decrease. Even though sending a soil sample to a state or commercial soil-testing service is the most accurate approach to determining soil pH, it is possible to determine soil pH yourself (see box, page Contractor 26).

* Phosphorus and potassium. Whether your soil is deficient in P or K depends in part on your location. For example, most soils in the Southeastern United States are grossly deficient in P but have moderate K levels. However, remember that urban soils-a polite way of referring to the dirt remaining after a completed construction project-usually bears little resemblance to the soils in the surrounding agronomic fields. Therefore, a soil test is the only way to determine if your soil needs P or K. Applying nutrients "just to be safe" without knowing if they are deficient wastes your time and your client's money. It canalso lead to nutrient losses, particularly with K. In addition, some nutrient excesses can cause problems just as deficiencies can.

P has several characteristics that are similar regardless of your location. P moves slowly in all soils. It moves so slowly that it may move only several inches after 50 years. Knowing this, what should be your major concern when you find you have a P-deficient soil? First, the ideal time to incorporate P into the soil is before you install plant material in a landscape. Because P moves so little in soil, a single application before planting can sustain the landscape for years to come. Second, if you apply P to an established landscape, you need to apply the most soluble P source you can find. Of all the dry fertilizers, diammonium phosphorus (18-46-0) is the most soluble P source. It is 2,000 times more soluble than triple superphosphate (0-46-0). Unfortunately, it may not be available in your area. if not, soil injection of a less-soluble P source is a good alternative.

A persistent belief is that P increases root growth. While it is true that applying P to soils that are deficient in P will increase plant growth (shoots and roots), the same response happens if N or K is deficient. Not a shred of evidence exists that adding P promotes new root growth if P is not deficient to begin with. So adding excessive P with the hopes of improving survival and growth will not work. I wish it were that easy.

* Nitrogen. Of all the nutrients, N has the largest impact on growth of shoots and roots of ornamental plants. In addition, N is the nutrient that is universally deficient. Unfortunately, while you can predict the P and K needs from a soil test, a soil test for N has little value. Therefore, N is the most difficult nutrient to manage properly. Maintaining the right quantity of N in the soil for growth while minimizing N losses depends on the landscaper managing the N rate and timing of application.

Traditionally, N recommendations have been generic and not based on the current state of the plant. However, all woody landscape species fall into one of three categories based on growth state:

1. Newly planted specimens that have lost a large portion of their root systems, such as with transplanted B&B specimens 2. Well-established plants in which you would like to promote growth 3. Healthy, attractive landscapes where maintenance is the priority and additional growth is of little concern.

First, how should you encourage root growth in newly planted landscapes to re-establish a good shoot:root ratio? Before determining the appropriate N rate to encourage root growth, let's examine how woody landscape plants respond to root loss. Each plant has a characteristic shoot:root ratio. This does not mean the shoot and the roots of the plant are the same size, just that the shoots and roots are adequate to support each other. More specifically, the roots can absorb enough water and nutrients to support shoot growth, and the shoots can manufacture enough energy to support root growth. While this explanation is greatly simplified, it does illustrate the basic relationship of the root and shoot systems. Root loss disrupts this ratio. The plant restores the shoot:root balance by increasing root growth and minimizing shoot growth. This is a natural reaction. Therefore, the role of the landscaper should be to enhance, if possible, a process that the plant already has set in place. The question is: What can you do to enhance this process?

Several research projects have concluded that increasing N rates suppresses root growth and increases shoot growth. Results from these studies concur with others that advocate zero or minimal N addition after root loss, whether transplanting or other circumstances caused it. The best option for the landscaper is to not interfere with a naturally occurring process-simply provide water to a reduced root system to prevent desiccation.

In the woodland environment, plants never experience anything other than a low-N environment. I believe the urban ecosystem depends on our fertility management because the urban environment bears little resemblance to a native woodland system. However, it is possible that a high-N fertility program can have negative consequences that we might not notice because the root system is not easy to observe. Moreover, clients often only notice the size of the above-ground portions of the plant.

What does all this mean? Newly transplanted material has minimal N-fertilization needs. I recommend no additional N or a maximum 1 pound per 1,000 square feet per year of a slow- or controlled-release fertilizer during the establishment period. The establishment period can range from months to years depending on the plant.

Although containerized trees and shrubs have intact root systems, I think you should treat them as though they have lost roots. Containerized plants rely on roots growing into the surrounding soil for establishment, but many of the circling roots will never do so. Therefore, I believe such plants benefit from a low-N environment that does not suppress new root growth.

Once a tree or shrub has re-established its shoot:root ratio, fertilization can increase growth. This is when N is most effective in increasing the growth rate. During the growth phase, the N rate should be 2 to 4 pounds N per 1,000 square feet per year. Within this range, base the application rate on the growth rate of the plant. For example, slow-growing (dwarf) plants are rising in popularity. It would be a poor management practice to apply high rates of N to plants such as these. By contrast, plants that may benefit from higher N are those with rapid growth rates. To minimize N losses, I advise limiting any single application to 2 pounds N per 1,000 square feet.

Maintaining healthy plants that are mature or at least an adequate size for the space they have to fill requires minimal N-just one pound per 1,000 square feet every 2 to 4 years. In addition, you should never have to fertilize any mature tree growing in fertilized turf.

So far, we have only discussed N application rates. However, several other decisions will affect the efficacy of your N application: timing, method of application and slow-release vs. water-soluble N sources. If you are going to apply your fertilizer just once annually, do so in the fall (October or November). My second choice would be late winter or early spring (February to April), preferably before bud break. However, a split application between fall and spring may be more effective and efficient than a single annual application. This is how I fertilize my home landscape.

Any N fertilizer, whether water-soluble or slow-release, can be effective and efficient if you consider the fertilizer characteristics. The biggest consideration when using a water-soluble N fertilizer is that the N becomes available immediately after application. A slow-release N source, by contrast, is released slowly, and the release rate depends on the N source and environmental factors. Proper timing and minimizing the amount of N you apply at one time are critical when using water-soluble N. As I mentioned earlier, research has shown that surface application of N is just as effective as other methods of application because N will move with water into the soil profile.

Fertilizer is a powerful tool for managing landscape plants. Use it wisely and you will get excellent results. Use it poorly and you'll waste the product.or the plant.

Dr. Stuart Warren is professor of horticulture at North Carolina State University (Raleigh, N.C.).

A simple way to determine the pH of your soil is to mix two parts (by volume) of distilled water with one part soil. Stir the mixture thoroughly and let it stand for a minimum of 30 minutes. Using a pH "pen" in the solution, you can get an approximate measurement of the pH of the soil. Many pH pens are on the market, and you should be able to get one from a horticultural supplier. You also can use pH paper, but this provides a less accurate measurement than pH pens.

Want to use this article? Click here for options!
© 2019 Penton Media Inc.

Interactive Products

Equipment Blue Book

Used Equipment Valuation Guide

Riding mowers, lawn tractors, snow throwers, golf carts


Grounds Maintenance Jobs

search our jobs database, upload your resume