CHOOSING THE RIGHT ORNAMENTAL FERTILIZER

WHEN AND HOW MUCH?

By Ray Buckwalter,

Agronomist for LebanonTurf 

Why Fertilize?

 

One of the main considerations when choosing a fertilizer is to determine the reason for fertilizing in the first place.  What do you want to accomplish?  The two primary reasons for fertilizing are to encourage growth, or to create a healthy, vigorous, attractive plant.  These goals are not mutually exclusive.  However, a high growth rate does necessarily equate to health.  Nurseries fertilize to stimulate managed growth to produce a large, attractive plant for sale.  Faster growth means reduced time to market and professional growers have mastered the science of managing growth by careful selection of varieties combined with controlled growing conditions and fertility programs.

 

Landscapers and homeowners, on the other hand, are typically more concerned with the long-term maintenance of the plant.   There is often a temptation to over-fertilize in the hopes of producing an even healthier, larger or more beautiful plant.  Be careful!  If you force a plant beyond its natural growth rate by over-fertilizing, you can cause it to grow too quickly.  This can result in structural problems, predispose the plant to insect or disease infestation, and reduce tolerance to drought or temperature extremes.

 

Evaluating the Need for Fertilizer: Consider Soil Conditions

 

The best starting point is a soil test.   Soil tests are inexpensive, typically about $10, but they can provide a lot of valuable information.  In the landscape construction often results in soils that differ from one house to the next or even from the front yard to the rear.  A good general rule of thumb is, if the soil looks significantly different in color or texture it is probably very different in it fertility as well.  Even if you’re landscaping in the same general area don’t assume the soil conditions are similar without first conducting a test. 

 

In the absence of a soil test, you can assume that most plants in a landscape setting require more nitrogen than they get from natural sources.  One reason is that the leaves that have fallen from the plant that could have put nutrients back into the soil are often raked and removed.  Ornamental plants often have showy foliage and exceptional blooms that are achieved by genetic selection.  They are grown in nurseries under ideal conditions.  These plants are less likely to be adapted to natural soil conditions and the more austere fertility levels that sustain their native cousins.  Most ornamentals will benefit from 2 to 6 pounds of applied nitrogen per season.  The exact amount, and ratio of other nutrients, depends on the type of plant and fertility of the existing soil.

 

 

Not all fertilizers are created equal.  So what’s the difference? 

 

There are almost as many fertilizers on the market today as there are plants to fertilize.  Trying to get beyond the marketing and hype and choose the right one can be a daunting task but understanding some of the basics can make it easier.

 

Every fertilizer has a guaranteed analysis that represents the concentration of plant nutrients.  Some products are more concentrated than others.  For example, most natural organic fertilizers contain about 3 to 5% nitrogen.  Synthetic organic or mineral fertilizers may contain 15 to 30% nitrogen.  You need eight times more of a 3% product to supply the same amount of nitrogen as a 24% product.  

 

Another factor closely related to the guaranteed analysis is the cost.  Let’s assume that in the example we just stated both products cost $18 per 50-pound bag.  With the 3% product there would be a total of 1.5 pounds of nitrogen in the bag, so the cost per pound of nitrogen would be $18 + 1.5 or $12.00.  With the 24% nitrogen product there would be 12 pounds of nitrogen in each bag so the cost is  $18 + 12, or $1.50.  The 3% material is actually 8 times more expensive.      You need to use 8 times as much to get the same amount of nitrogen.

 

Now we can consider coverage.  This is a measure of how many plants can be fertilized with each product.  Again we focus on nitrogen because that is the nutrient used in the largest amount by vascular plants.  If we want to supply ½ pound of nitrogen per plant the 3% material will fertilizer three plants and you will need 16.6 pounds of product for each one of them!  That’s a pretty big pile of fertilizer so you probably need to make several applications just to get that amount applied.  The 24% material will fertilizer 24 plants and you would need just over 2 pounds for each.  There are no rules governing usage recommendations on fertilizer labels so compare products based on the amount of nutrients they supply, not on the manufacturer’s coverage recommendations.       

 

Last but not least quality is important.  Quality is a function of particle size, consistency, lack of dust, and other physical characteristics.  However, most of the time we equate quality to the amount and type of controlled release nitrogen.   Of all nutrients, nitrogen is the most critical to control in order to provide an extended feeding.  Occasionally, potassium may also be provided in a slow release form. 

 

A slow release formulation is the most desirable because it feeds plants for a longer period of time and eliminates time and effort required to make frequent repeated applications.  There is less chance of problems arising from over fertilization since the nitrogen is released slowly and the plant is able to use more of the nitrogen so less is lost to leaching or volatilization.  This is often referred to as nitrogen efficiency and can also apply to other nutrients in slow release form such as potassium.

   

 

 

Slow Release Nitrogen and Low Salt Index Preferred

 

When choosing an ornamental fertilizer, an important consideration is the salt index.  Fertilizers with a low salt index, a measure of salinity that a fertilizer contributes to the soil, are preferred.  Almost all fertilizers have some salinity, but too much salt can prevent the plant from pulling moisture into its roots.  This type of injury is often referred to as fertilizer burn, but it is really an induced drought stress.   Typically, less expensive fertilizers have higher salt indexes.  If the plant being fertilized does not have a well-developed root structure, as is the case with a new transplant, it is very susceptible to moisture problems that may be exacerbated by a high soil salt content.

 

Meth EX® methylene urea, and IBDU® isobutylidene diurea are both excellent sources of contolled release nitrogen with low salt indexes.  Soil microbes release Meth EX, so it releases more nitrogen when soils are warm and moist.  This corresponds very well to when plants grow the most and need the most nitrogen.  IBDU releases by hydrolysis so it also relies on moisture, but is less dependent on temperature.  This makes IBDU a great source for plants that grow in cooler soils.  By using both Meth-EX and IBDU we can get a nitrogen package that works great under a wide range of conditions.

 

Methods of Application

           

Once you’ve chosen the fertilizer, what is the best way to apply it?  There are two basic types of fertilizers used for plants, liquid and granular.  Application methods vary but there are two basic methods, surface application and injection.  All of the feeder roots of plants are in the upper soil layers.  Surface applications distribute fertilizer evenly and it is then carried down to the feeder roots with water.  This is the most uniform application and it is very effective.  Injections are used either by auguring holes that are filled with granular fertilizer, or injecting liquids into the root zone under pressure.   Auguring or injecting fertilizer is often not justified.  Injecting or auguring concentrates nutrients and can actually damage roots at the site of the injection.  However, it can be beneficial if compacted soil requires aeration, if the root system has been buried due to a grade change, or if the site does not allow for broadcast application, street trees growing in the sidewalks for example. 

 

Trees growing in turf will benefit from the regular turf fertilizer although the rates may be increased slightly in the proximity of large trees.  Use caution when applying fertilizers with herbicides in close proximity to trees.  Most established trees tolerate the normal rates of herbicide used for turf weed control.  However, damage can occur if the plant variety is particularly susceptible, if repeated applications are made, if the plant is under stress from other factors, or if the plant is small and/or newly transplanted.

 

Liquids fertilizers are obviously solutions made with water.  The nutrient sources tend to be soluble rather than slow release and insoluble nutrients in liquid systems can be very problematic.   Soluble nitrogen is more prone to leaching and liquid injection is more equipment intensive than granular application.      

 

When is the Best Time to Fertilize?

 

Fertilizer should be a supplement to mother nature.  Fertilize so the nutrients are available when the plants are actively growing.  For most pants heavy growth periods usually occur in the spring and sometimes again in the fall.  If you use a slow-release fertilizer that lasts 8-12 weeks, it will probably be effective throughout the critical part of the growing season.  Remember that growth is not just based on temperature but on the amount of daylight, and growing degree-days.  While a record cold snap April may delay a few varieties, most plants will still bloom and grow on time.  Plan your fertilization accordingly.

 

Contrary to popular belief even mature plants can benefit from a fertilization program.  Since mature plants don’t grow as fast, they can actually be more susceptible to damage from insects or disease.  Take extra precaution to protect specimen trees that are more valuable and may be difficult or impossible to replace.  Mature plants usually only require one application of fertilizer in the spring.  Larger trees can store more nutrients so less frequent applications are needed.

 

Younger plants, and ground covers may benefit from two applications one in the spring and another in the fall.  Plants with showy flowers may benefit from post bloom fertilizer and annuals can be spoon fed weekly.  In temperate regions fall is that time of year when the temperatures start to drop and plants begin to prepare for dormancy or reduced growth.  In the north September is usually the ideal time for a fall application.  In tropical areas plants grow year-round.  If you’re in a tropical area where there is moisture year-round, you should be fertilizing throughout the year.  Palms and cycads usually benefit from 3 to 4 applications per season, 12 to 16 weeks apart.