To Fertilize or Not to Fertilize, That is the Question

To fertilize or not to fertilize; that is the question. 

At TimberPine we receive this question at least once a day and sometimes more.  I am going to walk you through the answers you should receive from any of our sales people you might talk to.

Customer, “When should I fertilize this new tree I just purchased?”

Sales Person, “All of our trees are treated with a slow release fertilizer early in the spring before the leaves come out.  Because this is an 8-9 month fertilizer, you will not need to fertilize this year.”

Anytime you purchase a new tree ask the sales person when it was last fertilized.  Over fertilizing can harm your tree and the environment.   You should use a root  stimulater to give them a little jump-start when planting and add water and mulch to help your tree get off on the right root 🙂

Customer, “When should I fertilize any existing trees I have?”

Sales Person, “Fertilization should be done early spring before the leaves start to shoot.  If you have very sandy soil you should apply half of the yearly rate in March and the other half in May.” If you live in Iowa the likelihood of you having sandy soil is very slim unless you live near a river.  Most soil here is a clay loam.  In new housing developments, you just have clay.

Timing is very important when fertilizing trees and shrubs.  This is the reason that we do not recommend fertilizing in the fall.  If you apply fertilizer too early in the fall you can cause new growth that will not be hardened off in time for winter, this will cause die back on your branches. 

You shouldn’t need to fertilize existing trees every year.  Fertilization is recommended every 2 to 3 years for deciduous trees and 4 to 6 years for evergreens.   There are several factors to consider before you apply any type of fertilizer.

  1. 1.        Do you fertilize your lawn?  If so your tree probably already receives enough nutrients.  Before you make an application to the tree you should take a soil sample.  Follow this link for soil sample info:
  2. 2.       Is the tree showing any symptoms of nutrient deficiencies?    ie

Yellow leaves with interior veins areas remaining green, marginal leaf browning, smaller leaves or reduced twig growth.  If we are seeing these things you may need to fertilize.  First take a soil sample as mentioned above.  Next, look at the shoot growth from last year.   How did it grow? Since you are probably doing this in early spring before the leaves are out start at the tip of the branch and measure back until you can see a bump and often a change in color, this is the new growth.  If the new growth is 6 inches + you probably do not need to fertilize.  If it is 2 inches or less and the soil sample recommends it you should go ahead and fertilize.  (This will not hold true with slow-growing trees such as Oaks.)


 On certain trees you might not notice problems until the leaves are on the tree and they are yellow.  You might see this on trees such as birch, pin oak, bald cypress, crabapple, elm, and London planetree just to name a few.   This is usually a sign of iron chlorosis. 

Please read more about chlorosis at:

To correct this problem you can:

  1.  Spray leaves with iron chelate.  This is a form of iron that can be taken up by the leaves.  This is only a short-term fix, only lasting one season.  It is ok to apply this during the year just follow the label on the bottle. 
  2. Decrease the soil pH by applying a powdered sulfur application.  Check the above link for suggested rates from Iowa State University or contact TimberPine.  We can help you green up your trees.  This is a more costly application, but it should last for 2 to 3 years.

Most of the information above was for deciduous trees.  Let me give you just a small shot on the evergreens.  They prefer an acidic soil.  Here in Iowa we have a more alkaline soil.  You will often see these trees showing yellow needles and having stunted growth.  They are reacting to the iron being tied up in the soil and not being able to take it up.  I would suggest using a fertilizer for acid loving plants when fertilizing any evergreen.

There are several methods to fertilize your trees.  I am going to touch on three.

  1. A broadcast of fertilizer is the cheapest and most efficient way to go.  It also takes the least time; just spread it and water it in.
  2. Fertilizer spikes work but in a slower fashion and they can be expensive.  You drill holes around the drip line of the trees and place the spikes.  The coverage is spotty with this method.
  3. The third method is hydraulic injection.  Fertilizer is injected into the ground on 3 ft intervals in a grid pattern.  This method is extremely expensive and often cost prohibitive.

For my money the broadcast method has never let me down.

Customer, “What type of fertilizer should I use?”

Sales Person, “After looking at the nutrient need recommended by your soil sample, choose the best fertilizer to fit your needs.  If you just want to shoot into the dark and skip the soil sample a good 10-10-10  fertilizer will work.”

Always remember to read and follow all label directions.  The old “if 1 tsp is good 2 must be better” is not the method to follow when working with any chemical.

Happy Growing.


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