Fertilizing evergreens

Fertilizer packages will include an analysis of the percentage of various nutrients and minerals that are included in the bag.

• A “complete” fertilizer — one that supplies the macronutrients nitrogen (N), phosphorus (P), and potassium (K) — is often recommended.
• The three numbers on the fertilizer bag represent the percentage of N-P-K
• A fertilizer analysis of 10-8-15 means the fertilizer has 10 percent nitrogen, 8 percent phosphorous, and 15 percent potassium.  
• Usually the percent of nitrogen content (N) will be the highest number. Nitrogen is a mobile nutrient that leaches through soil and needs to be replenished most often.

It is always best to submit a soil sample to the University of Minnesota Soil Testing Laboratory before purchasing and applying fertilizer, as most soil already has sufficient amounts of phosphorus.

Soil pH

Test results will also provide information about your soil's pH (7.0 is neutral, less than 7.0 is acidic, and higher than 7.0 is alkaline). The soil pH can affect whether certain minerals can be accessed by plants for healthy growth. Generally, evergreens grow better when soil pH is acidic; many nutrients may be unavailable to the plant when soil is too alkaline.

The best time to fertilize your evergreens is before new growth expands, around early April in Minnesota, up to about mid-July.

• Don’t apply fertilizer later than mid-July as this will stimulate new growth late in the season.
  • This new growth may not have time to “harden off” (become acclimated to colder temperatures).
  • New growth is much more likely to suffer winter injury and dieback. See Protecting trees and shrubs from winter damage.
• If a plant is nutrient-stressed, a slow or timed-release fertilizer can be applied in late summer up until late fall. This can help evergreens tolerate winter and emerge healthier in the spring.
• Water dry plants before fertilizing.
• Never fertilize drought-stressed plants as fertilizer can burn stressed tree roots or push the plant to put on new growth when it does not have the energy.
• If conditions continue to be dry after you've fertilized (e.g. little to no rainfall), be sure to water your evergreens regularly through late fall. See Water Wisely.

Fertilizer recommendations are usually given in pounds per thousand square feet. To find out how much to use, figure out roughly how many square feet your tree or shrub covers. For instance a spruce that measures 5 feet across would cover 25 square feet.

A common "maintenance rate" of fertilizer is two to four pounds actual nitrogen per thousand square feet of soil surface, applied every two to four years. For mature, slower growing trees, one pound of actual nitrogen is probably enough.

It's easy to figure out how much actual nitrogen is in a bag of fertilizer, because the numbers in the fertilizer analysis are actual percentages, by weight.

• Example 1: A 40 pound bag of 10-8-6 is 10 percent nitrogen. Ten percent of 40 is 4, which means the entire bag contains 4 actual pounds of nitrogen (plus phosphorus, potassium, and an inert carrier).
• Example 2: A 30 pound bag of 21-0-0 is 21 percent nitrogen. Twenty-one percent of 30 is 6.3, which means the bag contains 6.3 pounds of actual nitrogen.

Spreader

As long as the rate used is two pounds actual nitrogen per 1,000 square feet or less, you can spread the fertilizer with a drop spreader under the evergreen branches and slightly beyond, then water it in well. Any higher rate would burn the grass it was spread onto, even if it were watered immediately after application.

If your tree is mulched, spread the fertilizer right over the mulch and water in well. The granules will work their way down to the soil surface.

Incorporating fertilizer

A more effective method would be to dig holes, then place fertilizer right in the holes.

• Thoroughly soak the area where you will fertilize to soften the soil so it's easier to make holes.
• Use a crowbar, soil auger, metal rod or other tool to create holes roughly 2 inches in diameter. Make holes about 8 to 12 inches deep.
• Space the holes about 2 feet apart in large concentric circles, starting 1 1/2 feet from the trunk of young trees or 3 feet from the trunk of larger, more mature trees.
• Divide the amount of fertilizer needed, and place it equally into each hole.
• Then water the area. There's no need to refill the holes with soil.

Using these guidelines for making the holes, the following gives you a short-cut to determine how much fertilizer to place in each.

The amount of fertilizer to use per hole will vary depending on formulation. To get approximately two pounds actual nitrogen per 1,000 square feet, use:  

• 2 1/2 tablespoons for a 10-8-6 fertilizer
• 2 tablespoons for a 14-14-14 fertilizer
• 1 1/4 tablespoons for a 21-0-0 fertilizer

Using spikes or root feeders

Fertilizer spikes are a convenient and simple way to fertilize evergreens, and are effective when used in sufficient quantities. However, because each spike contains only a small amount of fertilizer, they are not cost-effective compared to granular products.

Applying liquid fertilizer through a root feeder is another option on all but heavy, poorly drained soil. But it can also mean additional work when applied to light, sandy soil. Liquid fertilizer leaches through sandy soil quickly, and may need to be applied several times throughout spring and early summer. Again, this method of fertilizing, though convenient, is more costly than applying standard granular fertilizer.

Keep evergreens healthy by mulching the soil surface under their branches and slightly beyond. The mulch ring should start at the root flare (where the main tree roots point out away from the trunk) and continue out from the tree trunk at about three or four inches deep.

Benefits of mulch

• Mulch helps hold in soil moisture.
• Mulch insulates roots by moderating soil temperature.
• A mulch ring around a plant will protect the stem / trunk from being damaged by lawnmowers and trimmers.
• Mulch gives a landscape a “finished” look.

Typically, evergreens are mulched with wood chips or shredded bark. However, any mulching material will do. Well-rotted manure or seasoned compost will add a small amount of nutrients as they break down further.

Never lay plastic sheeting under any kind of mulch 

• Plastic sheeting will prevent water from reaching the tree roots.
• Landscape fabric or “weed barriers” are not recommended under organic mulch such as shredded wood. 
• Mulch that is deep enough (3 to 4 inches around trees and shrubs) will prevent weed growth and adds organic matter to the soil as it breaks down.
• Landscape fabric is recommended under non-organic mulches such as rock and shredded rubber to prevent these materials from working their way into the soil.