The Denver Homeowner’s Guide to Spring Tree Fertilization

After winter, your trees and shrubs work hard to bounce back and produce healthy new growth. Looking forward to seeing your landscape turn lush green this spring? Give your trees the boost they deserve with well-timed and properly, professionally executed fertilization. Reap the benefits for the spring, summer, and fall months!

What Is the Purpose of Tree Fertilization?

The idea that fertilizer is tree food is a common misconception. Through photosynthesis, trees and shrubs naturally produce their own adequate food supply. However, the photosynthesis process can be hindered by poor soil, lack of available nutrients, drought, and other adverse conditions. 

The role of fertilizer is to act as a supplement, or vitamin, to trees and shrubs. Fertilizer is composed of minerals and nutrients that supply the ingredients required for successful photosynthesis and healthy growth. 

Fertilizer is not a magic pill for plants and trees that can cure all problems or diseases. Unhealthy trees that have been poorly planted or neglected for long stretches of time cannot simply be restored to full health with a cocktail of fertilizer. But when used strategically and correctly, fertilizer can help developing trees and shrubs flourish, growing strong and healthy with care and attention.

Do My Trees Need Fertilizer?

As mentioned, fertilizer is often misused. So, how can you tell if your trees really need it? To establish whether or not fertilizer is right for your trees, follow these simple steps:

  1. Soil Test: Step 1 is to have your soil tested. A soil test determines the acidity or alkalinity (pH) of the soil, along with the levels of nutrients that are present. Upon analysis of your results, you may need to add nutrients and fertilizer to make up for any deficiencies in the soil. 
  2. Check Growth: Inspect your shrubs and trees to look for signs of poor growth. This includes pale green or yellowed leaves, smaller-than-average leaf size, premature fall coloring, and leaf drop, minimal annual twig growth, or twig or branch dieback. Low levels of nutrients in the soil usually cause your trees and shrubs to display these tell-tale symptoms. However, stresses induced by insects, diseases, and weeds can cause these symptoms. Before you jump to fertilizing, consult with a trained arborist to determine the cause of the problem and correct it.
  3. Determine Planting Age: Fertilizer applications in the early years of established, transplanted trees and shrubs can speed up top growth and help young trees fill their allotted space in the landscape. Slow-release fertilizers are well-suited for recently planted trees and shrubs.

Too busy to play tree inspector? No worries! Our trained arborists can expertly conduct a quick assessment of your trees to determine their needs.

Why Spring Is the Perfect Time to Fertilize

The best time to apply fertilizer is when trees need it and when they are able to absorb the nutrients with their roots. This coincides in early spring when the weather is reasonable, trees have active root growth, and a decent amount of soil moisture. The conditions are highly conducive to plant growth. 

Typically, professionals avoid fertilizing trees and shrubs in the high heat of the summer when they can become stressed by drought. Fertilization is also not recommended in the frigid cold of winter because plants are often unable to absorb the nutrients.

With the right combination of timing and nutrients, you can help your trees and plants stimulate stunning new growth for the spring, summer, and fall months.

The How-Tos of Tree Fertilization

If you’ve determined your need and you’re ready to fertilize, the next thing you need to think about is how you want to fertilize. There are a variety of methods for fertilizing trees and shrubs and they vary depending on your goals and the needs of your plants.

According to Colorado State University’s Master Gardeners, fertilizer application rate depends on several things: the growth phase, soil content, and the area of the Tree Protection Zone or TPZ. To calculate a tree’s TPZ area, you have to determine the Critical Root Radius and then calculate the area in the TPZ using the CRR. If this sounds a little too much like algebra for you, our arborists are happy to help you! It’s important to get the calculations just right, too much fertilizer can actually be harmful to your tree, causing it to grow too fast and become stressed and susceptible to pests.

Next, you’ll need to select the appropriate method of fertilizer application. Each method strives to achieve a specific purpose and has its pros and cons. Here are descriptions of the most widely available options:

Deep-root feeding: Soluble liquid fertilizers are injected below the soil surface with this method of application. Deep-root injection can be highly effective for producing quick green results. Although it’s called deep-root feeding, 80-90% of tree roots are actually located in the top 8-16 inches of soil, so the fertilizer is applied in a more shallow area. One pro of this method is that liquid fertilizer is more soluble than granular and consequently reaches the root zone more rapidly.

Trunk injection: With this method, micronutrients are injected directly into the tree trunk. The injection can correct micronutrient deficiencies caused by elevated pH and high nitrogen levels found in the soil. It is possible for the tree to form a wound at the injection site which can become infected with fungus, and for this reason, it is sometimes used as a last resort.

Foliar feeding: If you’ve determined that your trees need micronutrients such as iron, manganese, and zinc, this might be the perfect application for you. In soil with high pH, this method is very effective for replacing the micronutrients that are unavailable. Foliar feeding uses a spray mixture that helps to increase the micronutrient uptake. 

Spiking: Small holes are either dug or punched in the soil and granular fertilizer is inserted into these holes. The hole pattern is scattered, reaching beyond the dripline and around the tree. One issue that can occur with this method is the high concentration of fertilizer in a small area. In some instances, the high salt concentration may burn roots at the fertilization site. 

Choosing the right type of fertilizer application is critical to the overall success of your trees. Trust our trained arborists to help you make an informed decision!

Organic vs. Inorganic Fertilizer

Nowadays, it seems like there are organic options for just about everything. The word organic has become synonymous with healthier, safer, and even better. But experts will tell you there are oftentimes very few differences between organic and inorganic, depending on the product. Fertilizers are available in many forms and combinations. Let’s break down the true differences between organic and inorganic fertilizers outlined by Purdue University Extension:

  • Organic fertilizers are made from plant or animal sources such as manure, grass clippings, leaf litter, and bone meal.
  • Inorganic fertilizers are made from nonliving sources. Some are mined from mineral deposits while others are made through complex manufacturing processes.
  • Plants absorb most nutrients as elements that are inorganic ions. Organic forms of elements must be converted to inorganic ions before the roots can take them up. So, organic nutrients are not immediately available to the plant.
  • A plant does not distinguish between ions that originate from inorganic or from organic sources. 
  • A fertilizer’s effectiveness depends on how soluble the materials are, how they affect the soil’s structure or pH, and how long they persist in the soil.
  • Organic fertilizer sources release their nutrients slowly because soil microorganisms must break them down before they are available to plants. This can be desirable because slow-release fertilizers provide nutrients over a longer period. 
  • If your primary concern is just supplying nutrients, inorganic fertilizers are usually preferable to organic forms. 
  • Inorganic fertilizers cost less per unit of nutrient, contain greater percentages of a given element, do not vary in nutrient concentrations, and are easier to handle and apply because they are more concentrated and less bulky. 
  • Nutrients in inorganic fertilizers are more quickly available to the plants and do not depend on the rate of organic decomposition. 

Early spring is the perfect time to fertilize your trees and shrubs. Give them the nutrients they crave with a customized fertilization plan crafted by one of our expert arborists. Call us today to receive a free consultation!