Trees We Care For: Walnut Trees

Famous for their beautiful canopies, prized hardwood, and delicious nuts, walnut trees are some of the most celebrated trees in the world. Fortunately for Michiganders, we live alongside two native walnut species and have the chance to grow a few other exotic species, which will thrive in our local climate. 

But whether you have a naturally growing walnut on your property or you’ve planted a few exotic species or cultivars, it’s important to provide them with the care they need to thrive. We’ll try to help you do exactly that below, by sharing some of the basic facts about these wonderful trees. 

Walnut Tree Species in Michigan: Natives and Exotic Options

There are certainly a lot of walnut trees growing within Michigan’s borders, and most are members of one of two native species:

• Black Walnut (Juglans nigra): The black walnut is the most common walnut species in Michigan, with the Walnut Council estimating that our state’s forests harbor as many as 8.5 million individuals. A large tree, it’s capable of reaching up to 130 feet in height under forest conditions, though it generally develops a shorter, broader crown in the open.    
• Butternut (Juglans cinerea): A smaller and slower-growing cousin of the black walnut, the butternut is found in various places throughout the state, though it’s a much rarer species. Relatively short-lived, the butternut generally only lives for about 50 to 100 years. This species is named for its fat-rich nuts, which were commonly eaten by Native Americans. 

In addition to these native species, the following exotics are sometimes planted or offered by specialty nurseries.

• English Walnut (Juglans regia): A wide-ranging species that is found from Western Europe to East Asia, the English walnut is also called the Persian walnut or common walnut. Capable of reaching 80 feet in height, this species is a common ornamental species and available in several different cultivated forms. 
• Heartnut (Juglans ailantifolia var. cordiformis): A cultivated variety of the Japanese walnut, the heartnut is named for the very hard-shelled, heart-shaped nuts it produces. This medium-sized tree is quite cold-hardy, making it suitable for many Michigan properties, but it remains fairly rare in the state.   
• Manchurian Walnut (Juglans mandshurica): Native to Northeast Asia, the Manchurian walnut is a very cold-tolerant species that’s hardy to -45° Fahrenheit, making it useful for properties in the Great Lakes State. It produces relatively small nuts and reaches about the same size as a heartnut — 80 feet or so. 

How to Care for Your Walnut Trees: A Michigan Homeowner’s Guide

Walnut trees are generally healthy trees, but they do require proper support to thrive. While most walnut species require relatively similar care, we’ll primarily focus on the requirements of the black walnut, as it’s far and away the most common species in our area.  

Plant the Right Tree in the Right Space

It’s always critical to plant the right tree in the right space, and that certainly applies to walnut trees. 

For starters, you’ll want to select a location that offers enough space for these sometimes-large trees to thrive. But that doesn’t just mean ensuring that there’s enough vertical space to accommodate them (i.e. they should never be planted beneath utility lines), but also enough space for their crowns, which sometimes spread more than 50 to 70 feet. 

In terms of soil, they’ll thrive best in places with deep, fertile, slightly acidic soil. Adequate soil moisture is important, as these trees won’t thrive in excessively dry soil, but perpetually damp areas must also be avoided. 

Walnut trees are not shade-tolerant, so they require at least six hours of full sun exposure per day. They may be able to survive in areas with partial shade, but they’ll never reach their full potential in these areas. Shade may also reduce their ability to produce full nut crops. 

Apply a Layer of Mulch

Like most other trees, walnut trees appreciate a 2- to 4-inch-thick layer of organic mulch above their root systems. Ideally, the mulch should extend to the drip line (edge of the canopy) but be sure that you don’t allow mulch to pile up at the base of the tree, as this can trap moisture near the bark and cause decay. 

Mulch provides a litany of benefits for walnut trees, including:

• It helps to retain soil moisture. 
• It moderates soil temperatures in the summer and winter.
• It helps to protect the roots from mower and string-trimmer damage.
• It helps prevent competing weeds from growing in the area. 
• It contributes organic matter to the soil. 
• It helps reduce soil compaction over time. 

Provide Supplemental Water When Appropriate

It’s always important to provide supplemental water in the years following transplantation. This helps to protect the roots and support the tree as it tries to establish itself. But after this period, most walnut trees will be able to satisfy their moisture needs themselves, provided they were planted in a suitable location.

However, it is also wise to provide your walnut with supplemental water during extended droughts — particularly if they occur during the summer. Just be sure to water the root zone deeply and infrequently, rather than providing a small amount of water several times per week. This helps encourage the development of deep roots.  

Fertilize Only When Needed

It can be helpful to provide walnut trees with fertilizer to help address any soil deficiencies and ensure they have the nutrients needed to grow properly. 

However, you shouldn’t simply apply fertilizer as a matter of practice — you should only do so after conducting soil tests to identify any specific deficiencies that are present. By doing so, you can select a fertilizer that’ll be most likely to help. 

Just be sure to select a slow-release fertilizer if possible and always provide plenty of water while applying it.  

Monitor Your Trees’ Health

Aside from selecting a suitable planting location, monitoring your trees regularly is likely the most important step in keeping them healthy and beautiful. Many health problems don’t present obvious symptoms until the tree is already suffering from considerable stress, but by being vigilant, you can get the prompt care your trees need. 

So, keep an eye out for any of the following issues and be sure to contact a professional arborist if you see:

• Unusual insect activity, including frass (insect waste) at the base of the tree.
• Premature color change or leaf drop. 
• Discolored leaves or leaves exhibiting spots or blotches. 
• Mushrooms growing on or around the tree. 
• Branches are dying in the canopy.

Trimming & Pruning Walnut Trees in Michigan

There are several times during your walnut tree’s life that you’ll want to have it pruned by a professional arborist. This includes the following situations:

• While the tree is young. Early in your walnut tree’s life, you should have it structurally pruned to encourage good form and structure. Typically carried out during the first 10 to 20 years of its life, structural pruning can help prevent dangerous and expensive problems down the line, such as co-dominant (“twin”) stems. 
• Every three to five years thereafter. Even after establishing a sound tree structure, you’ll want to have your walnut tree pruned periodically. Typically called “crown cleaning,” these pruning sessions should remove any dead, dying, decayed, damaged, or diseased limbs, as well as any that are likely to cause problems in the future. 
• Any time you notice a hazardous branch. While periodic crown cleaning will help limit the number of hazardous branches that form in your tree’s canopy, it’s always important to continue monitoring your tree and have potentially dangerous branches removed as soon as possible. 

Additionally, homeowners and property managers may also want to have trees pruned to suit their subjective desires. For example, you may want to have the following kinds of pruning work carried out by a professional arborist:

• Crown Raising: Crown raising is a technique in which the lower branches of a tree’s canopy are removed, effectively “raising” the canopy off the ground. This can be helpful for opening sight lines or improving access beneath a tree, but it may reduce a tree’s ability to remain stable during severe winds. 
• Crown Reduction: Crown reduction work is intended to reduce the height and spread of a tree’s canopy. It serves as an alternative to harmful techniques (such as tree “topping”) that are carried out in order to avoid removal when a tree is planted in a space that is too small. 
• Crown Thinning: Crown thinning involves the removal of selected branches within a tree’s canopy to help increase air flow and sun exposure. This can help allow more light penetration for nearby plants and help address some fungal leaf diseases, but it must be carried out carefully to avoid stressing the tree. 

A Dark Cloud on the Horizon: Thousand Cankers Disease

Black walnut trees are generally healthy, resilient trees that can thrive when planted properly and provided with the support they need. However, there is an insidious disease that’s currently threatening walnut trees across large swaths of the U.S., putting the species in grave danger. 

Called Thousand Cankers Disease (TCD), this issue is easily the most important threat walnut trees currently face. First identified in Utah and Colorado in the early 2000s, TCD is now found in several western states, as well as portions of Tennessee, Virginia, Pennsylvania, North Carolina, Ohio, and Maryland. It has not yet been identified in Michigan, but officials worry that this won’t be the case for long. 

To limit the spread of the disease, several states — including Michigan — now limit the movement of walnut trees and products made from them (including firewood). 

The disease itself is caused by the combination of walnut twig beetles and a fungus (Geosmithia morbida). The beetle becomes inoculated with the fungus, spreading it to trees while feeding. Once it has infected a tree, the fungus causes hundreds or thousands of small cankers to form. These cankers often fuse to form large cankers that can girdle twigs, branches, or the entire tree, preventing it from transporting water and nutrients properly. 

Unfortunately, there is no cure for this disease, though promptly removing infected branches may slow its progress. 

Other Common Walnut Tree Problems in Michigan

While TCD is the most frightening ailment that Michigan’s walnuts may eventually face, there are several other diseases and pests that are already present and cause problems for walnut trees. 

• Anthracnose: A foliar disease that’s most common following cool, wet springs, anthracnose can cause leaves to develop brown or black spots and also impact fruit production. 
• Walnut Bunch Disease: Likely caused by a virus-like organism, this disease causes trees to produce a profusion of small shoots (“witches’ brooms”). Afflicted trees usually decline gradually over time. 
• Walnut Caterpillars: Walnut caterpillars frequently defoliate walnut trees during the summer. Isolated occurrences aren’t usually serious, but repeated attacks can weaken trees. 
• Root Rots: A variety of fungi, such as Armillaria, can cause a walnut tree’s root system to decay, predisposing it to complete failure. The only sign of these infections is often the presence of mushrooms above the roots.  
• Walnut Husk Maggot: Walnut husk maggots feed within the green husk of walnuts, damaging the fruit in the process. However, these problems don’t often affect the health of the greater tree. 

If you see signs of any of these ailments — or any unusual symptoms at all — be sure to have an evaluation performed immediately, so that you can get your trees the care they require. 

Signs Your Walnut Tree May Require Removal

Because walnut trees are often large trees, it is important to monitor them regularly for signs indicating they may be hazardous. It can be disheartening to have to remove a beloved tree, but that’s often better than allowing it to cause injuries or damage property. 

While you’ll need an expert arborist to help determine whether removal is warranted or not, the following signs should prompt you to schedule an evaluation:

• A sudden lean or change in the angle of a tree’s existing lean
• The presence of mushrooms or conks 
• Large cavities or hollows
• Significant cracks in the trunk or major branches
• Dying branches within the crown
• Mounding soil near the base of the tree
• A pattern of branch shedding over time

Walnut Trees in Michigan: FAQ

Still have questions about walnut trees? Want to learn some of the biggest threats they face? We’ve rounded up a few of the most common questions our customers have and answered them below. 

What walnut trees grow in Michigan?

Black walnuts and butternuts are native to Michigan, and a few exotic species, such as the English walnut, can be planted in our state. 

Can you eat black walnuts in Michigan?

Yes – black walnuts are edible, though they may stain your hands. 

How much is a black walnut tree worth in Michigan?

The exact price you may be able to obtain for a black walnut tree will vary based on the tree’s structure, health, and size. However, mature walnut trees are generally worth between $300 and $1,000 (minus the costs associated with their removal, which are usually considerably higher than this).  

What tree is mistaken for black walnuts?

The tree of heaven is the species most commonly confused with the black walnut, but various sumacs and black locusts also resemble them. 

Do walnut trees prevent other plants from growing? 

While walnut trees do produce juglone — a compound that can inhibit the growth of other plants — the effect is often overstated, and many species tolerate it well under typical landscape conditions.

Treemendous Tree Care: Providing Walnut Tree Care in Clinton Township, MI

Whether you have a single walnut growing on your property or dozens, the professionals at Treemendous Tree Care can provide the care they need. Whether you need pruning, disease diagnosis, or removal assistance for one that’s unfortunately become hazardous, our team is ready to help. 

Contact us today to schedule your free on-site estimate and get the kind of expert tree care you deserve! 

Sources & References

• Michigan Department of Natural Resources – “Black Walnut”
  A state natural resources profile highlighting black walnut tree identification, habitat, and ecological value in Michigan.
  https://www.michigan.gov/dnr/education/michigan-species/plants-trees/black-walnut
• Michigan Flora Online – “Juglans (Walnut Genus)”
  A botanical database entry covering the taxonomy and species information for the walnut (Juglans) genus.
  https://michiganflora.net/genus/Juglans
• USDA Forest Service – “Walnut Distribution and Silvics in Michigan”
  A technical publication describing the growth characteristics, range, and silvicultural notes for walnut species in Michigan.
  https://www.nrs.fs.usda.gov/epubs/pdf/walnutmi.pdf
• Arbor Day Foundation – “Walnut Tree and Thousand Cankers Disease”
  An overview explaining the impact of Thousand Cankers Disease on walnut trees and associated risks to landscape and forest health.
  https://www.arborday.org/perspectives/walnut-tree-thousand-cankers-disease
• Michigan Department of Natural Resources – “Thousand Cankers Disease Identification Report”
  A state invasive species fact sheet detailing the biology, symptoms, and management concerns of thousand cankers disease in walnut.
  https://www.michigan.gov/invasives/id-report/disease/thousand-cankers-disease
• Michigan State University Extension – “Canker Diseases on Shade and Forest Trees, Part 2”
  An extension article discussing canker diseases affecting shade and forest trees, including symptomology and management practices.
  https://www.canr.msu.edu/news/canker_diseases_on_shade_and_forest_trees_part_2