What is a Black Walnut Tree?

Learn everything about the black walnut tree, what it looks like, where does it usually grow, how it reproduces and the ways to use the black walnut tree.

This is a close look at a large black walnut tree against a late afternoon sky.

Juglans Nigra

Known as the eastern black walnut, this tree is part of the walnut family (juglandaeceae). It is a deciduous tree native to the eastern side of North America, though they have also been cultivated on the western side, as well as in Hawaii and parts of Europe.

The black walnut is a very important hardwood tree in North America, both for its valuable and high-quality wood, and for its commercially harvested edible nut. They are one of the most abundant and resilient trees in America, mostly because they are resistant to many epidemics that plague other tree species.

They are semi-long living, with life expectancies of 150 years, and they are a large tree species, with the tallest one on record living in Sauvie Island, Oregon, which is 34 meters tall with a 44-meter canopy spread.

Check out these different Types of Fruit Trees or 101 Types of Trees from around the world to continue learning about these amazing woody creatures!

What do Black Walnut Trees Look Like?

Root System

Black walnut roots are both a shallow and laterally spread root system as well as a taproot system. Their taproot grows very deep into the soil in order to access moisture reserves in the earth during dry seasons.

They are a tree that is allelopathic. This means that their shallow roots release a chemical that harms other sensitive plants and trees, which gives the tree an advantage over forest canopy competition, and it prevents any weeds or parasitic fungi from robbing its nutrients. This is another reason why black walnut trees are so abundant.

Dimensions

Black walnuts are very large trees and can sometimes grow to heights of 30-40 meters. They also develop highly wide spreading canopies, which often exceed their overall height. Their trunks need to be quite wide in order to support this height and heavy branches, and trunk diameters can sometimes reach 4 meters around.

Growth Pattern

In forests, the tree will grow a very straight and tall trunk with a narrow crown in order to earn its spot in the forest canopy and needs to do so because it is a shade-intolerant species. In more wide-open spaces, their trunk will be much shorter and wider with a broad crown.

Bark

This is a close look at the bark of a black walnut tree.

The bark of young trees will be gray with very fine fissures. A mature tree will have darker gray bark with deep furrows and thin ridges that form a narrow diamond shape.

Foliage

This is a close look at the foliage and leaves of the black walnut tree.

The leaves, stems, and fruit husks of the black walnut tree have a characteristically strong spicy smell. A black walnut leaf is alternately arranged and pinnately compound; meaning that each compound leaf is comprised of multiple leaflets. Each leaf will have between 15 and 23 leaflets, with the largest ones occurring in the center, and growing smaller as they reach the edges.

Black walnut leaves have a rounded base that meets in a pointed tip with serrated margins. They are dark green in maturity with hairy undersides. They aren’t particularly known for their fall color and will turn brown before they fall. Leaf scars (the area where the leaf meets the stem) are indented with no ridge and 3 prominent bundles.

Leaves will emerge in the spring once the mean temperature of the day reaches 70 degrees Fahrenheit, and they will drop in the fall once the mean temperature of the day dips below 65 degrees Fahrenheit.

How do Black Walnut Trees Reproduce?

Flowers

Black walnut trees are monoecious, meaning that male flowers and female flowers occur on the same tree. However, a female flower will emerge before a male flower, which prevents them from self-pollinating. They are self-compatible and is occur on rare occasions.

Staminate flowers (male characteristics – pollen-producing) are borne as drooping catkins (flowers with indistinct or not petals) that form from the axillary buds of the previous year. A catkin is a light green in color.

Pistillate flowers (female characteristics – ovule producing) are borne in terminal clusters of 2-5 on the current year’s axillary. These clusters of flowers are white and delicate.

Fruit

This is a close look at a cluster of fresh black walnuts.

Once a tree has been either wind-pollinated or insect-pollinated, it will produce fruit in the form of a nut, or a walnut! They are spherical in shape and are green in their youth before they turn brown.

The meat on the inside of the fruit is edible, oily, and sweet. Surrounding the meat is a fleshy husk (or walnut hulls), which is then protected by a brown corrugated nutshell. The mature fruit will usually fall in the month of October.

Sexual Maturity

Black walnut trees are very fast-growing and will grow over 35 inches per year for the first decade of their life. They also reach sexual maturity quite quickly and start producing walnut seeds around 4-6 years of age. They will develop their most productive crops after 20 years.

Seedlings emerge in either April or May, and they portray impressive tree growth in this stage. A black walnut seedling is either dispersed through gravity or thanks to the digestion of animal species. Squirrels also play a huge role in distribution, who will often bury a walnut and forget about it, allowing it to successfully germinate.

Where do Black Walnut Trees Grow?

This is a close look up the tall black walnut tree.

Black walnuts are native to many places in North America and are most prevalent in riparian zones. They can be found in southern Ontario, towards North Dakota, Georgia, northern Florida, central Texas, and everywhere in between. They were also introduced to Europe in 1629 as a cultivar.

They are absent from coastal plains, North Carolina, and the Mississippi Valley. They also do not occur in areas where the frost-free season is too short, as walnuts require warm temperatures to germinate. The growing range of the black walnut tree has not extended past the interior of North America due to the conditions being too dry.

They grow abundantly in closed forests, and as well as along forest edges, roadsides, and fields. Because they are so resilient and have such deep roots, they are sometimes considered as either an invasive species or weed tree. The tree will thrive in warm regions with high water tables and very fertile lowlands.

They are most commonly associated in forests with these tree species:

  • American elm
  • black cherry
  • basswood
  • boxelder
  • common hackberry
  • green ash
  • hickory
  • oak
  • red oak
  • yellow poplar
  • white ash

What are the Growing Conditions of Black Walnut Trees?

This is a healthy mature black walnut tree in a field.

Soil

Black walnuts require soil that is very moist, they tend to prefer Entisol and Alfisol soil types. They prefer silt loam, sandy loam, and sometimes silty clay because these types of soils are able to hold more water than well-drained soils.

Sun Exposure

These trees are shade intolerant, and that is why they grow so tall in forest spaces, and wide in open spaces. Their nuts need warm temperatures and sun exposure in order to germinate.

Are Black Walnut Trees a Pioneer Species?

A pioneer species is a tree or plant that is the first to repopulate an area after there has been natural devastation. This can be anything from a hurricane, flood, tornado, forest fire, or draught. Pioneer species are very resilient, germinate quickly, and don’t have specific preferences of soil.

The black walnut is considered a pioneer species because it germinates quickly, it grows extremely quickly, and is resilient to many types of diseases that affect other trees.

These qualities also can make it kind of a nuisance, as it will grow pretty much anywhere, it develops very deep taproots, and it becomes very difficult to remove once it has established itself.

What Pests Affect the Black Walnut Tree?

This is a close look at a tall black walnut tree.

There are many pests that affect the black walnut tree, but they have proven that they can’t be taken down very easily. They are unaffected by other diseases that decimate other tree species, like the emerald ash borer, chestnut blight, butternut canker, anthracnose, or dutch elm disease.

That being said, they do have lots of pests that do attack them. These include:

  1. Thousand Cankers Disease – this disease is transmitted by walnut twig beetles (pityophthorus juglandis) and the fungus geosmithia morbida. These pathogens spread throughout the carvings made by wood beetles, and block the tree’s dripline, which inhibits the transportation of water and nutrients. This leads to leaf death, and ultimately crown and tree death.
  2. European Cankers (neonectria galligena) – these cankers act as open wounds which cover the tree. The spread very slowly and symptoms take a long time to show, but they will eventually kill the tree.
  3. Maggots – which form in the husk of the walnut. They remain in the husk and don’t affect the meat of the nut, but this tends to be a very large problem for walnut harvesters. They don’t damage the tree, but they are a big nuisance.
  4. Walnut Weevils (conotrachelus retentus) – adult weevils will suck plant juices through their snouts and lay their eggs in nuts. This damages the nut though does not kill the tree.
  5. Walnut Caterpillars (datana integerrima) & Fall Webworm (Hyphantria cunea) – these pests overgraze on the foliage of trees which end up diminishing photosynthesis and can greatly harm the tree.
  6. Walnut Lace Bug (corythuca juglandis) – the nymphs of this bug will suck the sap from lower surfaces of leaves, and deposit a sticky substance that is called “honey-dew”. This sticky substance will cover the leaf surface and eventually turn black which prevents photosynthesis, and ultimately kills the tree.
  7. Codling Moth (Cydia pomonella) – the larvae of this moth burrow inside walnuts and eat the meat of the kernels. This does not harm the tree but proves as being very troubling for commercial nut harvesters.

How are Black Walnut Trees Used?

Wood

This is a close look at the log of the black walnut tree.

Black walnut wood is straight-grained, heavy, strong, and shock-resistant. The wood is a lovely dark color and is considered as being one of the most durable hardwoods in the United States. Black walnut trees are grown popularly in the lumber industry, and wood is used to manufacture furniture, flooring, coffins, and gun stocks.

Syrup

Though not as popular as nut harvesting, black walnut trees also produced a rather sweet sap that is made to create syrup! They have tapped in the similar fashion that sugar maple is, and they produce a sap that is quite similar in sugar content.

The sap is then boiled down to greatly reduce the water content until the consistency of the syrup is achieved. Though not nearly popular as pure maple syrup, the black walnut syrup is a wonderful alternative.

Natural Dye

The bark of the black walnut tree is very high in tannins and a compound called juglone, which is very staining. Black walnut trees were often used for natural dyeing, as they produce an attractive yellow-brown color. These dyes were used by early American settlers to dye clothing, textiles, hair, and for artistic purposes.

Wildlife

This is an Eastern gray squirrel up a black walnut tree.

Much of the black walnut tree is grazed upon by animal species. The nuts are a very important source of food for squirrels and makeup at least 10% of their diet. Many types of rodents and birds also feed on walnuts. Leaves of the black walnut tree are eaten by white-tailed deer and rabbits.

Walnuts

This is a close look at walnuts ready to be harvested.

Black walnut trees are also cultivated for nut harvesting. Black walnuts are oily, sweet, and nutritious. They are sold in nut stocks and are used to make ice cream, cookies, cakes, pies, and many more baked goods.

What are the Nutritional Facts of the Black Walnut?

Black walnuts are a very valuable type of not. Not only that, they contain many nutritious elements. Here are the nutritional facts for every 100 grams of walnuts:

  • 5% water
  • 59% fat
  • 24% protein
  • 10% carbohydrates
  • 619 calories

They also include the following dietary minerals:

  • manganese
  • pantothenic acid
  • phosphorus
  • vitamin B
  • vitamin E

FAQs

Are black walnut trees protected in Ontario?

Black walnut trees are not protected in Ontario, or anywhere for that matter. Trees become a protected species when they are over-logged or endangered, and the black walnut tree is a very successful species.

Are black walnut trees invasive?

Though black walnut trees are next exactly considered as being an invasive species (because they are native to where they grow) some do consider them as being a nuisance. They are classified as weed trees because they can grow in many different locations, they grow very quickly, and they develop taproots that are very difficult to remove.

Are black walnut trees toxic to other plants?

Black walnut trees are allelopathic. This means that their roots secrete a biochemical which is toxic to certain plant species. They do this to help beat out other competing species or to deter ground cover which may inhibit their growth or health.

Do black walnut trees produce nuts every year?

The crops that black walnut trees produce are random. Very productive crops occur every 2-3 years, though this varies from tree to tree, and this varies based on growing conditions and temperatures of that year.

When do black walnut trees flower?

The black walnut tree becomes sexually mature after 4 to 6 years. They will start to produce flowers annually in order to become pollinated. Flowers will usually emerge in the early spring in either April or May.

Do black walnut trees kill grass?

Because black walnut trees are allelopathic, there is a chance that they will damage grass or a shrub or another susceptible plant that is planted under their canopies. Allelopathic trees secrete a biochemical from their roots that are toxic to certain plant and tree species.

How quickly do black walnut trees grow?

Black walnut trees are very fast growing in their seedling stage and can grow 35 inches in a year for the first 6 years of their life. After that initial growth, the speed will slow to under 24 inches in a year.

How long do black walnut trees live?

The black walnut tree is a semi-long-lived tree, with an average life expectancy of 150 years if it is growing in the optimal location.

How tall do black walnut trees get?

Black walnuts are very large trees and will obtain heights of 30 to 40 meters if they are growing within a forest. If they are growing in an open area, they will be slightly shorter with a thicker trunk and a wider crown.

When do black walnut trees lose their leaves?

The black walnut tree is of the deciduous variety and will lose its leaves seasonally in order to prepare for the winter months. The tree will usually drop its leaves in late autumn or fall. Fallen leaves are brown.

When do black walnut trees drop their nuts?

Once a black walnut has become fully mature, it will drop to the ground in the month of October, depending on the warmth of the seasons.

Is black walnut wood good for firewood?

Though black walnut wood is very strong and dense, it would be considered a waste to use it as firewood. Walnut wood is highly valuable in the lumber industry, and it has far better uses than firewood.

What is the difference between butternut and black walnut?

Butternut trees (juglans cinerea) are different from black walnuts in their overall leaf shape, growing range, and fruit shape. The nuts of the black walnut are rounded, whereas the nuts of the butternut are more ovular.

How should you prune a black walnut tree?

Pruning trees is true to the preference of the landowner. A tree does not need the help of a human in order to grow properly. However, if one was intent on pruning a black walnut tree, it is best to avoid pruning in the winter as the cut will continuously bleed sap.

Otherwise, reserve pruning for branches that appear as being sick or damaged, or if they are growing awkwardly into other branches.

How can you tell the difference between a black walnut tree and an English walnut tree?

The main difference between English walnuts (juglans Regia) and black walnuts is that English walnuts grow in orchards, and black walnuts grow in the wild. English walnuts also have nuts with a richer flavor, as they have been bred to produce higher quality nuts.

Is black walnut a hardwood?

Black walnut wood is considered as being of the sturdiest hardwoods available in the United States.

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