Walnut Tree

Walnut trees belong to the Juglans genus and the Juglandaceae family. They are deciduous trees, meaning they shed their leaves annually. Walnut trees are native to North America, South America, Europe, and Asia. They perform best in soils that are fertile, moist, and well-drained often near rivers and streams.

These trees are a beautiful addition to any landscape, providing shade in the summer and intricate branches in the winter. However, walnut trees can be troublesome for surrounding plant life. They produce a chemical called juglone that hinders the growth of nearby plants, so before planting anything near a walnut tree be sure to research which plants are juglone-resistant and which ones are not. More information this below.

Walnut trees emit a distinguishable spicy odor, which can be detected by crushing a leaf and taking a smell. They are relatives of the hickory tree family, which is why the two species are so often confused with each other.

How to Identify a Walnut Tree

  • LEAVES

    Walnut tree leaves are feathery with a rounded base, pointed tip, and serrated edges.

    They are green in color and have a fuzzy texture on the underside of them. The leaves appear in groups of 5-25 leaflets that are in an odd pinnately compound, which means that an odd number of leaflets alternate down the stem to collectively form a larger leaf. The leaf structure typically consists of larger leaflets in the center of the grouping with smaller ones appearing as you move toward either end. The leaf stems are typically two feet in length.

  • BARK

    Walnut tree bark starts out smooth and green-brown in color.

    As it matures, it gets more rough in texture and more gray in color. The bark itself develops into a furrowed pattern. If the bark is removed from a walnut tree, it will expose a dark brown color underneath.

  • SHAPE AND GROWTH

    Walnut trees can grow to be between 30 and 130 feet tall with broad crowns. However, if walnut trees are growing in a forest, then their shape could narrow out due to other trees competing for space.

  • TRUNK AND ROOTS

    Walnuts typically have a short trunk. These trees have a deep taproot that shoots down directly below it. Black walnut and butternut twigs have chambered piths, which means they are hollow on the inside with tiny chambers. When cut open, this can be a key identifier for a walnut tree.

  • FLOWERS

    Walnut trees have both male and female flowers growing on them. The pollen from the male flowers blows from wind and lands on the female flowers to fertilize them and eventually develop into walnuts. Male flowers are yellow to green in color and look like long cylinders of tiny buds. Female flowers are greenish-yellow and clustered in groups of 2-5.

  • WALNUTS

    The fruit of a walnut tree is the walnut. The walnuts are round with a protective green outer shell and are a little smaller than a tennis ball. Over time, the green shell fades to black and withers away, exposing the rough nut on the inside. The husks of the walnuts can cause health problems for dogs and cats if ingested. Walnuts are a good source of proteins, omega-3 fatty acids, vitamins, iron, calcium, and magnesium.

Various Walnut Species

Here are a some of the most common walnut species around the world:

  • BLACK WALNUT

    (Juglans nigra) One of the most common walnut trees, the black walnut is found primarily across central and eastern regions on the United States. It does best in more moist soils and full sun conditions. Black walnuts have darker bark and can often be identified by its bundled scars along its twigs.

  • MANCHURIAN WALNUT

    (Juglans mandshurica) Also known as the Chinese walnut, the Manchurian walnut is native to China, Russia, and North and South Korea.

  • SOUTHERN CALIFORNIA WALNUT

    (Juglans californica) This walnut species is smaller in height than most species as it typically tops out around 50 feet tall. Southern California walnut trees are often found in valleys or on slopes across the varying terrain of California.

  • BUTTERNUT

    (Juglans cinerea) Often referred to as white walnut, the butternut is native to the eastern U.S. and Canada. Butternuts have pale gray bark, dark green leaves, and elongated rather than rounded nuts. They typically reach around 60 feet in height and live for close to 75 years.

  • LITTLE WALNUT

    (Juglans microcarpa) It is no secret how this walnut tree species got its name. Little walnuts are shrub-like and usually only grow to be 20 feet tall. The walnuts that this tree produces are also smaller than those of other walnut species. It boasts a broad and rounded crown and can be mostly found across Texas and Oklahoma.

  • NORTHERN CALIFORNIA WALNUT

    (Juglans hindsii) Another walnut species native to California, the northern California walnut grows to around 75 feet in height. The wood from these trees is a popular choice for furniture makers.

  • ENGLISH WALNUT

    (Juglans regia) Other names it goes by include the common walnut or Persian walnut. They are fast-growing trees with a round, full shape, making them great for providing shade. However, if grown in a forest they grow taller and narrower. It is coveted for its thin-shelled seeds and quality timber.

  • JAPANESE WALNUT

    (Juglans ailantifolia) The Japanese walnut is known for its large, tropical looking leaves. It is native to Japan and usually grows between 40 and 65 feet tall.

  • ARIZONA WALNUT

    (Juglans major) This walnut can be found primarily in southwestern regions of the U.S. While most walnut species require strictly full sun conditions, the Arizona walnut can also perform well in partial shade.

  • BRAZILIAN WALNUT

    (Juglans australis) This species of walnut tree is native to Argentina and Bolivia and can grow to be taller than 80 feet.

  • ANDEAN WALNUT

    (Juglans neotropica) Also known as Columbian walnut, Ecuadorian walnut, and tropical walnut, the Andean walnut is native to Colombia, Ecuador, and Peru. Its canopy is more oval shaped and has bark that is reddish-brown in color.

Creatures Found in and Around Walnut Trees

Walnut trees are particularly popular among wildlife because they provide great shade and their walnuts are ideal harvests. Here are some of the creatures found near walnut trees.

  • Mice
  • Moths
  • Caterpillars
  • Woodpeckers
  • Squirrels
  • Foxes
  • Raccoons
  • Birds
  • Bears
  • Turkeys

Walnut Toxic Chemical: Juglone

This section focuses on the naturally-occurring toxic chemical emitted from the leaves, roots, bark, and fruit of walnut trees that can cause harm to all surrounding plant life. The most problematic of the juglone-producing trees are the black walnut and the butternut. Juglone is occurs naturally in hickory trees as well.

RANGE

Other plants growing in the affected area near walnut trees can die in as little as one to two months. The toxic area can reach as far as 80 feet away from the trunk of a walnut tree, but the range can sometimes be smaller. The toxin can even remain in the soil years after the tree has been removed.

  • DIRECT ROOT CONTACT

    One way that nearby sensitive plants can be affected by juglone from walnut trees is if the plant’s roots come in direct contact with the roots of the walnut tree.

  • LEAVES AND NUT HULLS

    Loose leaves or fallen nut hulls can also spread the toxic chemical if left to decompose near other plant life. To avoid this, promptly pick up or relocate tree debris after it falls.

Identifying the Effects of Juglone

Juglone can have a damaging effect on most surrounding plant life. Here are some ways to identify if Juglone is affecting your plants:

  • Stunted or deformed plant growth

  • Yellowing

  • Wilting

  • Death

Preventing and Managing Juglone:

Keep the soil surrounding a walnut tree well-drained and aerated by adding organic matter can help to accelerate the decomposition of juglone.

Make sure to plant any new plants as far away from walnut trees as possible.

Be sure to clean up any nut hull debris regularly to prevent any extra chemical spreading.

Do not use mulch that contains walnut debris.

Juglone-Tolerant vs. Non-Tolerant Plants

  • Some plants that are tolerant to Juglone:
    Some plants that are NOT tolerant to Juglone:

  • TREES

    • Ash
    • Elm(some)
    • Peach
    • Beech
    • Hickory
    • Plum
    • Birch
    • Oak
    • Chestnut
    • Maple(many but not all)
    • Apple
    • Pine
    • Birch
    • Silver Maple
    • Crabapple
    • Yew
    • Pear

  • SHRUBS

    • Dogwood
    • Forsythia
    • Honeysuckle
    • Wild Roses
    • Azaleas
    • Hydrangea
    • Lilac
    • Rhododendrons

  • FLOWERS

    • Aster
    • Dianthus
    • Lamb’s Ear
    • Bellflower
    • Ferns
    • Morning Glory
    • Clematis
    • Goldenrod
    • Pansies
    • Violets
    • Daisy
    • Impatiens
    • Peony
    • Wisteria
    • Daylily
    • Iris
    • Sunflower
    • Zinnia
    • Chrysanthemum (some)
    • Lily
    • Peony
    • Petunia

  • FRUITS AND VEGETABLES

    • Beet
    • Carrot
    • Onion
    • Corn
    • Squash
    • Melon
    • Asparagus
    • Cabbage
    • Potatoes
    • Beans (some)
    • Cucumber
    • Tomatoes
    • Blackberry
    • Eggplant
    • Blueberry
    • Pepper

Dangers to Walnut Trees

Here are some common diseases that can damage walnut trees and how to identify them.

  • WALNUT BLIGHT

    Small black spots on leaves

    Dieback of new shoots

    Damaged walnuts

  • FUSARIUM CANKERS

    Cracks in the bark

    Stained spots on stems

    Wilting of leaves

  • TARGET LEAFSPOT

    Brown spots that resemble targets develop on leaves

    Leaf spots combine and cause leaf to curl up and die

  • WALNUT ANTHRACNOSE

    Brown and black spots with a yellow border appearing on leaflets

    Nut has dry, dark, sunken areas

Uses for Walnut Wood

Walnut wood is a sturdy wood that is resistant to warping and coarse to the touch, but has a smooth finish when polished. It is a dense, tight-grained wood that is considered very valuable to furniture makers. Below are some ways in which walnut wood is used.

  • Furniture
  • Cabinets
  • Bowls
  • Veneer
  • Gun Stocks
  • Musical Instruments
  • Interior Paneling
  • Flooring
  • Wood Carving Projects
  • Shelving