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The Monday Garden
Great Americans: The Black Walnut

Issue No. 149 - January 30, 2005
by Sue Sweeney

Our native black walnuts are forest trees; they like to stretch their roots in deep, rich, moist, acidy forest soil. However, even the babies can’t tolerate shade, so new black walnut trees have trouble getting started except in clearings and at the forest edge. Further, the nuts are so large and heavy that the squirrels can’t transport them very far. Thus, the only way for the seeds to be disbursed over long distances is by water (hence, the prevalence of black walnuts along forest streams) and by human.

picture: mature black walnut with the back of the new courthouse in the background (Juglans nigraa),
Bedford Street Stamford, CT January 2004

While young black walnuts (Juglans nigra) can be easily confused with the alien ailanthus, and with other trees with palm-like pinnately compound leaves, such as the native staghorn sumac, ash, and locust, there’s no mistaking the lime green “golf ball” nuts on teenage and mature black walnut trees.


picture: mature black walnut (Juglans nigraa), Bedford Street Stamford, CT September 2004

Black walnuts have developed two strategies to help over come these reproductive limits. First, the young trees grow fast (for a hardwood tree, that is). This trait helps a sapling take advantage of a short-term gap in the forest canopy or other shade source. Second, like the ailanthus, the black walnut is an allelopath. It produces chemicals that literally kill off competing plants. Interestingly, the chemicals that black walnuts produce are particularly poisonous to the solanaceae family (tomatoes and kin- see Issue 117), several other garden vegetables, conifers, and the malus family (apple, hawthorn, serviceberry, etc). The conifers make sense because juniper, for example, would be an edge-of-forest competitor. However, as for the others, who knows what happened a million years ago when these survival traits were evolving?

In pre-Columbian times, there were absolutely HUGE black walnuts, 100 to 150 feet, gracing the eastern American forest from Zone 4 to Zone 8, west through the Great Plains. These mature black walnuts were massive, long-lived beings, standing tall and straight with a high, small crown in the forest; and spreading wide as a white oak when alone in a meadow. However, the dark, hard, satin grained lumber was so prized by the European settlers that, sadly, almost all of the old growth trees were chopped down long ago. The wood continues to be so valued today that medium-size trees are cut down to use for veneer – crafting objects out of solid black walnut is a thing of our colony past.


picture: massive, spreading form of a mature open-grown black walnut. Stamford CT January 2005.

Despite the wonders of the wood, the nuts may be the tree’s best and worst feature. The nut meats are delicious—we like them in deserts; squirrels and mice take theirs plain. However, getting to the meat is a challenge. The thick, soft outer shell covering which starts out lime green and ages to dark brown, contains a dye which will stain you hands and clothes. It’s not generally harmful to people but it does contain the black walnut’s allelopathic chemicals so don’t throw the husks in the compost heap. Then, inside the husk is a very, very hard shell. Try a hammer.

picture: fallen black walnuts, striped of the outer husk; one has been gnawed open by a squirrel or a mouse.
Stamford CT Winter 2004-05

The other problem with the nuts is that there are so many of them on an adult tree. Generally, many more than a family (and the neighborhood squirrels) can use, and what do you do with the rest? Remember, also that the nuts are big – do not plant this tree close to your house unless you’d like your decedents to stay awake nights listening to the thud of walnuts hitting the roof.

pictures: detail of nuts which persist on the branches after the leaves have fallen.

IDENTIFICATION: It’s easy to distinguish black walnuts from other trees when the signature nuts are present (which is most of the time, if you count the shells on the ground). With a little more information, you can tell the difference the rest of time, too.

FORM AND BARK: The young black walnut trees have the same up-reaching, blunt, twigless branches like ailanthus and staghorn sumac. However, there are clear differences. First, the black walnut develops rough, craggy back at a very young age, while ailanthus and staghorn sumac are still smooth-ish with prominent lenticels (bark pores). The mature black walnuts have a craggy, spreading nut tree shape—staghorn sumac don’t get this big and ailanthus are always up-swept, and comparatively lithe and curvy. The mature black walnuts have very deeply furrowed bark. Also, the bark is much darker than that of mature ailanthus and staghorn sumac.


picture: believe it or not; the young tree on the left is a black walnut and the one of the right is a slightly older ailanthus.


pictures: bark of 3”, 12” and 24” diameter black walnut trunks. Note that even the young 3” tree to the left has very rough bark. The base of tree to the right is encircled with an invasive euonymus. Unfortunately, even the black walnut allelopathic properties don’t deter this unwanted alien.

TWIG AND BUD: the winter twig and bud are very different from ailanthus and staghorn sumac. As you recall from last week, the ailanthus and staghorn sumac winter twigs are smooth and blunt with knobby buds, prominent lenticels, and large, flattish leaf scars, the black walnut has whitefish –grayish furry buds and 3-lobed leaf scars often called “monkey faces”.

pictures: close up of bud and of prior years’ leaf scars Stamford CT January 2005


Picture: buds and leaf scars. January 2005.

LEAVES: while the leaves of staghorn sumac, ailanthus, black walnut, ash, etc, look much alike for a distance, close up, the leaf detailing is different. Like ailanthus and staghorn sumac, the black walnut’s numerous stemless leaflets are opposite on the long leaf stalk and the leaf stalks are alternate.

However, there’s one leaf detail that only the black walnut has: a tiny notch on one side of the midrib just where the leaflet attaches to the stalk. In addition, the leaflets are toothed, with paler, hairy undersides; this rules out ailanthus which has a smooth leaf margin with a single lobe at the leaf’s bottom. The teeth are much finer than the sumac’s. In addition, the black walnut often lacks the single, terminal leaflet that ailanthus and staghorn sumac always have.


pictures: leaf detail; note the notch at the base of the leaflet, the fish tail like curve of the leaflets and the missing terminal leaf. (Some black walnuts have a small terminal leaf).


picture: in midsummer, the black walnut provides dappled shade. Stamford CT 2004

Wildlife: For reasons unknown, the eastern screech owl prefers to roost in black walnuts (perhaps they’re attracted by the mice nibbling on fallen nuts). Several birds are said to eat the nuts as do squirrels and mice. (Query: how do the birds get the nuts open?). Deer, rabbit, and mice will browse the young trees.

From The Monday Garden. Copyright © by Sue Sweeney. Reproduced with permission. 

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