It’s my understanding that both Morus alba (White Mulberry) and Morus nigra (Black Mulberry) are tolerant of the Black Walnut’s toxic juglone, but I’ve heard conflicting reports on Morus rubra’s (Red Mulberry) tolerance.
So that brings me to M. macroura (Long/Pakistan Mulberry). I can’t seem to find any info on this one’s tolerance, and would very much like to plant one near my neighbor’s walnut. Does anyone have experience with either M. rubra or M. macroura’s tolerance? I’d feel at least a bit better about risking it with the M. macroura if I got confirmation that M. rubra is also tolerant.
EDIT: Fixed taxanomic capitalization
Here’s a few things to consider
In the oft-cited paper by Brooks, he found that native Red Mulberry was one of several species tolerant of Eastern Black Walnut.
Effect of black walnut trees and their products on other vegetati.pdf (2.4 MB)
Morus rubra and Morus alba have the same chromosome structure (2n=2x=28) and readily interbreed. Morus alba was widely planted across the U.S. in the 1800’s in hopes of establishing a silk industry - a venture that failed due to labor costs. An unfortunate side effect is that the M. alba’s went feral and polluted the gene pool of native Red Mulberry. Few true M. rubra specimens are thought to exist in the U.S. today - especially in the nursery trade.
M. macroura is genetically different from M. alba, having chromosome structure 2n=4x=56, as is M. nigra with 2n=22x=308. All three are eastern hemisphere plants without Juglans nigra in their range, although certainly there are juglone-emitting walnut species in south central Asia.
Note that in the Latin binomial taxonomy of plants, the Genus name is capitalized (e.g. Morus) while the species portion (e.g. rubra) is never capitalized.
Thanks Richard! I didn’t realize alba originated from Asia; that gives me a lot of hope that M. macroura will do just fine.
I plan on talking with my neighbor to see if they’re open to having the walnut tree removed, but will report back in a couple years if it remains and the Macroura is doing just fine. It appears Juglone is biodegraded in as little as 2-6 months within soil, so I won’t be able to report much if the tree goes in after the walnut is removed.
Further research with Juglans species has demonstrated that root contact is necessary for toxic effects on many plants, and that the juglone concentration is negligible 6 meters from the edge of the tree canopy.
I didn’t mean to infer that M. alba is native to south central Asia, although certainly M. macroura and some species of Juglans are. The actual native range for M. alba is central and northern China - prior to human movement of the plant in the past 10,000 years. Oddly it is sometimes referred to as “Russian Mulberry”.
Soil type determines how fast juglone degrades. A heavy clay soil may keep it around 3 or more years. Juglone is present in leaves and roots and can accumulate in certain types of soil.
Oh I had looked it up after your initial post, didn’t mean to imply that you implied
That gives me a lot of hope for my pear trees! The closest is ~35 feet from the tree canopy, planted before I knew the final tree in my neighbor’s yard was a Black Walnut. Another thing going for me is that another larger tree is literally right next to the walnut. I’ve heard the roots of other trees create somewhat of a buffer as the walnuts roots have a harder time penetrating neighboring root structures.
Other readers of this thread should keep in mind that @kybishop is referring to Eastern Black Walnut (J. nigra), not the myriad other black walnuts e.g. J. californica.
Any walnut will secrete juglone because they are all grafted onto black walnut and in the same family. I have actually heard 5+ years it can remain in the soil. It is the reason I planted hazelnut bushes instead of a pecan or a walnut. Well the chemical factor and size. Another thing I noticed is one list said one thing and another list listed something else that was affected. I saw a video of a guy who has a lot of thriving apple trees and he planted walnuts thinking he could control it and a lot of his apple trees died. To me hazelnuts were a good medium because they can of include the taste of many nuts while staying small.