Mulberries of the Morus nigra species are generally regarded as the most flavorful. Laboratory tests consistently show them to be higher in sugars, antioxidants, and organic acids (a component of flavor) than other mulberries.
Here are some partial results comparing them to Morus rubra (also regarded as being very flavorful).
From: "Phytochemical and antioxidant properties of anthocyanin-rich Morus nigra and Morus rubra fruits", Mustafa Ozgen, Sedat Serce, Cemal Kaya, 2008
This stone wall was erected around an old Morus nigra mulberry in Lebanon.
Living for hundreds of years in favorable locations, Morus nigra mulberry trees often hold a position of reverence in European settings. Mulberries in general command little respect in the United States, but Morus nigras tend to be sought out over other mulberry rivals. As they are more desired, unwittingly or not, sellers quite often have other cultivars of mulberry labeled as Morus nigra. Look for named cultivars when purchasing: 'King James I' and 'Noir of Spain' are your best bet; 'Persian' and 'Black Beauty' may or may not be Morus nigra, depending on the nursery. The 'Dwarf Everbearing' is NOT Morus nigra.
Morus nigra CAN be grown from seed; the trick is to find the correct seed, stratify them for three months, plant, and wait for up to ten years for fruit (also, about a third of those plants will be male without fruit). This author made eleven separate online purchases in an attempt to obtain Morus nigra seeds; not a single purchase resulted in obtaining Morus nigra seed.
Morus nigra seedlings
Morus nigra seeds are 3 to 4 mm in length; compared to Morus alba, Morus rubra and Morus macroura, which range from 1.5 to 2.5 mm in length.
Diseased Morus nigra (before spray program)
Morus nigra appears more susceptible to fungal and bacterial spot diseases to a greater extent than other mulberry species. These organisms have their highest rates of infection in warm and wet environments. Not many are so foolish as I to attempt to grow Morus nigra out of its ideal range (I'm in Northern Florida), but for those who wish to give it a try, here is a brief summary of what I believe is necessary to maintain healthy growth.
Here in the United States, Morus nigra does well in the hot, but dry Southwest, and also does well in the wet, but cool Northwest coastal region, or wherever yearly temperatures have a typical maximum range of zero to 85 degrees F. In all other regions in the United States (in USDA Plant Hardiness Zones 7 to 10) Morus nigra struggles to survive. Here they most likely will require fungal, and perhaps, bacterial sprays and a good fertilization program to flourish and be productive. In the spring, examine leaves frequently and start spraying at the first sign of leaf spot disease. Under warm (above 85 degrees F) and humid conditions, weekly spraying may be necessary, and even more frequent during periods of excessive rain. A safe, non-toxic spray should be considered, especially through harvest. After harvest, switching to a more robust (not so environmentally friendly) spray may be necessary to contain disease progression. Be sure to spray the leaf underside. Keeping the plant well fertilized and watered will help keep it healthy and growing.
Update: Although it is important, if not critical, to use fungicide sprays initially, it appears that Morus nigra has the ability to adapt to a climate and its pathogens, as less disease occurs as the plant ages.
This video exemplifies typical disease problems. A plant pathologist felt it had a Fusarium fungal and an unidentified bacterial disease.
Morus nigra mulberry now disease free. No fungicidal sprays have been necessary on my North Florida Black Mulberry for over two years.
Note: As a carrer scientist, I hesitate to post information that is purely speculative (at this time), but proof could take a considerable number of years...So I am going to post my thoughts and perhaps it will benefit someone.
Mycorrhizae are fungus species that form a mutually beneficial relationship with plant roots. They greatly expand the plant's ability to absorb water and nutrients from the soil. In return, the plant supplies sugars and carbohydrates to the fungi. It is believed that over 95% of plant species utilize mycorrhizae.
Some mulberry cultivars have a difficult time becoming established and robust in their growth in a particular location or climate. While climate and soil conditions (moisture, nutrients, texture) play heavily in producing a healthy tree, the establishment of the appropriate mycorrhizae in the root system of mulberries is extremely important.
From purely a "climate" point-of-view, Morus nigra should perform OK in USDA plant hardiness zones 7 and warmer, but in the relatively "wet" southeast it does not thrive well. The first sign a newly planted tree generally displays is fungal "leaf-spot" disease, which usually indicates that the plant is not at peak health. I have noticed several different mulberry cultivars struggle with health issues when young but do quite well as the plant matures. I believe the main factor is relative to the amount of time that the young tree needs to find and establish the appropriate mycorrhizal that will benefit the mulberry plant.
As an experiment, I "air-layered" several Morus nigra branches and potted them in commercial potting soil. Some did all right their first year but inevitably all would die before ending their second year... until I mixed some of the soil from around the roots of my established (and extremely healthy) 'Persian' Morus nigra with commercial potting soil when planting one of the air-layered branches. In its second year, it is looking healthy and only has minimal "leaf spots". I believe this one will make it. I will need to repeat this many times before I have any authority to claim that the right mycorrhizal is important to Morus nigra (and different cultivars) but I wanted to "put it out there" in case others are desperately attempting to grow Morus nigra and are finding it difficult. Supplying Morus nigra with plenty of mulch or compost can also be beneficial to the plant, perhaps because it aids in establishing a healthy mycorrhizal community.
As a side note: I grafted several Morus nigra scions to different healthy Morus alba mulberry plants. The grafts would take but eventually would die (usually by the end of their second year). Just speculation, but I feel Morus nigra needs different mycorrhizal than the Morus alba plants.
Morus nigra: Note the large dark buds and berries that hug the branch.
It's difficult to pick ripe nigra berries without staining your fingers.
Morus nigra mulberry trees commanding respect in Lebanon.
Crushed Morus nigra fruit:
Viable seeds are seldom found in Morus nigra fruit in the United States, most likely due to the lack of pollinators, since plants bought here are usually dioecious females. This single mulberry fruit with many seeds was from a tree growing in Bulgaria.
Morus nigra videos on YouTube
Here is a list of YouTube videos that appear to be actual Morus nigra videos. Many videos on YouTube, perhaps most, showing Morus nigra or one of its synonym names, are something other than Morus nigra.