Even though silk never became a major commodity in North America, it indirectly made an impression on American gardening. The tree that was developed to most efficiently feed silkworms is now among the most popular of shade trees. The fruitless mulberry, Morus alba, wastes no resources producing fruit while providing only abundant foliage, which is the only sustenance for silkworms.
Young trees grow at a good rate to nearly thirty feet tall, and can eventually reach fifty feet. They are often pollarded (pruned severely back to the same burly ‘knuckles’ every winter), which causes them to regenerate stems at an alarming rate during summer. Shoots from mature pollarded knuckles have no problem reaching fifteen feet in all directions! Mulberries incidentally have the distinction of the fastest motion known to the plant kingdom, because they launch their pollen at more than half the speed of sound!
The serrate leaves are quite variable. Those of vigorous young shoots of pollarded trees are mostly about six inches long with rounded wide lobes, but can be nearly a foot long! They turn bright yellow and typically fall neatly from the tree within a limited time in autumn, facilitating raking. Leaves on slower growing stems of lightly pruned mature trees are mostly unlobed and less than six inches long. They begin to fall earlier in autumn and linger over a longer time, sometimes with slightly subdued autumn color.
Tony Tomeo 
We have M. alba in historic areas here, like at the Yorktown waterfront. I studied several trees in early spring, until they had developed enough for a positive id. They get very large and oddly shaped here. There are far more male trees than female, fruit bearing trees, perhaps by intent. They are beautiful trees, to me, but remain on some lists of invasive plants.
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Male trees may be naturally less numerous than female trees in the wild, but more numerous where fruit is not desired, or to feed silkworms. Fruitless mulberry is all male, but has become a problem for pollen production within some urban areas.
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That is interesting- I hadn’t heard about the pollen problem, but it isn’t surprising. All in all, this seems to be a prolific, outstanding tree. Until I studied the tree this spring, I had mis-identified it as a live oak from a distance, because of its shape and structure. I went looking for acorns in the fall and found none, of course, so I began trying to ID it from its winter twigs. Its early growth looked like mulberry fruit forming, then those buds opened into leaves. Soon, I was amazed to begin recognizing the bark and leaves all over the Yorktown area. These trees are very old, too- probably introduced at least a century ago.
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Fruitless mulberry was supposedly famous for its problematic pollen in the region of Phoenix. People with major allergy problems relocated to Phoenix because of the lack of pollen and the aridity that inhibits the dispersion of pollen. However, many of those same people planted fruitless mulberry for quick shade. It did not go well.
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Too funny. ‘Know your tree!’ I wonder sometimes why folks moving to AZ, NV, etc. feel the need to the species they left behind in more temperate areas. I just read today’s ad from Logee’s, and they are promoting a fall blooming tree from Korea called the ‘Bee-Bee Tree’ (Tetradium daniellii) on sale for only $16.95. A fast grower that produces a lot of seeds- what could possibly go wrong with an Asian introduction like this?
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I suspect that in Phoenix as well as Las Vegas, people, including natives, crave shade.
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