Mulberry (Morus sp.)
By: H. Wayne Shew, Ph.D.
NAB Certified Pollen Counter
Alabama Allergy & Asthma Center Collection Station—Birmingham, Alabama
The genus Morus is a member of the Moraceae family of plants. This is a large, economically important family with some 73 genera and greater than 1000 species. However, the family is mostly tropical and subtropical in its distribution. There are only a few genera present in the U.S. and the only native species of Moraceae in the Southeast is Morus rubra, Red Mulberry. There are several other members of this plant family found in the Southeast, but these are introduced and naturalized species. These additional species include Paper Mulberry, Broussonetia papyrifera, White Mulberry, Morus alba, and Osage Orange, Maclura pomifera.
Red Mulberry is a medium-sized tree that can reach heights of 60-70 feet. This species has a distribution range throughout the Southeast except for some portions of the Appalachian Mountains and the southern tip of Florida. It can be found in sporadic locations throughout its range.
The leaves on an individual mulberry tree can vary in morphology: ovate and unlobed with serrate edges, one-three lobes with serrate edges, and leaf bases that may be straight across or heart-shaped.Variation in leaf shape is most pronounced on young mulberry trees.
The flowers present on Mulberry trees are unisexual. A given mulberry tree may be monoecious (male and female flowers on the same tree) or dioecious (trees produce either male or female flowers but not both). The staminate (male) flowers are loosely arranged in spikes while pistillate (female) flowers are present in dense spikes. Unlike some of the other trees flowering in the spring mulberries produce flowers and leaves together, usually appearing in April, the peak period for mulberry pollen in our area. We will collect mulberry pollen starting in March and will continue to see it in our collections in May.Mulberry pollen ranges in size from 13-15 microns in Paper Mulberry, and 18-22 microns in Red and White Mulberry. The pollen is diporate, meaning that it has two openings (pores) in the wall of the grain. A diporate condition is not a common pore arrangement and is thus quite striking when seen under a microscope. See photos below.
The fruits of Red Mulberry trees are edible. The ripe fruits are dark red to black in color and are botanically described as a multiple fruit composed of numerous drupes; the fruit is the product of numerous flowers, each of which produces a drupe. The fruits have some resemblance to blackberries but blackberry fruits are aggregate fruits composed of drupes; that is, the fruit is the product of a single flower having numerous pistils. The fruits of mulberry trees are quite delicious, both raw and in pies. One can find recipes for a mulberry pie online. A few sites are listed below:
Mulberry pollen can be present in significant amounts in our area and should be considered an important allergy trigger for some people. If you develop hay fever every spring, then you may have an allergy to one of the tree pollens which is present in substantial amounts during this time, one of which can be mulberry. The only way to know for sure if you have a mulberry pollen allergy is to check with your allergist. Some parts of the country, notably the Southwest, have significant problems with mulberry pollen triggered allergies because of the substantial number of mulberry trees that have been planted in urban and residential areas. Male trees are often planted to avoid having to clean up overripe fruits that fall from the female trees, but also are the “culprits” when it comes to release of pollen that can cause hay fever. The irony of this situation is that people who were once told to move to the southwestern U.S. to reduce their allergies find their condition is now worse than it was in the place they formerly lived.
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Red Mulberry – Leaves
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White Mulberry – Leaves
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Red Mulberry – Pollen Grains
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Red Mulberry – (Diporate) Pollen Grain