Mulberry trees (Morus spp.) are typically dioecious, meaning male or female flowers grow on separate trees. Sometimes, however, they are monoecious, in which case male or female flowers grow on the same tree. Also, the flowers can change sex.
Flowers and Pollination
You will most likely encounter one of three species of mulberry: the black mulberry (Morus nigra), which is hardy in U.S. Department of Agriculture plant hardiness zones 5 through 9, the red mulberry (Morus rubra) and the white mulberry (Morus alba), both hardy in USDA zones 4 through 8. White mulberry is considered invasive in some U.S. locations; remove unwanted seedlings as they appear.
Mulberries are pollinated by wind. Some mulberry trees, including black mulberry and white mulberry, are self-fruitful, producing fruit without pollination from another tree. A tag on a plant nursery seedling should list whether or not the tree is self-fruitful. If that information is not present, then ask the nursery staff. California Rare Fruit Growers Inc. reports that mulberry trees growing in California produce fruit without other pollinating mulberry trees.
A nursery mulberry seedling typically bears fruit after two to three years. Fast-growing, fruitless, male white mulberry trees are often grown as ornamental plants, but they produce large amounts of pollen that may trigger allergies.
Tiny mulberry flowers grow in drooping clusters called catkins.
Difference in Flower Sexes
A male flower has four stamens -- the reproductive organs that have anthers, which bear pollen -- atop slender stems called filaments.
Each female flower has a single pistil consisting of a stigma, which receives pollen and sits atop a style. The style directs the pollen down to the ovary at its base.
Determine whether a mulberry tree is male or female by examining the tiny flowers on one of its catkins to see whether they have stamens, indicating the flowers are male, or pistils, found only on female flowers.
Look closely at the flowers on a mulberry catkin. If it is a male catkin, the tiny, lime-green or yellowish-green flowers will be slightly purplish near their tips. On these tips are tiny filaments or stems bearing anthers.
You may have to use a magnifying glass to see the filaments and anthers clearly.
If you look closely at the tiny, pale-green flowers on a female mulberry catkin, you won't see a filament, but you will see the stigma that curls outward from the tip of each flower. A stigma looks something like the curly horns on a sheep.
Using a magnifying glass may be necessary to see the stigma clearly. It's possible to confuse a male anther with a female stigma. If a flower has no stemlike growth, which indicates a male filament, then you're looking at a female flower.