Mulberry trees, belonging to the genus Morus, are deciduous trees known for their sweet, edible fruit. Determining if a mulberry tree is self-pollinating is complex due to its varied reproductive strategy. The fruiting potential of an individual tree depends entirely on its specific genetic makeup and flower structure, which varies widely even within the same species. Understanding this reproductive biology is key to ensuring a reliable harvest.
Understanding Mulberry Tree Sexes
Mulberries display sexual polymorphism, meaning different trees can have different flower arrangements. The simplest arrangement is dioecious, where a tree has flowers of only one sex (male or female), requiring two separate trees to produce fruit. Male-only trees produce pollen but no fruit, while female-only trees produce fruit only if they receive pollen from a nearby male tree.
The second common structure is monoecious, where a single tree has both male and female flowers, making it capable of self-pollination. A monoecious tree can produce fruit without a separate pollinator present. The three main species—White (Morus alba), Red (Morus rubra), and Black (Morus nigra)—can all exhibit either dioecious or monoecious characteristics.
Adding to this complexity is sex lability, where a mulberry tree can occasionally change its sexual expression. An individual tree may start as one reproductive type and transition to another as it matures or in response to environmental conditions. For example, high temperatures and long days can favor the production of male flowers. This flexible strategy means that a tree that has been fruitless might suddenly begin to produce fruit, or a heavy producer might shift its balance of male and female flowers.
The Mechanics of Mulberry Pollination
Mulberry flowers are inconspicuous, lacking the bright colors and strong scents that attract insects, because the primary method of pollen transfer is wind dispersal. The tiny, greenish-yellow flowers are grouped into dense, pendulous clusters called catkins. Male catkins are typically longer and looser than the female catkins.
The male flowers are structurally specialized for rapid and efficient wind pollination. The four stamens are held tightly bent inward in the flower bud, creating tension. When the flower opens, this tension is released explosively, launching the pollen into the air at high speeds, sometimes exceeding half the speed of sound.
This explosive release mechanism maximizes the distance the pollen can travel, increasing the chances of reaching a receptive female flower. The female catkins are positioned to catch this airborne pollen, facilitating fertilization. Because the process relies on air currents, strong windbreaks can interfere with successful pollen transfer, especially for dioecious trees.
Practical Steps for Guaranteed Fruiting
To ensure a reliable harvest, gardeners should focus on cultivar selection and propagation methods rather than relying on chance self-pollination. Choosing a variety known to be self-fertile or female-only is the most direct way to secure fruit production. For example, ‘Illinois Everbearing,’ a hybrid of Morus alba and Morus rubra, is reliably self-fertile and produces fruit without a separate pollinator.
When purchasing a tree, selecting one propagated from a cutting or grafted onto a rootstock is necessary to guarantee its sex. Trees grown from seed are genetically variable and their sex cannot be determined until they mature and flower, which can take up to ten years. Grafted trees are clones of the mother plant and often begin bearing fruit within the first few years.
For a known female-only tree, planting a compatible male tree nearby is necessary to ensure cross-pollination. Since mulberries are wind-pollinated, the trees should be planted within a reasonable distance, generally no more than 50 feet apart, to maximize pollen transfer. This proximity ensures the female flowers receive enough wind-borne pollen to produce a full crop of fruit.