Why Are Some Mulberry Trees Illegal To Grow?

Mulberry trees have a long history as landscape trees that are attractive to birds and wildlife due to their large leaf canopies and abundant berries. In the United States, there are native red mulberry trees (Morus rubra), as well as two species that originate from Asia: the white mulberry (Morus alba) and the much less common black mulberry (Morus nigra). In recent years, white mulberry trees have begun to crowd out the native red mulberries, hybridizing with them and creating fast-growing, invasive mulberry trees. In addition to having invasive root systems, these newer hybrids also cause allergic reactions from pollen. Various issues with invasive mulberry trees have caused them to be declared illegal in a number of cities and states.

Despite being popular trees for many decades, the recent change in the character of these trees resulting from the hybridizing of native and non-native varieties has given them a bad reputation. They're known to spread rapidly and aggressively, and once established, these "weed trees," as they're often called, grow as much as two feet in one year. 

In 1992, the city of El Paso in Texas declared the planting of mulberry trees to be illegal.  They were later joined by the cities of Las Vegas, Tucson, and Albuquerque. Many residents complained of unpleasant allergy symptoms from pollen, while others found the trees' rapid growth and invasive spread to be a problem. States that have completely outlawed mulberry trees include Illinois, Kentucky, and Tennessee, and they have official invasive status in a number of states, including Pennsylvania.

There are a few different issues with hybridized mulberry trees that have started spreading in urban areas over the last several decades, and these issues should make you think twice before planting these bird-attracting trees in your yard. Though there are white berries on some white mulberry trees, the hybrid trees that mix genes from white and red mulberries tend to produce red berries, which ripen to a black-purple hue. This deep color gives the berries their antioxidant nutritive value. It also stains sidewalks and patios when the berries fall or are deposited in bird droppings, which is another issue that gives these trees a reputation as a nuisance. The pollen is also a problem, but this tree's biggest threat lies in its rapid growth and spread.

These trees spread via fast-growing roots that can break through concrete and squeeze into the cracks of foundations. They also spread via pollen grains in the tree's catkins. This invasive tendency explains why these trees can overtake an urban setting quickly and why some cities have responded by making them illegal to plant. Their ability to gain height rapidly also makes them a potential nuisance around power lines and other structures.

The white mulberry is not as selective about its growing conditions as the red variety, so this is one reason the white mulberry hybrids tend to spread aggressively. There is also some evidence that the white mulberry tree can pass a root-damaging disease to red mulberry trees. 

The leaves of mulberry trees are jagged on the edges, often with curved lobes, and the roots of seedling trees are often yellow-orange, making them easy to identify. More mature trees have a slightly orange hue to the pale tan bark. The seedlings should be pulled up rather than mowed, as mowing is not an effective method of control and can even stimulate root growth. Sometimes cutting down a mature mulberry is the only solution to lessening its impact on health and property. But cutting down a tree that provides shade and food for wildlife feels like a real loss, so other options should be considered for planting in its place. Some native, non-invasive alternatives to white mulberry can be planted instead of this problematic tree. In the southwest region, where major cities have banned its sale, other indigenous trees have been suggested by arborists as substitutes, including Mexican redbud, honey mesquite, juniper, and desert willow.

Depending on your preferences and location, other replacements for white mulberry could include flowering trees like Eastern redbud or wild gum, fruit trees such as cherry, pear, or serviceberry, or nut trees like hazelnut or hickory. Check with your local tree nursery, arborist, or cooperative extension to get advice on which trees are best in your area. There are a few things to consider before planting a fruit tree, such as its mature size, growing conditions, and how much care and maintenance it needs.