When I include mulberry trees in my clients’ permaculture plans, I often get lots of questions. “Aren’t they bad?”, “Aren’t they illegal?”, and “Will they damage my property?”. To help answer these questions, I did some research and put together this guide.
So, are mulberry trees invasive, and if so, which varieties?
White mulberry trees are highly invasive and grow aggressively in disturbed soil such as fields, forest edges, and roadsides. Their canopy and roots can grow up to 40 feet. They often outcompete red mulberry trees, which are native to the US. While the two are similar in appearance, they can be identified separately.
Let’s take a look at what exactly white mulberry trees do to their environment and how to tell the difference between the invasive white mulberry and the native red mulberry.
How Did Mulberry Trees Become Invasive?
White mulberry trees are natively from China and were brought to the US during colonial times to establish a silk industry (mulberry leaves are the sole food of silkworms). However, it became too expensive and the project was abandoned.
Unfortunately, the white mulberry trees remained, growing aggressively in every US state except for Nevada.
Ecological threat: Invades open forests, woodland edges, prairies, fields, and disturbed areas. Tolerant of a wide variety of conditions; salt-tolerant, withstands drought and wind-resistant. Outcompetes and hybridizes with our native mulberry, replacing those populations.
White Mulberry, Wisconsin Department of Natural Resources
While controversial, invasive species don’t automatically mean bad species. For example, if a bird carries seeds from one continent to another, is it invasive?
Many times land that is highly disturbed benefits from the pioneering and hardy nature of invasive species.
These pioneers are experts at growing in poor soil, where they amend it and create partial shade, paving the way for more sensitive, slower-growing plants.
This process is called ecological succession.
However, this doesn’t mean that white mulberries are innocent and should be allowed to grow everywhere.
They often outcompete the native red mulberry and cause other issues such as excess pollen (one of the reasons why mulberry trees are illegal).
The problem is, it’s difficult to tell the difference between red and white mulberry trees.
A white mulberry tree’s fruit is actually red to purple when it’s fully ripe, so you can’t go off of fruit alone.
Here are some key ways you can tell the difference between red and white mulberry trees:
| Part of Tree | Red Mulberry (Native) | White Mulberry (Invasive) |
|---|---|---|
| Leaf Size | 4-10 inches | 3-4 inches |
| Leaf Color | Dark Green | Bright Green |
| Buds | Black Band on Each Scale | Smaller, Brown Band on Each Scale |
| Twigs | Pale-Tan | Pinkish-Brown |
| Bark | More flat, grayish with tannish inner bark. Scaly ridges | Thick, tannish with yellowish exposed inner bark. Braided ridges |
| Fruit Color | Purple-Black | White, Red, Purple |
Should You Remove Mulberry Trees?
If you have white mulberry trees, and they’re beginning to become damaging, it’s a good idea to remove them. Their root systems are aggressive and they can dominate a landscape fairly quickly.
For example, a single mulberry tree is estimated to produce twenty million seeds. And since mulberry fruits are a popular food for birds, these trees spread fast.
You can remove white mulberry trees by pulling any saplings or cutting the entire tree to the ground and grinding the stump.
If the root system is left intact, the tree will likely try regrowing, but cutting the regrowth down regularly exhausts the stored energy in the roots and eventually kills the tree.
Mulberry Tree Roots
How Long Do Mulberry Tree Roots Grow?
Mulberry trees are medium-to-large fast-growing deciduous trees, growing up to 40’ tall with a rounded crown roughly equal in width to its height. The rope-like roots typically stretch from 15 to 45 feet long, with some smaller roots reaching over 75 feet.
Even though red mulberry trees are native to the US, their root systems grow quickly, so they should be planted with some planning.
Mulberry trees spread their roots horizontally over the ground to seek water, air, and nutrients and to provide stability. These lateral roots typically grow outwards in proportion to the tree’s height. In some situations, the roots can extend along the ground up to 3x as wide as the tree is tall.
Although they can travel far, root thickness tapers off sharply within 6 to 9 feet of the trunk of a mature tree.
Beyond 9 feet, the roots are no thicker than 2 inches in diameter and most are much finer than that.
Aside from the tree’s size and genetic predisposition, factors like soil fertility and compaction influence how far the roots extend to ensure adequate air, nutrients, and water. The poorer and more compacted the soil, the further they will go to get what they need.
For example, a dwarf mulberry growing to a height of 15 feet will likely have roots reaching out laterally at least 15 feet from the trunk and up to 45 feet in less-than-optimal conditions.
When choosing a mulberry, take note of the variety, its expected height at maturity, and your soil conditions. This way, you can estimate how much space the roots will take up over their lifetime and make good decisions on the tree’s placement in your garden or orchard.
How Deep Do Mulberry Tree Roots Grow?
Over 90% of mulberry roots are found in the top 2-3 feet of soil. Like most trees, mulberries primarily have a horizontal root structure, so you’ll likely find a larger length of roots compared to depth.
The deepest roots of the average mulberry occur close to the trunk, forming off of the lateral roots. These are called “sinker” roots, whose primary role is stabilization rather than water or nutrient uptake.
Often less than half an inch in diameter, sinker roots move down through the earth, branching into non-woody growth the deeper they get. It is unusual for these roots to grow more than 2-3 feet deep.
In most yards and fields, the soil has been compacted by years of foot and machinery traffic and low concentrations of organic matter. The root system’s depth is influenced by the type of soil the mulberry is growing in. Given loose, well-aerated soils, roots can extend deeper into the ground.
If you’d like your mulberry’s roots to trend deeper rather than wider, plant it in optimal soil conditions with plenty of nutrition and aeration and water it regularly.
Be sure to mulch around the base, 3 feet in all directions and at least 6 inches deep, after planting to suppress weeds and promote healthy soil as the mulch breaks down over time.
Mulching is also good for established trees. Spreading a thick layer of leaves or wood chips under the canopy increases the presence of soil-improving insects and beneficial microorganisms. As the soil quality improves, the roots will be less likely to branch outward to meet their needs.
Pairing a nice layer of mulch with a regular watering schedule (I suggest only watering plants when the top 2-4 inches of soil gets dry) is the best way to ensure a more compact, efficient root system.
Can Mulberry Tree Roots Damage Structures?
| Mulberry Variety | Root Damage Potential |
|---|---|
| White Mulberry | High |
| Red Mulberry | High |
| Black Mulberry | High |
Mulberries get a bad reputation for having invasive roots that wreak havoc on the built environment. They are said to lift sidewalks, crack driveways, and damage garden walls and sheds. And they are accused of undermining foundations and invading septic systems. But there is some truth to it.
Mulberry roots are not strong enough to penetrate building materials themselves. They cannot drill through concrete, stone, or even thick plastic like PVC.
However, because they typically remain just below the soil surface, they can press up, damage paved surfaces, and upset lighter-loaded walls like garages and sheds.
When it comes to a house’s foundation, mulberries can cause damage indirectly by dehydrating the ground and compromising the soil’s integrity. It all depends on the land the house was built upon.
Some types of soil, like certain clays, are more prone to shrinkage and expansion than others. When growing in shrinkable soil, mulberry roots can absorb moisture and shift the soil structure, causing the foundation to lose support, buckle, and warp.
In the case of septic systems, keep in mind that mulberry roots are opportunistic and look for the most accessible sources of moisture and nutrition. While the roots are not strong enough to crack pipes, a leaking pipe could attract thirsty roots, which could get into the line and worsen the problem.
If you are considering planting a mulberry in the vicinity of your septic, confirm that the distance is at least as great as the tree’s height at maturity (usually around 40 feet).
Also, regularly mulching and watering your tree will prevent it from seeking nutrition and water further afield.
In all cases, damage can be prevented by ensuring your mulberries are adequately spaced away from built structures. Although they can have far-reaching root systems, once the roots reach 9 feet from the trunk, they have tapered to be no thicker than a garden hose.
Remember these smaller roots do not have enough force to cause damage. They follow the path of least resistance and go around obstructions rather than pressing through.
How Far Away to Plant Mulberry Trees
Consider your soil type and the tree’s size at maturity. Plant your mulberry at least 9 feet from your home or even further away if you have shrinkable clay.
Space the trees 30-50% further apart when you want more sunshine for understory plants such as companion plants.
When planning an understory of shrubs and herbaceous plants around your mulberries, design so that surrounding plants will be near the outer edge of the canopy. Plants can be spaced slightly closer in or further out depending on the sun and water requirements of the individual plants.
Plants that are both shade-loving and drought-tolerant, like columbine and bluebells, can be planted closer in. For those that love the sun and need proper moisture, like comfrey and daffodils, plant them further out.
When planting mulberries with other trees, a good rule of thumb is to use the “crown touching rule” and space them so that the crowns will barely touch one another when they reach maturity. This ensures less root competition and stress.
It also means each tree gets maximum sunlight for optimal fruiting.
This also applies where the soil is poor and you want to limit competition. As previously mentioned, poor soil causes the roots to stretch further to meet the trees’ nutrient and water needs.
Overall, how far you should plant your mulberry from other plants depends on how large it will be at maturity, the soil quality, and the surrounding plants’ needs.
Following these guidelines for spacing and proper tree care promotes the health of your plants and protects your structures.
Final Thoughts
While mulberry trees have some cons (especially white mulberries), they’re still a great choice to use in many gardens and backyards and bring benefits such as attracting pollinators and beneficial insect predators.
Their quick-growing nature also provides partial shade for other, more sensitive plants. Being deciduous, they’re great to use with trellises or espaliers to add passive cooling and heating (blocking the sun in the summer, and letting sunlight in the winter when the leaves are shed).
For more reasons to grow mulberry trees, check out this video by The Garden Nerd.
If you’d like some native alternatives to mulberries, try growing red maple (Acer rubrum), black gum (Nyssa sylvatica), hackberry (Celtis occidentalis), and sassafras (Sassafras albidum).
Sources
- https://www.trees.org.uk/Trees.org.uk/files/61/6181f2b7-e35d-4075-832f-5e230d16aa9e.pdf
- https://www.natesnursery.com/how-do-tree-roots-grow/
- https://elibrary.dcnr.pa.gov/GetDocument?docId=1738766&DocName=WhiteMulberry.pdf
