How to manage invasive Mimosa trees

How to manage mimosa and invasive species trees. How to kill mimosa trees and what uses do mimosa trees have.

The mimosa tree (Acacia dealbata) is beautiful and valuable with many uses; however, in Portugal, it is highly invasive. This invasiveness, combined with its flammability, makes it a significant fire hazard. As a result, planting mimosa is illegal, and managing or removing it can be quite challenging. We initially tried to eliminate it from our farm but have since found peace with the tree. Now, we work in harmony with the mimosa to reduce its fire risk over time while benefiting from its various uses.

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What is an Invasive Species?

Invasive species are non-native plants, animals, or organisms that enter a new ecosystem and cause harm to the environment, economy, or human health. They outcompete native species for resources, disrupt natural ecological processes, and can lead to the extinction of native species. Invasive species may be introduced intentionally or accidentally through human activities such as transportation and trade, and they can quickly spread and establish themselves in their new environments, often with devastating consequences.

Advantages of Mimosa

• When in bloom, mimosa has a pleasant scent and attracts numerous bees.
• It is a nitrogen fixer, and its leaves or wood chips can enhance soil quality and increase nitrogen levels.
• We use its branches in construction through a technique called wattle and daub.
• It grows quickly and regrows after being cut, making it a useful addition to our firewood supply. It lights easily and is perfect for starting winter fires.
• The leaves are very useful as a forage for sheep
• It serves as a beneficial, fast-growing shade tree.

Invasiveness of Mimosa

In Portugal, mimosa spreads vigorously through seeds. Each year, millions of seeds fall from the trees, resulting in many new saplings. Additionally, the tree spreads by its roots. Notably, when cut down, it tends to grow back even more vigorously and bushy.

How to kill Mimosa trees

We have tried various methods to eliminate mimosa: cutting it back entirely, digging it out with a bulldozer, ring barking and more. Whatever we do it keeps coming back. The only effective way to control it is to keep sheep permanently in the infested area; however, this doesn’t work for us since we rotate our sheep around the farm to regenerate other land.

Managing Mimosa

When designing a new forest, food forest, agroforest, or native forest, I like to work backwards from what I hope to achieve. 

**Ultimate Aim**: To allow large native trees to shade out the mimosa in a silvopasture managed by sheep. To achieve this, I will manage the mimosa trees to create a mimosa forest with large trees spaced 12 meters apart, each with an expansive canopy.

Year One of Managing Mimosa

We have observed that removing all the mimosa stumps from a clump leads to more vigorous regrowth. Instead, we remove all the trunks from one clump but leave just one. I then introduce sheep into the mimosa area to eat the baby mimosa saplings and any reachable leaves on the trees, converting my mimosa problem into a source of meat (mimosa is high in protein and serves as excellent fodder). During the dry summer months, when the pasture lacks sufficient feed for the sheep, I remove any lower branches from the mimosa trees and provide them to the sheep as feed.

Year Two of Managing Mimosa

I revisit the mimosa trees and check for any that are touching. If trees are in close contact, I thin them out to ensure they do not touch. If unsure, I avoid cutting down trees, as more cutting often leads to greater regrowth. Twice a year, I put the sheep in to eat any regrowth, allowing me to collect firewood and sheep feed from this process. Additionally, mimosa makes excellent nitrogen-rich wood chips for gardens.

Years 3 to 10

As the trees grow and their canopies begin to touch again, I continue to thin them out so the canopies do not make contact. This encourages wider growth. Each year, I repeat this thinning process and remove lower branches, ensuring that once the trees grow large, no branches are lower than three meters from the ground. It is crucial to train and prune the trees in a way that minimizes wind damage. If I suspect that a tree might fall due to wind, I can pollard it at three meters above ground. Remember to put the sheep or goats in twice a year to manage any new growth.

Year 11

By now, the mimosa trees should be large, with massive canopies, creating a shaded area. This is the stage we are currently at with a small section of our mimosa forest. Our goal is to expand this to the entire area before employing the next steps: planting future trees that thrive in shaded conditions during their early years, such as walnut, elderberry, chestnut, oak, and cork oak.

Years 15 to 30

The focus during these years will be on managing the forest for firewood and shade while preventing any new trees from growing. Throughout the process, I have been removing mimosa trees at their bases whenever the canopy of one tree touches another. This strategy encourages wider and larger growth among the remaining mimosa trees.