Invasive Thoughts: A Battle with Conscientious Consciousness
I did a lot of research. I hummed and hawed, danced around the issue, came up with a veritable pros and cons type of list. Ultimately, the base of our food forest needed some fast-growing, nitrogen-fixing trees, and whether or not they were native came to feel somewhat moot. I found a good tree species. It was readily available (and free). What difference did it make if folks had deemed it invasive?
The mimosa tree, aka the Persian silk tree, officially the Albizia julibrissin, has been in the US for centuries now. A drive down any road here in North Carolina will likely involve passing several dozen along the side of it. If ever a tree were firmly established, invasive or not, the mimosa has become a naturalized citizen of the US. It ain’t going anywhere, and from what I can tell, no real efforts to make it disappear are underway. (Thank goodness, as they seem to typically involve the negligent spraying of toxic substances.)
That said, Albizia julibrissindoes appear regularly on the invasive species lists here in the United States and is listed second on the “severe threat” list in North Carolina. Also on the list are Russian olive and autumn olive, which, like the mimosa, are fast-growing, quick-spreading, nitrogen-fixing pioneer species. Once there, these types of trees sprout up everywhere. I get that.
Here’s where I struggle:
Many leguminous trees are pioneer species. They don’t grow in the forests and challenge established trees, like other members on the list, vines such as kudzu (Pueraria montana) or bittersweet (Celastrus orbiculatus). What the nitrogen-fixing trees are more prone to do is hit the open plains (yards, roadsides, disturbed spaces) and rejuvenate the soil. The problem, then, seems to me that they aren’t challenging forests as much as they are establishing themselves where their niche remains open. In those cases, they can be very domineering trees.
The Persian Silk tree seeds itself like crazy. My friend’s yard, where I harvested seedlings for the food forest, has sprouts coming up in any area that he doesn’t regularly mow. I fully expect, somewhat plan, that new seedlings will be popping throughout our food forest. In my mind, they will be a plentiful source of chop-and-drop mulch and a constant application of natural fertilization for the fruit and nut trees. These trees were of particular interest to me because of these qualities, qualities many “invasive” nitrogen-fixing legumes seem to have.
Furthermore, it’s not as if I’m bringing some type of tree that isn’t already well-established in the area. There are several just down the street, growing just beside the road, where forest was cut and cleared the way. The mimosa tree was introduced to North Carolina in the late 1700s. No, it isn’t native, but if we all only grew native plants, then there’d be a lot less corn and soy fields around here. The state isn’t objecting to those.
Photo: A Dandy Tree
Here’s the other thing(s):
Additionally, the mimosa tree is just a dandy specimen to have around. It’s beautiful. The leaves are lacy and resemble fern fronds. It puts out pink pom-pom flowers in early summer. Not only are the flowers eye-catching, but also they smell absolutely fantastic. The trees stay relatively small, about 12 meters or less and bush out roughly the same. The leaves provide dappled shade that isn’t a horrible thing for cultivating young fruit trees.
And, there’s more. The mimosa tree can be pollarded or coppiced, allowing continual chop-and-drop mulching over its lifespan of around 15-25 years. Its flowers are edible and can be cooked like a vegetable, and its leaves are edible and can be cooked like a pot vegetable in soups and stews. The tree has also been used medicinally for centuries.
On the checklist of things I’m after, this tree has a lot to be said for it, and though some may consider it invasive, I kind of consider it ideal.
Here’s why there’s guilt:
Even so, I know that some people adamantly oppose planting the tree around these parts. I know that, in those open spaces where it thrives, it may out compete lesser natural flora for its space in the peeking order. Normally, I’m the type of person who fights to promote natives, and actively pursuing an exotic tree has left me defensive.
The other afternoon I was sitting with another friend who is into her native plants, and she saw a jar lid of seeds drying on our outdoor table. She asked if they were redbuds, so I begrudgingly confessed that they were, in fact, mimosa. She commented on the invasiveness, and I responded with the facts about how long they’ve been here and so on. It wasn’t ugly, and the topic dropped. But, it was a moment in which I felt the required to justify such a choice and doubted as to the efficacy of that justification.
The thing is that native redbuds are part of the plan as well, but, though they are in the pea family, they aren’t nitrogen-fixers. And, of the property my wife and I are developing, 3.5 acres of the 4.66 acres is in natural forest. The remaining area had already been cleared when we bought it. We looked for years for such a property so that we could protect a small bit of forest and have an area to develop our gardens without razing any trees to do so. Isn’t that doing our part for native vegetation?
Here’s the rest of it:
We’ve planted the first of the mimosas, over twenty along the roadside border of the house, growing them to work somewhat like a privacy hedge along which we will grow other edible trees, such as the non-native Kousa dogwood (Cornus kousa), which has edible berries (and leaves) unlike the native dogwoods, which will also be around. I’d like to put in goumi berries, a nitrogen-fixer from the same genus (Elaeagnus) as Russian olives, throughout the food forest as an understory shrub.
But, plans don’t stop at trees. I’ll probably want to grow tomatoes, which are fantastic self-seeders, as well as many other non-native vegetables and fruits, most of which I hope will flourish and reproduce freely across the acre we are developing. I hope arugula, amaranth, mustard, dill, nasturtium, and so on grow wild and free. The potential of them doing so is part of what makes me want to plant them. I’m seriously looking into edible bamboo as well.
I guess the difficulty with the notion of invasives that I’m finding myself in is that, like the term weeds, the criteria don’t always seem to apply evenly across the board. The labeling sometimes feels misplaced or missing the point: If we clear forest and introduce a plant and it does what it is naturally programmed to do, i.e. begin to rebuild forests, has it invaded to challenge natural ecosystems, or have we just supplanted the normal regional sequencing? Is it wrong to do that?
The mimosa has been in North Carolina for as long as North Carolina has been part of the United States, at what point is it counted as part of North Carolina’s current ecosystem?
Header: Flowers in Bloom
This is an important area of discussion. We live in Hawaii where invasive species is a big issue. At our elevation the dominate pioneering nitrogen fixing legume is leuceana leucocephala, a fast growing shrub. Many dislike it but it is good fodder for ruminants, makes excellent mulch and iis here to stay, so why not use it. This is true of many plants here. Often the methods used to eradicate them are very harmful to the environment. I think the state should do more to prevent the introduction of invasive (like NZ does) and learn to make the best use of those that are established here.
I think that while it might already be established in your area, it makes a bad problem worse. You say since it doesn’t invade mature forests Mimosa is not bad, which ignores the importance and ecological well-being of other habitat types that are heavily affected by this (and other) invasive species. These n-fixing invasives might offer some soil-related benefits in a controlled setting but in disturbed native habitat they usually halt ecological succession, form monocultures, displace natives, and futher encourage use of herbicides to combat their spread. I just don’t necessarily see the design advantage with loading up on invasive nitrogen fixers, because when you look at North Carolina’s native mature forests you speak of, the forest system continues to produce bountiful mast despite Nitrogen limitation, in the absence of huge populations of native nitrogen fixing shrubs or trees. In fact, mature forests that have been invaded by n-fixers like Mimosa and Russian Olive are negatively impacted as the shrub/sapling layer can come to dominate the herbaceous layer. I think in a larger garden situation Mimosa could end up being a well-established pain that could escape into someone elses garden. I hope that you will reconsider your approach :)
I’m a mimosa fan myself but I’d add that Amorpha fruticosa is a great choice for your area and also native.
In Australia we are blessed with many nitrogen fixing pioneer species & first choice for me would be the local species such as Acacia, Pultenaea, Hardenbergia etc. Having said that I would still consider non local species in situations where their cultivation is helpful & they are easy to manage.
While I love the look of the mimosa tree, especially when in flower, I know from personal experience what a pain it is. My aunt had one in her urban front yard and even with lots of concrete to keep it in bounds, it sent up seedlings everywhere. I’d imagine introducing it to a more suburban/rural setting might be a recipe for a mimosa forest.
I understand Bill Mollison once said that he uses only native plants, native to the planet earth!
Sounds like you’ve made a thoughtful decision and are creating something that is positive for the environment, even if not perfect in some peoples view.
I believe Nature has a vision which is far vaster than our meagre timeframes, and it holds no prejudice or discrimination as we do. Each and every species fills an ecological niche, as designed by Life, and all are used as required. Less fighting against it and more cooperation is required. That isn’t to say we should indiscriminately introduce species across regional boundaries, but to waste resources fighting a species that has already become established? I think Nature has chosen…what makes people think the forest, as Nature, hasn’t done the same?
“The mimosa has been in North Carolina for as long as North Carolina has been part of the United States, at what point is it counted as part of North Carolina’s current ecosystem?”
Very important discussion needing to happen here, worldwide, especially considering the ongoing onslaughts of herbicides/pesticides and resources being increasingly used in order to come back to some ideal of “before”. Who gets to determine this ideal? And at what cost to the planet? A wider angle look would go beyond futile attempts at eradicating the thing that is here, to looking at WHY the thing is here, to how it’s presence can be harnessed/ Tao Orion’s book “Beyond the War on Invasive Species” and “The New Wild” by Fred Pearce are good, well researched eye-openers on this topic of invasive species. Turned my head around.
I love your plan. We are from southern California but moved to North Eastern Texas because we wanted some land and less regulations. We got just that. Now we are building our permaculture orchard. Our property came with some well wooded, okay, incredibly overgrown forested/wooded acreage too.. In that wooded area,, along the tree line, there are mimosa trees. They are quite beautiful. This year we are taking cuttings from them place into the orchard. I can’t wait until we get a little warmer as I see the trees are greening up already. Once it’s a little warmer, cuttings will begin and thus will transplants.
Along the Roadsides we also ha e dogwood and Redbird. Not exactly the species I was hoping for, but certainly those which will suit our purposes. I contacted the city and they were happy to allow me to take small trees as well as cutting from some that were more mature. So, I have some grafts coming along very nicely.
I’m quite excited for this spring.
I enjoyed your article. Thanks bunches for sharing. We’d love to see updates on your endeavors.
Lots of luck to you!
Hester House Hobby Farm.
Introducing an invasive species is risky — however, if you can ensure this helps to increase biodiversity, you may have a good reason to introduce it.
My recommendation — grow lupins, perhaps, not trees, or shrubs, not edible, but decorative and controllable, can be composted, cut and dropped.
But, if you want food forest climbers/pea family — grow peas — pretty and edible. Grow a classic variety that can climb up 6 foot high — and eat the peas — most modern varieties stop a 3 – 4 foot for mechanical harvesting. These are nitrogen fixers — so are fava beans — also decorative and edible.
It may be necessary to introduce alien species to help the biosphere cope with climate change — bringing the risks of disease and invasiveness. Done carefully and monitored is the way I would recommend.
Been gardening for 60 years — worked for a while in commercial horticulture — did postgrad in environmental studies and was involved with the lead up to the Rio Summit in a very minor way. Have read Agenda 21 and COPs etc. and am about to do more relevant postgrad study.