N-fix 8: Albizia julibrissin

Terrible Tree #1 — Mimosa (Albizia julibrissin) What’s wrong with it: Weedy, short-lived, insect- and disease-prone, invasive roots, unattractive most of the year.

Comment: Yes, I know. You grew up with mimosas in the yard (sniff), they remind you of Meemaw’s garden (sniff, sniff), and they’re so pretty when their fluffy pink flowers open in early summer. But let’s get real. The flowers last about two weeks. Then they’re replaced by scads of these large, ugly, brown seed pods that hang there until the next spring. So for two weeks of beauty you get 50 weeks of gross. Plus, seedlings from your tree will sprout in everyone’s yard within a quarter-mile.

-Southern Living Magazine

Once the naysayers settle down to let me get a word in edgewise: Mimosa -specifically Albizia julibrissin or silk tree -in my climate at least, does not own up to any of these accusatory standards. To boost, it’s quite a proficient N-fixer.

As to their longevity: There are several specimen of silk tree in my area, many of which are over 30 years old, some, in really protected areas, nearly twenty feet tall, and only all the more gorgeous for it. Perhaps we are outside the range of its pests. On the other hand, winters around here often frost silk tree back to the ground. Such dieback actually extends their likely lifespan, because the wood is renewed. For plant cells, this is the equivalent of turning back the biological clock, and can be done over and over again.

Short lived flowers? Perhaps it’s the cooler weather, but the bloom lasts far more than two weeks around here. Some do only bloom for three or four weeks, but several I see start in late July, and continue without stop until October.

I should mention some trees of course have more color to their plume-like blossoms. The one below my neighbor found as a seedling even further north, from a parent tree whose flowers were this undesirable color. It came out alright I think.

As to “ugly,” “brown,” pods: They’re tan, if not gold. I find them very attractive. In fact, even less attractive pods like these on any plant are often highly sought by advanced garden designers for “winter interest” they provide. Of course, a barren landscape in winter is more a northern problem than a southern.

Seeding all over the place: In the south silk tree is considered a noxious weed, per its willingness to pop up everywhere. It’s doing its job of turning open space into riparian field, to become productive forest. Shame on you for hogging up petroleum, riding around a growling hog to keep open spaces it’s just trying to repair. But I’ll stop my rant there.

If this seeding were an issue in northern climates I would only be all the more delighted to grow this plant; but I can say from experience that we only find a seedling or two every year, quite close to the parent tree. These do not always survive to adulthood, and so are quite precious, usually given as gifts.

There are rumors of a few trees in this climate that seed especially well, but these are interestingly enough, ugly white-flowered specimens.

These considerations are for tall trees though. My neighbor has allowed her tree to reach the upper story of her forest garden between a magnolia,  Montmorency cherry, and serviceberry tree. For most food forests, silk tree has much more practical functions.

Soft design aside, the tree makes N. It grows footage of branches every year, which in our climate helps it bounce back from winter-kill. In the case of a food forest, this means lots of delicious mulch for healthy soil.

The USDA cites this plant as a medium level nitrogen fixer. One study noted it made far more nodules than other N-fixers, although only had medium level bacterial activity as pictured below. (Notice all the species aside from silk tree are decidedly tropical). View the full study here.


This study in particular pointed out that nitrogen fixation is heavily dependent on sunlight for enough energy to run the nitrogen-fixing process. As a result, cloudy climates may not see such vigorous growth. But Ohio certainly isn’t the most sunny state; and many fast growing silk trees I have come across do so in half day’s shade.

I recommend taxing the tree of its growth for mulch at about time of bloom. You could just snap off the flowering branches with your bare hands. In the case that your tree has not reached blooming age but is getting out of reach for easily collecting mulch, just wait until about midsummer to begin snapping off branches.

One more practical accusation to address is the invasive roots. This refers to roots choking out plants below it, and roots becoming new plants. I should also mention that many accuse mimosa of allelopathy -manufacturing poisons for the surrounding plants (See Notes on Juglone for more info on allelopathy). Soil type, and climate can have major effects on how these attributes are expressed. I must report I find they don’t show up with the local specimen. My neighbor in particular has ferns, hosta, and tree peony below her mimosa. They have no complaint but for the slight dryness due to the thick branches above. Otherwise, they’re quite happy.

Studies have been conducted on mimosa and its relatives’ ability to inhibit germination in seeds. Although only a limited range have been placed in controlled studies, this study, for example, did not find any inhibition of germination.

View the rest of the text this image came from here.

The best way to propagate mimosa, aside from seeds which just need water, warmth and soil in most cases, is actually to dig up pieces of the root and let them sprout branches. I have tried cuttings, and they don’t root. If you are hacking up the ground with a shovel around the base of a silk tree, then perhaps you’ll get a sprout. We never have been lucky enough for this to happen around here.

When placing mimosa in design, I give it especially visible positions, where people can interact with it. For one client, I have mimosa at the corners of a grotto design where they will be allowed to grow as large as winters allow. The intent is to create a dome of arching branches. In winter these will quietly rustle with golden pods.

Placed where you might brush by the leaves, silk tree is especially entertaining, because it folds its leaves in response – a character which in plant terminology is called thigmonasty. The tree also folds its leaves at night, or in the dark before storms. This response to light is called photonasty. Below are some photos taken by my client of her young trees.

My client asked why the mimosa had stopped folding its leaves when she petted it. The fact is they can learn.  In The Hidden Life of Trees a study was cited which used dripping water as a means for stimulating a mimosa’s thigmonasty, causing it to shut. After many days of applying water in regular intervals, stopping, applying again, it learned this was not a threat, and got on with photosynthesis.

An intelligent tree like this is most desirable for adding to food forests as pleasure for the eye, delight for the curiosity, as fountains of mulch and fertility, for vibrant, healthy, cold climate jungles. Thank you for letting me state my position.


The rules of spacing

I was at a Christmas party in conversation with a local Timken engineer who, hearing I design food forests, wanted to pick my brain on apple trees. He had six trees in two rows of three, well spaced in his backyard. He was throwing out terms about the mainstream organic sprays he was using, and framed his questions expecting me to know some super organic spray, or spray regimen, that would fix his problems of pests and low vigor in general. I don’t think he expected the answer I gave: ‘What’s planted around the trees?’

We often think of the rules of spacing as rules for keeping other plants away from each other. In practice I find the lines blur between species, and enters a much more broad science: it’s what should be included near the plant, as well as what shouldn’t. Between these two aspects, you make or break the majority of fruit tree problems.

The lines often blur between species because, let’s face it, plants don’t grow in a vacuum and always have something growing up against them. In this guy’s case, his trees were planted right into his lawn. They were in competition with the grass.

Looking at their history, grass and trees are in most cases nemesis of one another. Trees make forest; but grass needs open space. The setting in most yards of trees with grass between is quite artificial, and only exists because we keep the grass mowed. In any other situation, trees would take over.

The prairies are the kingdom of grass, and these occured because of rain shadows, or areas where circumstances such as the Rocky Mountain range messed with the winds that carry rain, creating droughts in one part of the year, and near flooding in another. Trees don’t like that, because most have relatively shallow roots, as much as 80 percent residing in the top three feet of soil depending on the kind and its conditions; but prairie plants, such as the grasses, and N fixers like senna hebecarpa, put roots down unusually deep, so reach the water table whether rain comes or not.

An experiment showing the root growth of Red Delicious apple tree two years after planting.
Have you ever wondered as you pass woods how the trees survive so close? If you were planting an oak tree in your yard that would someday reach a hundred foot tall, can you imagine the spacing recommendations? They would be over fifty feet apart. Most yards couldn’t fit more than one tree. But in the woods they stand on top of each other, growing for hundreds of years, happy, and healthy.

Studies have shown that trees can grow their roots deep into the ground, but prefer to keep their roots higher in the soil if possible. There is more organic matter, hence nutrients and water, in this layer. If there isn’t, trees will try to put in the work to grow deeper. This is a lot more work, and certainly isn’t their first choice.

What trees really prefer is building networks in which they share and preserve resources. For instance, trees have what is called hydraulic redistibution, which is a fancy term for moving water not only up for their own use, but back down into the soil for storage, and horizontally to other plants. Peter Wholleben, in his book The Hidden Life of Trees recalls his surprise when he found a ring of roots from a beech tree that must have been cut down well over a century beforehand, but still had green, living roots showing above ground. It had no leaves, and the stump was gone. As he explained, citing various studies, the living trees around this ancient (should be dead) tree were feeding it sugars made in their leaves, keeping it alive. Likely, they got some kind of kickback from the extended root system because it allowed them access to more resources.

This is in ancient, established forests, so conditions aren’t quite the same for our young transplants. We can get some similar effects by growing fruit trees in more open settings, or riparian zones. These are zones similar to fencerows and overgrown fields where grasses are just converting to trees. These zones are iconically untidy and wild; but skillful gardeners know the elements of these zones, like clay in a potters hand, have the best potential to form the most beautiful, lush gardens.

Riparian zones have many layers, with notably high numbers of low growing herbaceous and woody shrubs, many of which are nitrogen fixers. The quickest way to simulate this ecology is making ‘guilds’ of plants right around your fruit trees. Here is my manual of bed building for info on quickly clearing grass without tillage. Plan on expanding these plantings every year until the beds around your trees meet. If the tree is older, and larger, the bed should extend at least a couple feet beyond its drip line.

An example guild: 1. Fruit Tree 2. Comfrey 3. Siberian Peashrub 4. Amorpha fruticosa 5. Japanese Wineraspberry 6. Honeyberry 7. Blueberry 8. Turkish Rocket 9. Crambe cordifolia 10. Stepping stones, (or in this case, stepping logs). The green base is a ground cover of mint.
Any guild should include at least 2 woody nitrogen fixing plants, about 5 plants that do not fix nitrogen but can be cut for mulch, such as comfrey, or a groundcover of something like mint, then several fruiting shrubs like raspberry or honeyberry, and some perennial vegetables.

This is the best method if you already have fruit trees in the ground, like our engineer friend. If you’re just planning your food forest, Robert Hart, the father of the northern food forests, recommended planting full size or standard fruit trees at recommended spacing for their size, in rows like any orchard, but then semi standard or medium trees, then dwarf trees, then shrubs, then herbaceous plants, then vines to climb and fill in the cracks between them.

Photo credit: Graham Burnett.
I’d recommend mulching as much as you can, and planting that area with a complete planting like this. The space should be completly filled with plants, and will establish faster with less work overall.

This system gives quite attractive results that are increasingly less cost and labor than serial applications of even organic, clay-based sprays, pyrethrums and neems, let alone the more harsh chemicals. There is work later on, but this is of course dabatable, because its mostly harvests of fruit. Sounds like pleasant work to me.

The Garden of Mortal Tree

Such gardens arrive after some years of trials, where species themselves indicate their preferences, often in defiance of the dictates of literature. It is fortunate indeed that plants cannot read!

-Bill Mollison Permaculture ll

The sun bleeds a hotter light as it sets in the west, and this slope leans into it. It’s face to the setting sun, backed by trees to the east that swallow the gentle light of morning, days begin in shadow that lingers almost to the heat of midday only to be seared by the red light of evening.

The plants grow from these elements. Red-tan brushes of broomsedge grass (Andropogen) speckles the front in tall clusters like an artist’s idle tools, while at the back, messes of honeysuckle cling on piles of logs below scraggly chokecherries. Near the center, in contrast to the lush green Catalpa tree down the hill, stands the skeleton of what was once an apple tree. This is Mortal Tree, the sun scorched slope next to my house that’s becoming a vibrant, lush, productive food forest.



You might say I’m starting from scratch on this fourth acre of scrubby land; but I don’t plan on doing it alone. I know that every plant and tiny animal living here intends as much as I do to make the place burst with life. The aim of all nature is to create more life. My aim is not to take over, it’s to encourage and strengthen the process already at work. All this through carrying out a design.

Design works for me as the bridge between growth the garden wants, and yield I want. The more my design mirrors nature’s design, the more successful the project will be. In Mortal Tree, that design is a self-sustaining organism, every part interacting to make the whole, every part feeding the others, in care and nutrients, demanding little intervention from me.

Mortal Tree is the center of this design. Paths radiate from this point in fractile, lightning-like arms, and around this point the intensive planting starts -at the “nucleus” of the garden, growing out. The rest I let grow wild, mowing the grass and weeds with the quiet swing of my scythe, gathering excellent mulch. This mulch is fertilizer, moisture retention, and a tool for clearing space my edible plants and fruit trees need. It’s my main tactic, directing free growth of the ecology into my designs of hungry crops.

I’m experimenting with a no-till grain patch, fine-tuning PASSIVE gardening, enjoying the productive beauty in polycultures full of fruit trees and perennial vegetables. Nut and more fruit trees are scattered through the wild growth further down the hill that would perform far better if I had the mulch for them, but there lies my biggest problem:

It’s easy with the desire to power more plantings to overtax the free growth, and damage this delicate ecology. The current design has its limits. Over time, the area of free ecology I harvest for mulch is becoming smaller than the area producing food. What will happen when the need for fertility is greater than the supply?

I watch for what the garden offers. Mirroring the designs of nature in the yields, the design is improving. I’m using running plants to manage weeds, plants that produce massive amounts of mulch in the organized plantings to cycle nutrients and provide cover that protects the ground, and nitrogen fixers that can meet high demands of the hungry production plants. Slowly but surely, the two ends of the equation -wild carefree and productive, labor intensive -are drawing close.


I have to remember design is only so good as the nature that informs it. Nature is only so good as the life that fills it. No matter how refined my designs become, they are still nature. They house the light that bleeds from the sun, stretching up as growing plants, spuring on change in every aspect. Sometimes that change is good, sometimes not so good; but with the hope of achieving an ever more harmonious design to harness this light, through the changes each day brings, this garden, and its designer, lean into it.

For those of you new to this blog, “The Garden of Mortal Tree” used to be my About page, but after placing it in the Visual Archive, it fell out of existence. This is a major overhaul of that page that better reflects the present Mortal Tree.