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. (Update: The Nepal Journal of Science and Technology is no longer hosting back issues of their journal. I was unable to find another copy of the study on the web.)

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 (Update: The college hosting the study no longer hosts it on their database. I was unable to find it elsewhere on the web.) for example, did not find any inhibition of germination.



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.