The most used nitrogen fixer in my food forest? Amorpha fruticosa. There are myriad reasons why: First and foremost, because it is easy to grow from seed. No scarification. No hot water or acid baths. It doesn’t even need stratification. But it grows. I planted a small handful of seed this spring, and I couldn’t even get around to potting up all the sprouts. Thankfully they didn’t all come up at once, giving me a larger span of time for transplanting. For such convenience, there is hardly compromise in quality. The USDA puts Amorpha fruticosa in the “medium” nitrogen fixer category, meaning it has 85-160 lbs per acre nitrogen fixing potential. This is very good for a native.
Being native, my seedlings found the right bacterial clients without any inoculation. For anyone not in its native range, it would be wise to inoculate to be sure you have them. I started finding nodules within eight months from planting the seeds.
I imagine having or not having the bacteria would explain why some people think of this plant as slow growing. My established plants can grow as much as six feet from the ground up in one year. In their first year from seed, the fastest growing plants grew to just under three feet. Even in its native range the ground might not have the right bacteria. So if your plants grow much slower, perhaps consider getting some inoculant.
Such fast growth doesn’t equate to a huge plant. It only grows to a maximum of 12 feet. Most people cite 6-10 feet as a more average height. But again, maximum benefit will be achieved from coppicing. So it shouldn’t ever get over 6 ft, right? It has no complaints being coppiced either. An experiment in the southeast US was able to coppice the plant four times a year.
This management conveniently keeps the plant from setting seed, from which it propagates very well, as I mentioned. I plan to coppice mine just once a year, in late May. This is right before the flowers bloom so it doesn’t make seed, and makes for the best mulch material, since flowering time for a plant is when the most nutrients are active in its upper parts.
If you want to keep the above ground growth, such as for letting the plant establish, I have been successful just popping off the blossoms by hand, since the plant doesn’t rebloom.
I wouldn’t pluck them too early though, as the flowers are really very beautiful. On top of that, I have a love affair with healthy legume foliage. So between that, the flowers, and all the uses, there’s no wonder why I can’t get enough of it.
Mortal Tree 
Anything you have shared on spacing of these shrubs when using to fix N? I am experimenting and put them at spacing ranging from 5 to 10 feet from different trees. How close do you space these with trees you hope to benefit as you coppice down the Amorpha?
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Hi Stuart. My apologies for the delay on reply. I was just reviewing the comments folders for the site and saw this got lost in my pending comments.
Ideally, the amorpha are planted ahead of your crop trees by a few years. If you already have some trees in the ground, and for quick boost in N, I have found good results from as close as 3 feet away. This entails you are cutting the amorpha at least annually. This can get a bit messy. So gauge what your crop tree tolerance for shading may be. A tree at least 3 feet tall I would generally feel comfortable pitting against an amorpha like this if the amorpha is cut at least once a year in May/flowering as I often recommend. By year 3 or 4 for the amorpha, depending on how vigorous your amorpha grows, you should start cutting 3 to 4 times per year with intent of killing off the amorpha. This gets fastest results for increasing N for your crop trees.
For long term N, 5 to 10 feet is fine. Throw annual/multiple coppice per year material near the drip line of your tree, or as you like lower story species suppression. From an anecdotal perspective, I generally notice superior growth of plants growing within 3 feet of amorpha plants.
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