I am growing what I call a mini food forest (basically a tree filled urban kitchen garden). I am using many herbaceous nitrogen fixers like lupine, clover, peas and beans for ground covers for a consistant supply of nitrogen and organic matter. I am aware that N fixing shrubs and trees are a much better source. The only one that seems to be best in my mind is Caragana, but I am trying to stay away from having so many of one kind.
I do have one "quicksilver" russian olive in the back corner of my yard. This is helping the surrounding bamboo, roses plums and other plants around it. So far it isnt suckering, which is why I am trying to stay away from seedling russian olive. I could strike some cuttings but again I am trying to limit the amount of same species/type trees as much as possible. I was tempted to get a honeylocust cultivar. They are around 60 bucks but I am not sure if honeylocust fixes nitrogen (I know there is more evidence against black locusts ability to fix nitrogen but an unaware of honeylocust). They also tend to become pretty big, and I am being very selective of tall species since I dont have much space to work with. I could chop and drop them to control the size.
I was going to get some sea buckthorn but decided not to due to what seems to be a relentless suckering habit. I also thought about alder, but am aware of how massive they can get.
Anyone else have any suggestions? The above is all that I can think of.
Alder is native here as well, they are just hard to find in nurseries and get massive for the most part.
I've got three small alders in my small suburban garden. They're native here as well, and I see lots of saplings in the localwood, where it is of course not allowed to take plants from, ahem. Like my fruit trees, they are subjected to pruning to keep them small.
Ceanothus is another N-fixer--I think it's hardy to zone 4. Also laburnum (golden-chain), ulex (gorse), and cytisus (scotch broom)--but check zone hardiness for these. Though cytisus may be illegal in some places, as it's considered an invasive. It's another native here.