Looking for nitrogen fixer ideas for zone 7b

I'm in the tip of zone 7b on the border of 7a.  I've got 2 6ft stumps in my yard that I'm using as a trellis to grow grapes/kiwis.  However the span between them is about 15 to 20ft.  I already have a raspberry and chokeberry established kind of in the middle between them.  What I'm looking for is a nitrogen fixer I can plant dead center that I can cut way back in the winter but still hang my grapes and kiwis on.  I want to keep it almost like a straight pole just to hole up the vines and not shade out the other shrubs.  Plus I can't have it getting super tall, maybe 10ft or so.  I don't mind if it wants to get to higher than 10 but I'll be cutting it to keep it low enough to pick the fruit.  Any ideas what I can use?  

I don't particularly mind if it's not edible but that would be preferred.

The 2 6ft stumps were oaks that will eventually rot (one is completely hollow already, haven't cut open the other).  My plan was to plant something (right now chewing on a weeping mulberry) in the center and let it climb up and out of the stump.  by the time the stump fully rots there'll be a new tree w/ a thick trunk to hold up the vines.

This post by Paul suggests Honey Locust and Alder:

Toby pointed out via a mailing list that Honey Locust makes just enough for itself and does not share.  But black locust makes enough to share.

Further, I think it was Dave Boehnlein (of bullock brother's farm fame) was giving a tour and mentioned that if he grew an alder and then cut off branches on one side, that within weeks he would see greening of the area under where those branches used to be.  As if the roots tat would mirror the branches underground had died.

https://permies.com/t/1161/Nitrogen-Fixing-Plants#6626

I don't know about height though these could be coppiced for firewood, etc.

scotch broom (shrub) and dutch white clover (9inch height)

I really am a sucker for dutch white clover. I started a mostly clover free yard and now have gotten to about a 25%-50% mixture. It's small size makes it easier to spread and incorporate into the landscape.

One note, it will take some foot traffic but I would avoid it in HEAVY foot traffic areas at least on its own. Mixed with other grass seems to help in walkable pathways.

Take a look at siberian pea shrub, I’m not sure how it would stand up to cutting back but it’s done pretty well for us in Vermont.

Criteria:

Nitrogen fixer

Capable of holding up kiwis / grapes

Zone 7b

Straight and pole-like

Doesn't shade out neighboring shrubs

Able/willing to be cut back in winter

Black locust meets your listed criteria, good sir.  

Here's a post  -- of a post! -- with some observations from my own farm.  If you plant one and prune it thoughtfully every year, I bet you'll be able to get a good standard trunk shape that provides a sturdy pole within 5 years.   I've been hanging my hammock sleeping between such young trees without fear of breaking.  And since hardy kiwis are known to be mighty heavy vines, such strength is warranted.  Permaculture farm "Tangly Woods" currently uses black locusts to trellis grapes.  Consider eventually adding some kind of strong cable between your posts, unless you plan on using a chainsaw or milling the wood.

My maintenance recommendations:

Plant at your own risk due to its opportunistic nature.  Check regularly for suckering plants at the outer edges of the root zone/drip zone and lop them.

Be extra mindful of thorns.  Wear sturdy leather gloves (gauntlets even) and eye protection.   Be especially careful if lopping overhead branches.  Or don't and accept the regular and painful feedback

Prune in winter to encourage development of a single standard trunk aka "central leader".  Cut up the pruning bits as ramial chipped wood "chop and drop" for higher value plants

I second the black locust recommendation.
I have one that grows to over 8 feet, from being cut to the ground, in a years time.

Hey William, how thick is your regrowth in that first year?

It's about an inch thick.
It was about 2 inches thick the first time it was felled, and I've coppiced it every year since.
If it were in a better spot, I would pollard it rather than coppice it.
I have successfully transplanted one sucker over to my yarden.
It's one year old, and a few weeks ago I pushed it flat, scraped back some bark and pinned it under some soil.
That sapling was about 8 feet tall when I "pleached" it.

I also have a siberian pea shrub.
I was excited for it as self replicating chicken fodder, and maybe food,but it hasn't produced any off spring.
Not the first time I've been disappointed by an "invasive" plant.


I have mimosa that grow here, but they are not very fast and they tend to die back before they get strong enough to support much.

It's not a nitrogen fixer,but my favorite tree for scaffolding is the mulberry.