Red alder in the garden

I've been transplanting these guys around my property for years, they're a great free resource of firewood, a quick privacy screen, leaves in the chicken run etc
Last year I planted a few in the garden, about 2 ' tall. I'm thinking that they'll be a benefit by fixing nitrogen, give a bit of shade, build the soil, etc people look at me like I'm crazy when I tell them this? Am I nuts? Would there be any downside? My thinking is I can always drop them in a few years and chip em.
I not noticed that of the two white walnuts I planted about 5 years ago, the one that's surrounded by alders is 3 or 4x as tall. I thought maybe the extra nitrogen had something to do with that
Any thoughts?

They are apparently good for coppice, so they might be hard to kill off later.
Tree roots can aggressively invade garden beds,out competing the annuals.
Some nitrogen fixers are said to do little or no fixing of nitrogen is already available, so you might not get the benifits you want.

All that said, I build raised lasagna beds right next to fruit trees.
I feed the bed annually and get no  tree roots in the annual root zones.

In your posit, I would plan on pollarding them annually,  or even continuously.

Thanks,  I guess there's better ways to add nitrogen and provide shade.

Mike, I am doing this as well, so I think it is a good idea. The annual pulsing of roots and leaf biomass decomposing does improve soil, hence the ability of later succession trees to grow where they could not before alders’ help. I think if in a place with decent soil already and minimal sun it may be less helpful, but where I am using them I have very poor soil to start with and plenty of sun and wind for the alders to help moderate.

The above post points out a good reason not to waste nitrogen fertilizer on beds around the alder, but I do not see a reason not to try this. If they grow back from coppice, then just keep cutting them back and get a proportional amount of root die back which will decompose in place, fertilizing the soil. I use the trunks and branches as trellises too.

Ben, exactly what I'm thinking.  I cut them back every couple of years, they won't take over. We get alot of sun in the summer , sunrise to set is 16 hrs right now, much of the garden gets too much afternoon sun in the summer. If they're causing a problem I get rid of it. I've had issues of plants going yellow and not growing so I'm thinking nitrogen is an issue. Comfrey tea helps but doesn't solve the problem

I don't think of Red Alders as coppicing easily. I've probably seen some suckering from roots. But, no, when they fall over they tend to be dead and done. Granted, I am thinking of trees that were already well grown 30 years ago when I was a boy and first got to know them. Younger ones could be different. But I can't think of one that came back after we cut it, either.

Are they yellowing on older leaves, younger leaves, or both relatively equally? If you cut back alder branches, does this seem to help with yellowing? If it does help, it may be due to nitrogen released with proportional root die back by the alders (most plants balance their root to aboveground growth).

Sounds like you are at a pretty high latitude, whereabouts are you? It can help us bring  some context to our answers if a general location, elevation, and annual precipitation is in your profile or signature.

I’d echo Jeremy’s comment on red alder coppicing, as it does seem the whole trunk dies when losing their leader. The do handle thinning lower branches well though, as they do naturally when any given leaf or branch does not get adequate light (6hrs+).

The yellowing is on my plants in the garden. The alders transplant no problem. The ones in the garden are young,  I'd probably chop them when they're 10' or so. There's no real plan though, if everything around it dies, I'll cut it. If they thrive, I'll leave it. I like to experiment.

I didnt mean to discourage your plan at all, rather quite the opposite!
My experience with coppicing/pollarding black locust   catalpa and mulberry made me warn about the potential difficulty of terminating such a tree.
I have no direct experience with alder of any kind, but a quick google indicated they were good for coppice.
That suggests they will grow back from stumps,which could be good or bad, depending on the scenario.
The source I read also said to coppice on a relatively short cycle, as trees cut at older than 10 years would not readly grow back.
If black locust is any indication of how nitrogen fixing trees grow,  a 10 + year old alder could be huge.
I chop down one particular black locust sapling every year, and t grows back strong every  year.
Based on this I have started siberian pea shrub cuttings in my tomato and cucumber beds  but NOT black locust.

Alder is clearly not locust, but it may still ressist dying on cue.

Thanks for the clarification William, but I did see some wisdom in your post regardless.

As for the yellowing leaves, older leaves vs younger leaves yellowing on the same plant indicate different things. Older leaves naturally yellow as the die off, but can indicate N deficiency if it is premature. In some cases N can be locked out by PH out of the tolerable range for that plant. If its the whole plant yellowing, it could be a temperature issue or over/under watering. If on tips of upper leaves most exposed to sun, it could be sun-scald. Regardless, I would likely not blame the alder, though it may not be performing the miracle of immediate soil restoration/formation we hope for. I might experiment with thinning lower alder branches in an experimental intervention vs control (no thinning) setup to see if that alters the yellowing issue. Maybe another intervention would be a really good compost top dressing or extract.