Hi,
I am going to plant a mixed orchard of olive and almond trees and I'd like to interplant, in-between every two trees in the same line, a legume shrub for N fixation.
I am looking for suggestions on legume shrubs with the following characteristics:

- hardy to zone 7-9, so they need to endure winter temp of up to -12º - 15º C
- drought hardy in summer months. Usual precipitation rates range from 600mm/m2 to 300 mm/m2
- high summer temps, up to 40+º C
- can grow in alkaline soils
- tolerates high calcium levels
- can grow in heavy soils; lime-clay or loamy-clay

I had a look at the typical legumes that grow in my region (NE Spain) and all of those that I have checked upon usually grow in a different type of soil, that is they need light/draining soils/gravelly/stony soils not so much compacted clay or loamy clay (the result of conventional agriculture usually). So I am starting to look at non natives as well.
Any ideas is welcome
Cheers

Siberian Pea Shrub is the first thing that comes to mind for me. It seems to meet most of your requirements though I don't know how it does in drought situations.

http://tcpermaculture.com/site/2013/05/13/permaculture-plants-pea-trees/

I am not positive about this, but I think perhaps you might look to black locust or honey locust. They aren't shrubs, per se, but they coppice and pollard, and so will take regular pruning to keep the size down, providing nitrogen-rich mulch as a result.

Have you thought about pasture strips containing a viable polyculture including no more than 20% alfalfa? The alfalfa will fix nitrogen, drop root systems down six to eight feet, and make fodder or an excellent chop-and-drop that you could mow along with the rest of your pasture, or graze with animals in a more traditional approach.

This doesn't obviate the use of a nitrogen-fixing bacteria host shrub, only adds livestock, probably goats, that will trim them down for you and cycle nutrients faster, as well as taking care of any suckers on your crop trees.

-CK

This is something I just came across this morning, not exactly what you were looking for, but it's pretty interesting anyways.

It's a nitrogen fixing corn from Mexico:

https://espanol.yahoo.com/noticias/sierra-mixe-el-extrano-maiz-mexicano-que-podria-salvar-al-mundo-182253949.html

Does it have to be a shrub? I've had issues growing woody anything but I have 4 acres of sainfoin growing with 11 inches of moisture a year. It's a nitrogen fixer, beautiful, and easy to grow.

Antonio,

it would be good to know the soil type as well.

Elle has a great suggestion on sanfoin, but it is low growing. Vetches are often good for low cover as well.

My favorite would be lespedeza, which is really hardy. Some species are low, others are shrubs.

I doubt caragana would perform well in a dry climate until well established, but it is a great plant. It doesn't like shade until it's bigger.

Three to try (cheap and probably easy to find seeds) would be

Caragana (peashrub)
Bicolor Lespedeza
Eleagnus (goumi, autumn olive, or russian olive-depends on restrictions in your area)

There are several very hardy trees that can be coppiced in your zone from the broader acacia family, like mimosa, and the mentioned honey and black locust. There are numerous others with similar habits.

Plant from seed to get the best drought tolerance for sure.

Tj Jefferson wrote:

Elle has a great suggestion on sanfoin, but it is low growing. Vetches are often good for low cover as well.

It's lower than a tree or bush for sure but mine is waist high 3 years in. I expect it to get a bit bigger.

I'll second what Chris said.  Black locust seedlings survive here in portland for the entire summer with zero rainfall and they don't even look phased.  A tiny top growth is supported by a massive root system.  They do also seem to coppice well.  I'll also add that autumn olives seem pretty drought resistant and they're a shrub.  

Hi there and thanks for all of the quick replies!

For sure I am going to integrate cover crops / green manure in the aisles. I already pinned down which ones I can use in my situation which are vetch and oats, which is quite typical in my area, but intermixed with mustard since I need to continue the soil decompaction. Thanks for suggesting the alfa-alfa. I should look more into it for varieties that are more drought resistant. As for the sanfoine, I didn't know about it, it seems to be a very interesting crop to try out. The only thing is that the seed company that I was suggested says that it needs a freely draining soil, which is not exactly what I have there. But it surely is a crop I'll consider now when the soil type is appropriate.

I mentioned the soil type, but I'll be more specific now: silty-clay loam in the first 30 cm and clay loam deeper down (I think I specified it really badly in my initial post...apologize)
Thanks for suggesting so many options for woody shrubs or coppiced trees.
I think the siberian pea shrub can be a candidate for sure, but I'd need others.
Elaeagnus umbellata might be one of the or even Elaeagnus ebbingei if, as TJ mentioned, there is no restriction)

I think Caragana needs a more humid situation that the one we are in, and lespedeza bicolor also seems to need a more draining soil type. One of the big doubts that I have is if species that are said to be thriving in a specific soil type can't adapt at all to other soil types, for example if those that are said to be needing soils with good drainage might not adapt to clay soils....at the end of the day I don't need these plants to be productive in the same sense as the fruit trees will need to be, but I guess if the soil type is too challenging will eventually die or just their growth will be very stunted.

As for the suggested trees, I would rather not use them, even if just for the fact that need pruning every so often (especially the black locust). I prefer to reduce the amount of work if possible
and don yet have animals to do this type of work for me, nor would I want to introduce goats, for example, in a newly plated olive plantation.
So thanks so far for all the suggestions.
If anybody has more ideas please go ahead!

If you can take note of the types of shrubs and trees in your area that thrive without any human interaction...they could make good candidates.  You may likely find some/most that aren't legumes, but they are hardy and adapted to your climate which can help you grow your forest.  Often the most hardiest ones will be labeled as invasive.  And some of them will be very quick growing, especially if you give them a little care and help them along.  I wouldn't know of any species to look for in your area, but if you keep your eyes open for those trees and bushes thriving on utter neglect, which nature has generally planted, I'm sure you'll find a few good ones to use.  And even though they may not be legumes and nitrogen fixing, they will still drop tons of mulch for you and you can over plant them and do a bunch of chopping and dropping while you nurse up the long term trees and bushes you intend to keep.

Those legumes are interesting

TJ, you were suggesting to actually sowing those legume shrubs directly on the ground in order do become more drought resistant (and develop a better root system as well I guess).
Can you recommend any guidelines to do this well? or you just 'stick' the seed in the ground, irrigate it and hope for the best?
I guess some seeds may need scarification or other pre-treatment before sowing....but what after sowing?

TJ, you were suggesting to actually sowing those legume shrubs directly on the ground in order do become more drought resistant (and develop a better root system as well I guess).  

Antonio,

Precisely. Seeded native roots are going to be the best, the natural growth from seed to mature root will produce a better water-seeking taproot. This is very important in dry climates. Once that as been pruned for transplantation my understanding is that subsequent penetrating roots will not be as effective. I have dug up some transplants and they do get roots going deeper, but nothing like a true taproot. My transplanted oaks (for instance) grow at maybe half the speed of the ones the squirrels plant. By about year 3 the seeded trees are superior.

Two major issues growing from seed in my limited experience- something will outperform them or the seedling will not last the first dry season

First-protection. I use tree tubes, they keep the animals from getting to the stem, and especially with a modest amount of mulch (either pea gravel or a thin layer of bark mulch, they can still emerge. Then the tube acts as a greenhouse, extending the season (I get growth much earlier and thus the trees are about 2.5x taller than controls) and they trap some humidity in dry seasons. I spend more on the tubes than the plants by far. My experience is that they can be reused, I leave them on until the plant is taller than the competing stuff. Then I remove the tube, do not stake because that makes them have weak stems, and they get marked with an orange flag so I don't mow it.

Second- nursery. The tubes help a lot, but in dry areas you can make a zai pit or similar trench, filled with whatever organics you can get, to extend the time before the ground is dry and to promote deeper rooting by a couple mechanisms. The first is that the act of making the pit breaks up the soil below, and also the microbes harbored in the decomposing organics will create a living web for the new root, allowing nutrient access. This is well described on the epic soil thread, sort of a FAQ for how to mimic living soil and produce more vibrant plants. Compost tea would also be great, especially with discrete zai pits I think it would kick start the soil life. All this stuff needs to be protected from drying out and UV, and that is why some surface layer that keeps moisture in and UV out is so important. I don't think they need anything, I don't treat them if they are out over the cold period.

Lespedeza bicolor has been quite drought tolerant for me. I also have clay soils. I don't think the caragana is as appreciative. The Eleagnus does very very well in clay,it is a total weed here. I let it grow profusely. It feeds birds too which is great for your disease control. It is a rock star plant. I am gradually replacing the umbellata with multiflora because they are incredibly tasty, but the umbellata is very hardy.

Clay has the ability to store huge amounts of water. Earthworks are much more effective in clay, because the amount of time it takes to infiltrate is higher. Even small ones are effective. Erosion does become an issue because you basically have a sealed surface, you must be careful with your placement.

Antonio Scotti wrote:Hi there and thanks for all of the quick replies!

For sure I am going to integrate cover crops / green manure in the aisles. I already pinned down which ones I can use in my situation which are vetch and oats, which is quite typical in my area, but intermixed with mustard since I need to continue the soil decompaction. Thanks for suggesting the alfa-alfa. I should look more into it for varieties that are more drought resistant. As for the sanfoine, I didn't know about it, it seems to be a very interesting crop to try out. The only thing is that the seed company that I was suggested says that it needs a freely draining soil, which is not exactly what I have there. But it surely is a crop I'll consider now when the soil type is appropriate.

I mentioned the soil type, but I'll be more specific now: silty-clay loam in the first 30 cm and clay loam deeper down (I think I specified it really badly in my initial post...apologize)
Thanks for suggesting so many options for woody shrubs or coppiced trees.
I think the siberian pea shrub can be a candidate for sure, but I'd need others.
Elaeagnus umbellata might be one of the or even Elaeagnus ebbingei if, as TJ mentioned, there is no restriction)

I think Caragana needs a more humid situation that the one we are in, and lespedeza bicolor also seems to need a more draining soil type. One of the big doubts that I have is if species that are said to be thriving in a specific soil type can't adapt at all to other soil types, for example if those that are said to be needing soils with good drainage might not adapt to clay soils....at the end of the day I don't need these plants to be productive in the same sense as the fruit trees will need to be, but I guess if the soil type is too challenging will eventually die or just their growth will be very stunted.

As for the suggested trees, I would rather not use them, even if just for the fact that need pruning every so often (especially the black locust). I prefer to reduce the amount of work if possible
and don yet have animals to do this type of work for me, nor would I want to introduce goats, for example, in a newly plated olive plantation.
So thanks so far for all the suggestions.
If anybody has more ideas please go ahead!

Just thought I'd give you some of my growing stuff. I have caragana growing in as the backbone of my wind protection tree line and sainfoin on 4 acres and spreading. I have heavy clay soil, highly alkaline. I'm pretty high altitude. Very dry. 11 inches of rain a year. I also have vetch growing, though it does not reseed and regrown as easily as Sainfoin has for me.

I am the type of person to plant absolutely everything and just see what grows. Lots of failures but some startling successes too! Plant some. You might like it.

Elle, sanfoin is super cool. No bloat, takes a beating.

Along with alfalfa, in my experience it needs an alkaline pH, and won't even sprout in acidic soils. I wish wish wish I could grow it here, both those are great late season pollen sources. They are a great choice for the west.

I saw a big bag of it at the feed and seed, like 50# they can't sell for $15. No one even tries here. I did initially but have not seen even one plant from 10# of seed. Trefoil does poorly here too.

I think you get it, just sprinkle in some species in decent starting areas and see what works. I spent a lot of money on exotics and then my best producer has been red clover, which I didn't plant at all!

This one maybe doesn't meet all your requirements but I've found Ceanothus velutinus to be so tough and adaptable it might be worth a shot. Ceanothus in general I think prefers acidic soil over alkaline and I see it growing most in sandy rocky places. There are many species though and, like I said, the C. velutinus I have seems to be able to handle anything. Drought and weeks of near 40° temps don't faze it. And the leaves make nice tea and it smells amazing..

I'm planting Pride of Barbados, Coral Bean, Esperanza, Desert Willow, Leadplant, Indigo Bush, and Honey Locust.  Not sure how many of these actually fix nitrogen, but in any case I can use them as chop and drop mulch.

My Pride of Barbados planted last week are already coming up.  I hope they can put on enough growth to be able to die down and survive the winter as a crown with root system.  My Esperanza were able to survive a pretty cold winter this way.

Cytisus scoparius aka Scottish Broom, despite Wikipedia I am seeing it on clay soil everywhere.

In Spain maybe rather Spartium junceum..

elle sagenev wrote:
Just thought I'd give you some of my growing stuff. I have caragana growing in as the backbone of my wind protection tree line and sainfoin on 4 acres and spreading. I have heavy clay soil, highly alkaline. I'm pretty high altitude. Very dry. 11 inches of rain a year. I also have vetch growing, though it does not reseed and regrown as easily as Sainfoin has for me.

I am the type of person to plant absolutely everything and just see what grows. Lots of failures but some startling successes too! Plant some. You might like it.

Hi Elle,
thanks for your feedback, I think you are right. Perhaps I should just experiment more.
I am starting to feel that many plants that are described as "not apt" for what I consider to be "my situation" might actually work.
Cheers

Spartium junceum and Cytisus scoparius are actually some of the plants I had initially considered, but put aside because of the soil type
I'll probably give them a go anyway and see if the actually work in my field.
Thanks for suggesting them
Cheers

TJ, thanks for explaining how to go about the direct sowing and the lead on lespedeza.
Cheers