Thought this might be of interest to some. It was a great find for me because I'm considering a test plot of ultra northern hardy pecans, and was thinking about all of the species in this study, wondering which would work best.
locust, false indigo, and smooth false indigo seedlings had high foliage nitrogen concentrations and have
been shown to quickly produce effective root nodules in association with native rhizobial populations?
"Honeylocust and Kentucky coffeetree, but not redbud, had intermediate foliage nitrogen concentrations."
"The low-foliage nitrogen concentrations for redbud were unexpected...Perhaps efficient
compatible rhizobial bacteria were not present within our old-field soils, or redbud is incapable of nonnodular
I live in a desert area where russian olive has become rampant. It's amazing how it can fill in a piece of non-irrigated desert land and once the stand has achieved a decent size the microclimate is changed and the grass begins to grow around it and the soil begins to increase in organic matter. Very humid in a moderately thick stand on a summer day.
I think you are best to not choose only one but to use several and see which works best and alter the plan as you go. If you don't do this you will never figure out what you didn't know when you started. All you'll know is that one nitrogen fixer is or isn't helping.
My tentative plan is to have a row of pecans running east-west, planted at the recommended 50 ft distance. A nitrogen fixing tree will be planted at the mid point between each tree, with a nitrogen fixing bush planted in the middle between each pecan tree and the adjacent nitrogen fixing tree. The n. fixers will be cut back periodically and used as mulch around the pecan trees.
I'm also considering putting mixed rows of nitrogen fixing trees and shrubs at the mid point between each row of pecan, to be heavily pollarded or killed once the pecans need the room and/or start to get shaded out. I have to do some more research around this before I settle with this plan. I'm not sure if this'll be overkill, or if the plan in the above paragraph, coupled with some herbaceous dynamic accumulator mulch plants will be enough of a mulch source.
Consider the orientation of the rows carefully in the overall context of what is trying to be achieved. It seems to me that north-south orientation with a wider spacing east-west would have some sunlight advantages the deeper the polyculture is surrounding the pecans. If only a saleable yield from the pecans is planned and the other plants are all designed to support increased yields from the pecans then orientation may not matter as much. It seems to me that since we know edges are the most productive that there is an advantage to designing more edge into the system which will get full sun part of the day rather than being on the north side of a row of pecans and subject to a greater amount of shade during the day.
If I were attempting what has been outlined I would plant basically as described except I would also plant sacrificial support plants closer to the pecans which would be turned into mulch as the pecans grow as well as more shade tolerant plants which can stay in place as the pecans begin to yield. I would avoid hauling pruning or leaf litter from the between pecans nitrogen fixer but would rather plant additional ones closer which can be sacrificed. I’d design it to be chop and drop if at all possible. Let the between pecans nitrogen fixer feed another plant near it until the pecan roots are large enough to reach it.
All of it depends on how deep the polyculture will be and what type of yields are expected other than a pecan crop. I believe that the deeper the polyculture is at the beginning the more control will be possible as it progresses in that a greater latitude should exist as to what plants to keep and what to let go by the wayside as the system develops. And there will be a bonus of earlier yields from another plant.
I don't think I was clear with what I meant in my last post, sorry. I meant that to move 25 feet from the n. fixing tree to the pecan to 'drop the chop' seemed excessive/too long a distance to be efficient. I agree that planting sacrificial trees closer makes more sense but I struggle with coming up with the planting distance from the pecan trees.
Normally I would want a north-south orientation as you suggest but was thinking of east/west orientation because of space and microclimate limitations. I'm really going to be pushing the limit of range for these pecans, so I need the warmest possible spots for them. I only have one field that is suitable; a south facing hill that is about 500-600 feet east-west, and about 200 feet north-south. At the bottom of the hill is a mature pioneer forest comprised of poplar, cedar, ash and birch, which I imagine will block the flow of cold air and create a frost pocket at the hills bottom. To avoid the frost pocket, I figure I'll have to keep the pecans on the upper half of the hill, which means 2 pecan trees running north-south at most.
I haven't really got as far as thinking about the herbaceous layer yet, beyond a green manure, dynamic acccumulators, maybe a permanent white clover dominated cover with sprawling herbs mixed in, and/or some squash. Not really sure at this point. It's at the very farthest corner of the property from my house, so I couldn't plant much of anything that needs a lot of attention.
Exactly. 25 feet makes chop and drop into chop, walk, drop, walk. The distance you should plant the sacrificial all depends on what you choose. If it utilizes space at the same height the pecan does that will be more difficult. Choose one that is separated by height and/or breadth. Have your plan in place far ahead of time as to how you will eliminate the sacrificail and what it will be replaced with.
I have no experience with grape vines as of yet but as I get time I want to look into planting a dense row of Russian olive on the north side of grape vines and use the pruned Russian olives for support for the grape vines and nitrogen. I need to decide if the sun reflecting off the light colored foliage of the Russian olive is a positive or negative among other things. This will be a science project to learn from.
In the study, the nurse trees/bushes were planted 3 feet from each pecan, on either side of the row. The rows were 14.7 feet apart. The nurse crops were coppiced 4 years after planting. I wonder if that three foot distance was chosen with a real-world scenario in mind, or simply for the purposes of the experiment. It just seems really close, even though they were pruned to a single dominant stem. That leads me to think that if I'm going to let the pecans grow without doing any 'shaping' pruning that if emulating this study, I'd have to go from 3 feet apart, to either 6 or 9. On the other hand, looking at Lawton's forest gardening dvd, his nurse crops seem to be about 3 feet or so from his fruit trees.
Do you, (or anyone else reading this) think it'd work to plant the nitrogen fixing bushes 3 feet from the pecans, the nurse trees 6 feet, and another at 12 feet, 18 etc, filling in between the rows of pecans? As the pecans grow and need the space, (or as I need to supress grass growth) I could 'sacrifice' the very closest nurse trees, and bushes or at least heavily coppice them. If this doesn't seem sound, what would you recommend?
My tentative species list for nurse trees right now are...
*range of planting distances recommended for these are 20-30 feet apart.
speckled alder (planted on the south-west side)
poplar (planted on the northwest side) (I realize it only fixes nitrogen into its stem and not roots but they grow like crazy on the land here, with hundreds to transplant)
kentucky coffee (planted on south-east side)
The study indicates that the nurse trees were expected to provide side shading to the pecans (see last paragraph). I think you need to determine how much shade you’d like the nurse tree to provide and let that determine the distance from the pecan you need to plant the nurse tree. 3’ does seem too close if you are not training to a single stem.
IMO planting a sacrificial bush nitrogen fixer at 3’ is reasonable as long as the bush doesn’t outgrow the pecan early on and shade the pecan out. Being the inquisitive type I would be tempted to plant an annual legume near the pecan the first year and not harvest the crop and then plant the bush late in the first year or the second year. I would definitely try additional strategies on a few pecans. You never know what you will learn. Will you be planting additional bushes further from the pecan trees which will remain in place as part of a polyculture?
Filling the gap between the rows of pecans with nurse trees seems impractical to me unless you are going to obtain a yield you desire from them. If your plan is to sacrifice the near pecan nurse trees and leave the between row nurse trees to create a more divers polyculture then it may be the right thing to do. Is there some other yield which would be useful to you which can be grown in the early years and remain as part of the polyculture?
I'll email the nursery and run the scenario by them, and ask how much shade the pecans will need, if any. I've already emailed two of the authors of the study to ask the same.
Since I haven't cover cropped the area, I was thinking of planting peas and/or white clover and/or fava beans. If any woody plants are to be 3' from the pecans, I think I'd plant them at the same time of year as the pecans, otherwise I'd be worried about damaging the pecan roots. I'm also thinking of planting some herbaceous dynamic accumulators in or adjacent to the mulch zone (eg. docks, comfrey,
Due to budget restraints, I'll only be getting 4-6 pecan trees now (2 grafted, and 1-4 seedlings from two different nursery sources). I don't want to bet too much of the pie on this tree until I know it'll work in this zone. If they take off after a few years, I may put several more in. Since one of my goals is to see which one works, I think I need to stay relatively uniform with the surrounding support species and mulching.
The reason I was thinking of filling the gap between the rows of pecans with nurse trees was to cut them down, use the stems as mulch around the pecans, and laying the trunks on contour and leave them to rot and house fungi. I see that as being a yield of sorts. I could also always use some for firewood, as we have a wood burning stove in one of the houses on the farm. I was thinking of using mostly poplar and basswood aka linden, since there are plenty around here dig em up for free.