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I have several young (1-4 years old) fruit trees and am considering planting nitrogen fixers around/underneath:

  • white clover (ground cover)
  • lupine (herb layer)
  • siberian pea shrub (shrub layer)

It seems intuitive that the N-fixers will only provide a benefit as far as their roots extend.

I don't want to damage the tree roots by digging too close. I think the tree roots will extend further outward over time as the trees grow.

Does it make sense, and is it safe for the trees, to dig planting holes for the lupine just outside the drip line of the trees? Or is there more potential for harm than good?

If I go to the effort of digging there (digging isn't easy here), I'd probably amend the general area with manure.

Similarly with the clover, I'm thinking about scraping away the mulch and scattering seed underneath the trees then covering with 0.5" or so of composted manure. (Or covering the area with 1-2" of compost and spreading seed there.) Are there any potential problems with this?

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My understanding of nitrogen fixing plants is that nitrogen is retained by the plant for it's own use. Farmers will sow clover and legumes but to make the nitrogen accessible they plow it in. This is called green manure and works well for fields with annual crops.

I see a problem with your idea due to the long term nature of an apple tree. The clover will compete with the tree in the same root zone and you cannot plow the cover crop in without damaging the apple tree roots. However the clover will attract bees who will pollinate your apple tree if the timing is right.

Nitrogen is highly mobile within the soil but mostly it is mobile in one direction, down. So rain will wash it out but a source must be close to roots for it to be picked up.

Are there annual nitrogen fixing crops? Surely some of the soybean crops would fit. Then you could harvest the beans and mow the rest of plant into a green mulch.

Edit: I reviewed that post that you referred to that indicates that nitrogen transfer does take place from nitrogen fixing plants to the roots of other species. If your apple trees have or can host the microrhizae required to facilitate the transfer then you are in a trade off situation.

Pros:

  • the clover provides some nitrogen at low levels in a continuous feed
  • the clover will attract pollinators when it flowers. This may assist if the clovers flower at the same time as the apple trees.
  • clover will add organic matter, keep the soil from being compacted, reduce water runoff
  • clover is not eaten by those Japanese Beetles that are problematic if you grew grass instead

Cons: - clover is competing for the same water and nutrients that the apple tree roots are

It's very hard to assess whether there is a net benefit. I agree with violadaprile that you do not see shrubs grown close to apple trees. Farmers appear to prefer monoculture, possibly for maintenance reasons or just habit.

  • This answer talks about sharing without having the N-fixer plowed under. – bstpierre Apr 9 '13 at 11:33
  • Agree with Kevinsky, but also would add, if your fruit trees are on dwarfing rootstock, any under 3 years old should not have anything growing within 3 feet of the trunk. – Bamboo Apr 9 '13 at 17:19
  • @Bamboo: why not? – bstpierre Apr 9 '13 at 18:13
  • ok - why not? Because dwarf root stocks have a bit of difficulty competing for nutrients during their early years, so anything growing within 3 feet may take what the tree needs before the tree does. – Bamboo Apr 10 '13 at 11:48
  • Another benefit to clover should be that it helps prevent water from evaporating from the soil. In my semi-arid area with our soil type, that seems to be more important than the risk of plants competing for water, since the weather can dry the soil faster than plants may use it, if there's nothing covering it. Bare soil is generally very dry here in the frost-free growing season. Soil with many plants in it is often moist, even if I don't water it. Plants may utilize dew, too. – Brōtsyorfuzthrāx Oct 4 '17 at 12:25
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I have interplanted goumi bushes between my dwarf fruit trees for this reason. I can't say for certain whether the nitrogen from the goumis is helping the fruit trees, but they are growing well and appear very healthy.

My trees are about 8 feet apart. I planted the goumi half way between the trees about a year after the initial planting. At the time, they were outside of the drip line, but now they overlap with it. I think your idea of planting just outside of the current drip line is a good one. If it doesn't work, lupine is easy enough to pull out, right? I might be a little more careful with the white clover, since it is harder to get rid of if it were to cause a problem, in my experience.

  • I believe the goumi bushes are described here en.wikipedia.org/wiki/Elaeagnus_multiflora and they do fix and share nitrogen – kevinsky Apr 11 '13 at 10:50
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    Yes, I know for certain they are nitrogen fixers. I just have no way of proving that the fruit trees I have planted near them are growing better than they would be had I not planted the goumis. – michelle Apr 11 '13 at 19:20
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The starting point of studies here was the observation that legumes, and species of this family are able to deposit nitrogen in the soil in which they live. Furthermore, the type of nitrogen that deposit, which is of an organic nature (manufactured by living beings), is not washed away by rain, as instead happens to inorganic form.

To avoid damage to the roots of plants, legumes are seeded prior to implantation of the plants. Then the soil is turned, burying the plants usually covered with aphids (more nitrogen). Only after that are implanted plants.

Later, the soil around the plants is kept clean (for more air). The sowing of nitrogenous is at aside, in a small special field or on the sidelines, or even directly in the compost area. At maturity the new compound is turned and distributed around the plants, without touching the roots.

Here in italy (nor in French) you wont see fruit trees surrounded by shrubs or herbs of any kind.

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you can chop and drop comrey. a lot of permaculture farms grow rhubarb around their fruit trees

  • rhubarb is not known as a nitrogen fixer, more as a heavy feeder. – kevinsky Oct 4 '17 at 0:54

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