Or if you want something really low, clover or birdsfoot trefoil?
You can also keep some N-fixing trees to a shrub size with pruning, I have shrub sized elaeagnus
If you're willing to plant each season, there are a lot of productive bush forms of beans. They'll not fix as much nitrogen as if you were tilling under before beans form but would provided another option.
FROM: Eric Koperek = email@example.com
SUBJECT: Ground Covers for Dwarf Fruit Trees
DATE: PM 7:47 Monday 23 May 2016
1. Seed Dutch White Clover = trifolium repens at 12 pounds per acre. Dutch White Clover grows only 6 to 8 inches high so you never have to mow it. Dutch White Clover grows vigorously = it is highly competitive = it blots out most weeds. Dutch White Clover also tolerates shade and field traffic so it is a highly persistent living mulch. Remember to inoculate clover seed with nitrogen-fixing bacteria before planting. Dutch White Clover is often used as a ground cover in commercial orchards. Other low-growing clovers include: Crimson Clover = Trifolium incarnatum and Sub Clover = Trifolium subterraneum.
ERIC KOPEREK = firstname.lastname@example.org
With the clover and the beans what you probably need to do is actually chop and drop. When you cut the top, the corresponding root die back releases nitrogen into the surrounding soil. Your tree roots are also growing there, ready to immediately access this resource. No tilling required, in fact you benefit from minimal root disturbance all around.
With perennial plants you have the added benefit that the still living plant will regrow and capture more nitrogen to repeat the cycle with in the future. With annuals you have to replant to repeat.
If you are in the bay area and zone 3, you must have a snow machine or defunked icemaking business. I'm guessing you're really zone 7. See: http://www.sfgate.com/business/article/USDA-releases-new-Plant-Hardiness-Zone-Map-2699544.php#photo-2170804 for a map.
Looks like most of the suggestions so far are appropriate for the bay area though. There are also perennial pea vines I would check out, as well as some clovers. the pea vines are often edible, or part of the plant is edible (hyacinth, runner beans) but clovers are usually not. Your season might be long enough for peanut. There's also apios (potato bean), but that one you have to disturb root area for, unless you just focus on the pods. Dry beans may also work or chickpeas (which used to be grown not far from there as an ag-crop)
Also, stay tuned cus i'm about to post that poly culture thing
Amit Enventres wrote:Ah! Chris- that makes more sense Permaculture zone - lol. Good luck with the plants. I'm no expert on root nodules, but I think the science isn't perfect on them. I believe that when you grow an N-fixing plant, no matter what you will get some additional N in the soil. If you chop and drop the top of the plant, what you drop is going to add N to the soil. Some say the nodules will fall off and add more N to the soil too. I'm susupicious of that- I would think the plant would just use them to make more leaves. Some studies show plants and soil buggies share nutrients, so there's another possibility.
My understanding is that when you chop the top of the plant, you have root die-off and those root nodules die too and add N to the soil. The roots that are still alive contribute to the plant growing back and growing new leaves, as you said.
Also innoculating the area with mycorrhiza can help your fruit trees access that nitrogen in the soil.