Seed choice for a permanent living mulch setup

I've had some mandatory work done on a field (about 40x80') and now that part of land is covered with some kind of sand with lot of rocks in it (quite big). There's also the septic tank part, I think it is called the purification field? It is about 20x30' where I've been told I should not grow anything with deep root and grass should be cut.

I was thinking about turning most of that field into a permanent living mulch production. I don't know if I really need to care about what kind of plant I grow in the septic tank part, I suppose as long it isn't a tree, bush or other kind of larger root plant, I should be fine.

I've looked through the Master Garden class and I've found the following plants for living mulch:

- Clover
- Alfalfa
- Grass
- Oat
- Peas

I was thinking about using the following:

- Red Clover
- Alfalfa
- Grass
- Maybe some peas
- Confrey (where I can grow deep root)

For now, I would mow using a scythe.

I was thinking about seeding when it would start freezing so it would start growing at the right time in spring. Maybe I should put some hay on top?

Appropriate timing. I am in the prosses of final mowing of my mulch production field.  When I have tall grass I use my scythe but right now it is dry standing grass which I am cutting with the riding mower that blows it into a wagon.  That gives me my winter mulch for raised beds.  
Comfrey is good along an edge.  I really like red clover but it needs to set seed to be perpetual. Alfalfa is persistent from the roots.  Vetch makes a large mass for me in late spring though it can get tangled when mowing with the scythe.
My zone is 7 so the vetch and grass is starting to grow now and will grow all winter.   I do not mow in summer because it will be hot and dry so the soil is unprotected unless the grass is allowed to stand and go to seed.  Comfrey roots will reach down to the gravel of the leach field but they are not the sort that will invade the pipes but should give you living mulch harvest then in dry periods.
So I basically have a green spring harvest and a fall dry harvest.  You will have to observe what works best for your climate cycle.  
P S As you rake the seeds in rake the rocks out you don't wand to be hitting them with the scythe.

Great advices!

As for the rocks, I tried to remove some before, but it doesn't seem to do anything since there's other big rocks under... I suppose it would help to let some of the living mulch there to make more soil above at the beginning.

Hans, does alfalfa have deep roots?

I like your idea for growing plants to chop and drop to use as mulch from your drain field.

Usually, drain fields don't have pipes where roots might clog.  At least mine didn't.

Winter rye would be a good plant.

Wildflowers might be good, too.

It is quite unlikely that the drain field would clog (at least mine). The wholes are under the pipes and there is geotextil above, then rocks.Although, thinking about it, there might not even be soil there, only rocks... have to take a look. Maybe I should have asked for soil above it?

From the sounds of it, you might consider sourcing free-fill dirt (without rocks) and dumping several loads to build up a couple of inches of topsoil. Then I might plant alfalfa with Sunflower to grow up out of it. There is nutrient value to feed the wildlife, and it beautifies any property. The fall cutting can be used as compost to feed poultry or let it lay where it is for the winter.

Lotus might be a good choice if your summers aren't dry. It's a perennial legume but without the deep rooting habit of alfalfa.

I was also wondering, I believe the Master Garden course is advising for soil testing (a complete one)? Is it a necessity when using a living mulch technique (for the garden)? Would you recommend it?

Phil Stevens wrote:Lotus might be a good choice if your summers aren't dry. It's a perennial legume but without the deep rooting habit of alfalfa.

Isn't lotus a water plant?

Alfalfa does have deep roots so it can take advantage of the water in the drain field.  My understanding is that different than tree roots they are not apt to form a mass in the drain tile and cause a blockage.  As mentioned modern drain fields have a fabric barrier above the gravel so the roots should go to the subsoil beside the gravel and absorb the water.

Hi Jordan. The lotus I'm referring to is the legume Lotus corniculatus, commonly known as birdsfoot trefoil. Great to have in pasture and orchards, not so nice in garden beds.

I'm gonna go for lotus, seems like the perfect choice on drain field. I suppose lotus seeds isn't something I will find in all feed store?

I'm currently looking for winter rye. It seems like it is called fall rye in french. How can I know it is the right one? If it is for green fertilizer, does it mean it is the right one?

Also, I suppose I can start seed in fall as soon as the temperature is cold? Do I need to wait for a freeze?

The lotus will most likely be call the birds foot trefoil as mentioned above. Then decide if you want rye (cereal grain) or ryegrass.

Ryegrass or rye will both be at almost any kind of farm store. You might have to order the other unless you have a seed store close by.