Asparagus Bed - Green Manure

To be honest I don't know and I don't grow asparagus but heres a couple of things that came to mind.

When do asparagus spears start shooting - you'd want the beans out by then.

You wouldn't be able to dig the beans into the soil, although you may be able to chop and drop mulch the tops or could bung them in the compost bin.

Do legumes fix nitrogen over winter - it may be too cold for the bacteria to do much which means that they'll be taking nitrogen away from the bed rather than fixing it in. Of course they'd be using nitrogen instead of it being washed away by rain and if you chop & drop or compost and mulch you'd get that released back over over the year.

Field beans are sold as an overwintering green manure so i presume they fix Nitrogen in the winter but i have to say i have no idea.

The plan would be to remove them in late March/ Early April before the Asparagus appeared

I don't really know the right answer but (same as Jay) a couple of things that came to my mind were:
You may damage the emerging spears while clearing away the beans. Even if they aren't showing they could be just under the surface.
Asparagus are a relatively early crop and will need all the nutrients/water they can get...I'm not sure something else growing at the same time will be giving more than they take away. Usually you hoe off and dig in manures a few weeks before planting..you can't do this in the time frame you have.
What's wrong with a decent mulch over winter?

The alternative is of course to just mulch the bed but i was curious if anyone thought that the green manure would be beneficial or not

Fingers crossed someone with more info on green manures willbe along soon...what type of soil are you on, how long have the asparagus been in situ for?

Personally! I don't see a plus side to it! But agree with the angle you are coming from, but surley if a long standing crop such as Asparagus can reap you with the reward from a simple mulch of well rotted manure/compost to carry it over the winter months! I'd be very reluctant to grow "Green Manure's" which really have to be cut down n chopped in for them to be effective, and as others have said, I'd be fear of damaging the crowns!

Asparagus is a fairly shallow crop so not sure it would welcome the competition of a green manure. I generously top dress ours with home made compost at the end of the season (to protect the soil over winter) and feed with chicken manure pellets after final crop cutting in mid-summer.

Asparagus had been in since this spring.

I think Field beans fix the nitrogen in nodules on the roots so all i would need to do was chop off the tops and leave the roots in the ground to rot.

I'm not sure that legumes will fix nitrogen per se, I think under certain conditions they may not.

Legumes fix nitrogen by having nodules on their roots which hold certain specific bacteria, it's the bacteria which fix the nitrogen and pass it along to the beans in exchange for sugars from the beans. Production of these sugars is an extra burden on the plants resources.

If there is an abundance in the soil of nitrogen then the plants can just use this nitrogen without having to spend extra resources on manufacturing the sugars to feed the bacteria. If there isn't any competition for the available nitrogen then they may not need to fix their own.

Even with overwintered legumes, their main growth is going to be in the spring which is when the majority of nitrogen will be needed - they may not need to go to the effort of producing the sugars in the autumn/winter to get the extra nitrogen.

Legumes need the right kind of bacteria to be present in order to form the modules and have it grow in their roots - if the bacteria isn't there they won't be able to fix nitrogen.

Bacterial activity slows down with temperature so the bacteria may not fix nitrogen if the temperature drops below a threshold.

In a growing legume plant the majority of the nitrogen will be in the leaves and stems of the plant. The nitrogen is used to make the proteins that make up the plant structure and the chlorophyll needed to create sugars. The roots hold some of the nitrogen in the same way that other plants roots have nitrogen in their cellular structure and tissues. The nodules are the factories and will have some additional nitrogen but I assume that gets transported to the bits of the plants that actually need it asap.

I also wonder if growing legumes in an area does make more nitrogen available to other crops as I can't see them producing loads of sugars to pay the bacteria for the nitrogen then discarding a pile of it. I assume that the nitrogen enters the cycle and becomes available to other plants through die off and decomposition so wouldn't be available until after the legume has died or shed it's leaves.

Thanks for all the advice - it sounds like a standard mulch is the better option

No idea about green manure, but can you get your hands on seaweed for a mulch???...Tis the best thing by far apparently. (told to me by a commercial grower)

Oh...that's just reminded me!
Years ago we planted out lettuce on top of the asparagus mounds and covered them with fleece tunnels.
They were just ready for taking as the spears appeared.

I probably can get hold of seaweed at some stage. I heard banana skins are very good as well - getting hold of enough might be tricky though !