HUBAM white clover as winter killed green manure

Here's my situation.

I am looking for a fall cover crop primarily to add organic matter and N to soil and secondarily to prevent fall weed growth. I intend to no-till in the spring, or at most hand cultivate if needed. This is a raised bed vegetable garden in zone 4. Due to short growing season, most vegetable crops are planted as early as possible, too early to allow a cover crop to grow in the spring. I also don't want to have to use herbicide (obviously) or till to kill the cover crop.

What I've got in mind is using an annual white sweetclover such as HUBAM or similar, seeded late this summer or this fall, as areas of the beds become available as crops are harvested. I really want something that will winterkill, so I don't have to till or spray in the spring. I realize that I lose a significant amount of N and organic matter by letting it winter kill, but that's a tradeoff I'm willing to make at this point so that I don't have to allow it to start to flower in spring so that I can adequately kill it. Rolling also isn't an option unless whatever I plant can be adequately killed by May 1 at the latest.

So here are my questions for anyone that has experience with this.

Am I completely off base here?
Is an annual white clover going to fix and add enough nitrogen in say 30-40 days to provide any benefit?
Is it possible that I'll actually reduce available nitrogen if I don't let it mature enough?
Will an annual white clover winterkill in zone 4 so that I don't have to deal with it regrowing in spring/summer and becoming competition for vegetables?
If it does try to re-grow, will grass mulch suppress it?

If all else fails, I will till in spring, so if what I'm trying to do is better suited to a different cover crop that would need to be tilled to kill it, let's not completely eliminate that as a possibility.

Thanks.

What about oats or barley or an oat/barley mix?

David Hartley wrote:What about oats or barley or an oat/barley mix?

That's an option too, but I was hoping for something that would provide at least minimal nitrogen fixation. I know my soil has only adequate nitrogen as it is, certainly nothing that supports stellar growth. It's getting better every year by adding compost and chicken manure, but it's a pretty large garden and I never have enough compost and manure to amend the soil the way I'd like to. I'm also not going to add any fertilizers at all, organic or otherwise, because I have an aversion to any external inputs. Nitrogen fixation solves a lot of problems for me if I can pull it off.

Hmmm... What about fava/broad beans? They winter-kill at right around 20degF, iirc.

David Hartley wrote:Hmmm... What about fava/broad beans? They winter-kill at right around 20degF, iirc.

I hadn't considered any types of beans, because my understanding was that their nitrogen fixing was inferior to clovers, but I've been reading into it since you suggested it and it looks like it might be a good option. What I'm finding suggests that most legumes nave nodules present at 2-3 weeks post-emergence, and should therefore spend some time fixing nitrogen if they're allowed at least 60 days before killing frost or they go dormant. What I can't seem to find is how much they have fixed by that point, and if it provides any actual benefit.