Preventing lawn grass from invading hugels

I converted a large chunk of our yard to hugelkultur gardens, three large mounds running across the gentle slope, about 6 months ago. The crops in it this year had modest results, in spite of me not getting the mounds built until after most of the rain had come and gone many things still thrived without irrigation. I undersowed everything with dutch white clover and then have recently added a cover crop of red clover, mixed California native clovers, and winter rye, as well as letting mustard, kale, calendula, and radish drop seed. In spite of all this the lawn grass (mostly fescues I believe) is encroaching heavily and I don't know what to do about it. Will my covers eventually marginalize it enough to make it negligible? Should I just bite the bullet and sheet mulch the mounds and resow my covers? I am in zone 5 I believe, very temperate, rarely freezes, lots of rain for the next 6 or 7 months.

It depends on the conditions of your soil, but yes, mulching heavily is an option, and not one that should be looked upon with scorn. You will have to reseed everything you want later, but in the meantime, you will amplify your hugels' natural tendency towards acting like a worm and soil life generator. If you haven't already, you could cut a path around your hugels, tear up the grass, invert it atop your hugels, layer with partially finished compost, dig out the paths and replace with ramial wood chips, and mulch the tops of your hugels with the same.

Also, how bad is it to have grass in the mix? You can easily incorporate lawn grass into a meadow/pasture mix and use it as living mulch, or incorporate it into a chop and drop green manure regimen.

Finally, you could find something likely to outcompete the grass, anything from squash to physically shade the ground with its large leaves, to buckwheat, which grows quickly and hosts nitrogen-fixing bacteria.

-CK

Is the grass coming in from the sides or up from the lawn you covered? If it is coming from the sides I would recommend pulling it from your hugel beds and then sheet mulching a buffer around each of the hugel beds. But if it is coming up from the hurried stuff and it is too much to just pull by hand I would sheet mulch the beds. It will save you work in the long run.

If you have good fertility, clover should trump most grass.  Just don't let the existing grass get tall enough to go to seed.  Clover can be cut a few time a year without losing ground, but there are certain wild plants that will out compete it.  Mowing or scything keeps them and the grass down.  Also, seed the clover a little heavier than the recommended rate.  If you see the clover losing ground to the fescue, do as Daron suggests and spot mulch.

I used a deer mix on my hugels (mostly clover)  and  I used way more than I needed.   One of the beds was like a green carpet.  

To prep the beds for the fall I covered with wood chips.  

I started to plant perennials and flowers and they started spreading and taking up the area.  What is it Sobkowiak says, "if you don't plant something nature will."   I slowly plant in patches and what I want takes over that area on the hugel.    I know the clover enriches the soil but it also holds the spot for when I plant what I want.  

This is the first year for these hugels.  

I have more issue with the area around the hugels than the hugels themselves.