A couple of questions:
1. Should we avoid adding grass cuttings w/seed to the Interbay Mulch mix?
2. Can I mix/layer the native soil into the IB mix?

Interbay Mulch

As long as you have a good mixture of varied ingredients, without too many browns or too many greens predominating, Interbay Mulch should work for you.

It's also important to make sure the ingredients stay moist. (Like the ingredients in a compost heap, somewhere close to the dampness of a well wrung out sponge.) And like a compost heap, it helps if the ingredients don't pack down so much that there is no air flow. You want air to continually circulate so the decomposition stays aerobic.

You ask about mixing native soil into the layers of the pile as you're building it. If it was me, I'd drop the extra soil at the bottom of the pile, and keep any mixing to a handful of soil per square yard, per 1-2" layer deep.

This will help the ingredients to be primed with the existing local soil microherd, but it's really not necessary -- the local microherd is already omnipresent on all the living and dead things in our gardens, fields and forests. :)

However, some gardeners advocate for adding clay soil to compost heaps as a growth site for nitrogen-fixing soil bacteria. Personally, I've done both and haven't noticed much difference. YMMV.

Any grass, including grass with ripe seeds, which you add to an Interbay Mulch pile, is likely to get consumed pretty darn quick by the microherd. (Although if you have persistant stubborn perennial weed grasses like crabgrass, you may want to keep them out, or keep them to the center of the heaps. Others here could likely advise you better on that issue, if it may be a problem for you.)

Some years ago I built an IM heap in the fall. It was originally stacked up about 4-5 feet tall on an 8' x 4' raised clay bed, but it slumped a foot or two within the month. Then I built it back up to the original height, covered it, and left it alone until the following spring.

When the snow melted and warm weather arrived, I removed the cover and started raking thru the largely decomposed ingredients. Aside from fresh wood branches over 1/4" thick, nearly everything had broken down into a rich brown-black compost.

The breakdown was far more rapid than I usually see here with standard passive compost heaps -- and far more uniform throughout the pile. Even the larger wood branches which remained whole had been stripped of their bark, and the bark reduced to fragments the size of small wood chips or less.

There was about 8-10" of compost left once all the branches were removed. I raked it smooth, covered it with a leaf mulch then a wood chip mulch, and transplanted thru it all into the soil surface. The tomato, pepper and basil plants in that area of the garden produced bountifully the following summer, with no weed, pest, or disease problems.

Just make sure you follow the standard composting guidelines. Varied ingredients with a good balance of browns and greens; damp as a wrung out sponge; pile loosely so there's room for airflow.

Summer rainfall here where I live is enough to guarantee any IM or other compost heap usually stays moist. Depending on your local weather, you may want to check the heap once every few weeks to make sure it hasn't dried out.

I found IM a surprisingly fast, efficient and low-effort way to produce large volumes of compost. But if I was doing it again, I'd reduce the number of green branches over 1/4" thick. Though older and thinner deadwood broke down quite well, for me.

Good luck!

All the best,
-Patrick

p.s. When I built my IM heap, I had no burlap handy. So I covered the pile with a single layer of the heavy brown paper from torn-open yard waste bags. Weighted down with light boards and small stones, they kept the heap dark and moist enough to keep working through the winter.

You know your local weather conditions best. If not enough rain falls to keep a standard compost heap damp, then an IM heap may not be practical for you, for the reasons you described.

However, it may help to keep in mind that the covering material helps hold water in the heap. Rowley's original method used burlap, but heavy paper from yard waste bags worked quite well for me.

Perhaps you could try a test run this year and see how it works, for you.

All the best,
-Patrick

Rather than thinking in terms of putting water ON the area think about putting it IN the heap as you build. It can be nearly impossible to moisten through the pile once it is built. But if you spray with a hose each time you add a layer of a few inches, that OM will hold moisture for a long time. If you add some compost or some partially composted stuff, that seems to work well.

Karen