Trench Composting for a forest garden

I plan to start introducing organic matter to an area of my garden that is quite clay ridden, but very rocky and if we have consistent rain here in the UK, prone to some flooding. Given the resources I have at my disposal, trench composting is I hope a good solution for now to introduce, organic matter and break uk the soil some. I may also spread a green manure type cover crop, as at the moment its all grasses and weeds.

The intention for this area is a forest garden, so I want to make sure I have good soil / soil depth for planting this sort of a system and (this is just a guess) I might mitigate some of my flooding issues, by changing the soil (or at least the topsoil) a bit further away from being clay), increasing permeability and therefore, opening up the types of trees I can plant here in the next couple of years.

I was hoping to get some broad ideas as to the best approach to trench composting for the above intention and have a few specific questions. Hopefully someone can offer some advice on:

How deep to dig down and bury the matter? Normally I believe 1/2ft but is that enough given I will plant trees in the space.

When backfilling, I've had advice that I should tread down the soil to compact it a little, but should this still apply is a clay soil already seemingly fairly compacted?

The rocks and stones are plentiful. I dug a small hole circa 3 ft long and 2 ft wide. Every shovel down hit a rock - generally fist sized, some smaller. Some people seem to have removed rocks, other left them suggesting that others will only appear in their place later. Any view on whether these should be filtered out?

Some of the organic waste comes from other plants in the garden. I've seen it suggested that weeds, seed heads etc all can be composted in this way - but is it not prone to then being weedy / plants popping up from the seeds?

In particular I have Euphorbias elsewhere in the garden that I absolutely want to get rid of. They are poisonous, take up enormous space and have spread seed all over the garden. These species also don't seem to flower so not alot here for pollinators either. I am not sure if I can add these and all their milky sap into trenches / general compost. If I can trench compost these, fantastic - but its so pervasive that I really don't want to risk it returning just in a new spot. What can I do with problem plants if not composting? Burn?

Many thanks for any advice Keen to get off on the right foot.

In my experience, rocks aren't a problem for trees, they just grow around them.  Shallow trenches should work fine for composting, since the trees will mainly be getting their nutrients from near the surface anyway.  Organic material can just be placed on the surface, also, unless you need to bury it to discourage varmints or flies.

Not sure about the Euphorbias.  I tend to see rampant weeds as a benefit, because they are busy producing organic material.  But if they are going to seed, they might need to be composted away from the tree planting area first.

Trench composting..........great method. I've used it for years with good results. And I'm still using it, plus top dressing, in my own orchards and food forest. As I accumulate organic debris, I'll dig an appropriate sized hole or trench, fill it, then cover it with the dirt I have removed. My trench is 8"-10" deep (if I can dig that deep because of the lava rock). When I back fill with the dirt, I'll just smack it down with the back of the shovel to level things out a bit. I don't tromp it down with my feet. I've been doing it this way for 15 years and it works fine.

Rocks.......unless you intend to use a rototiller or plow, I would leave them. The rocks will break tilling equipment, and will by annoying if you do a lot of shoveling. But they are needed for soil mineralization, drainage, and soil stability, and anchorage....especially as your soil gains more organic content. In my food forests and orchards, I'll remove any rock bigger than a football only because I want to use it in the rock walls that I'm building. And in my vegetable gardens where I use a rototiller, I'll leave any rock bigger than a hen's egg. All other rocks stay put.

Sounds like an excellent project. I reckon a food forest would be a good plan over rocky clay. As has been said rocks are good for drainage, stabililty and minerals.  Trees and shrubs love clay once they get their roots down in our UK climate.

To tread down backfilled areas or not; when planting trees and shrubs, they don't like being wobbly when planted so treading is good. Annuals are less bothered by fluffy soil.

I have an invasive euphorbia in my front garden, I feel your pain. Good news is the tops compost just fine. If they are a running type I would bag up the roots to rot on their own then incorporate later.

As far as burying seedy weeds, lots of them will sit under the surface waiting for light to trigger germination, so if they're always in the dark they won't hurt you. It's weeds with fleshy or running roots that you don't want to bury.

All the best, hope this helps.