Before I wind up too much, I will throw out a disclaimer. I am not encouraging making silage out of your weeds!
However, you can use some of the silage-making techniques to kill the weeds for eventual composting.
Farming on a croft, you probably know this, but for others that might not be familiar, silage is essentially vegetable matter that has been fermented to preserve it as animal feed. In older times, it was often made in a deep trench and covered with soil. For many years now, it has been made wrapped in plastic. You sometimes see it when moist round hay bales are stored in a pasture or hay field, looking like giant marshmallows!
Silage can also be made from other materials, such as green corn stalks, sometimes mixed with grain. To ferment properly, and preserve rather than spoil silage is maintained in an anaerobic state during processing (keep air out as much as possible). Silage-making also produces heat, so the anaerobic condition and heat should kill seeds and any roots or shoots.
Another way to do it is to lay vegetative material on large heavy-gauge plastic sheet (you need 2 sheets, a top and bottom sheet). Put the moist vegetation mostly in the middle of the bottom sheet, making a pile. It is better if the material is first chopped before it is placed on the tarp. (lawn mower with a bagger would work). When you have piled up everything you can, lay the second sheet of plastic on top of the pile. Fold up the edges of the plastic sheets with the goal of keeping air out. Use stones (or something like old tires) to weight down the plastic sheets.
The pile will heat up, but at some point, will also cool down. Although it might be safe to compost when it has cooled, I would leave it wrapped up for a while longer to be sure all weeds, roots, and shoots are dead.
I am uncertain about adding more chopped vegetation at a later date if you want to add more to the pile. It would need to heat up and cool down again.
Since it was anaerobic for a while, expect an odor when you open it up. Not roses!
