Hi
Why on earth would you want to spray your compost heap? The whole point of a compost heap is that when fully composted, you are returning nutrients to your garden.
Whatever about the residue's from fruit/veg that you buy in a shop, at least they are occasionally tested. If you spray your compost heap, how do you know how much residue is leaking backing into your own home grown veg?
I just don't fully believe the argument that weed killers are biodegradable or neutralised in the soil. By how much? How long does it take?
Round Up and all the other Monsanto rubbish should be confined to the bin.
Apologies for causing offence, but I just dont see the point in spraying, especially needlessly.
But to specifically answer the previous question, I bring all unwanted weeds: counch grass, dandeline, etc. that could possibly germinate to my nearest green waste facility. If not available to you, I would suggest composting for non-veg purposes, i.e. for areas that you dont mind as much if the grass/weeds return.