Local stables lets us take all we want. It's free of straw and has wood shavings in it. How long should I leave it before using it to bulk up my clay based soil?
If I put it in a trench and fill up over the top with soil, what can I grow in it? Or what must I not try to grow in it?
A good year. Some of ours is 20 plus years old and we have people fighting over it. Even when its well rotted we will only grow our heavy crops straight in it (they are more forgiving). Otherwise we mix it in with the soil as an improver.
Yes leave it 'til next year and plant some squashes in it, they'll take off like rockets ;D