Cultivated (Button) Mushrooms. Many uses for this versatile fungi. First make the compost with 5kg of Horse manure compost mixed with 20% wet straw (takes about 20 days, or buy the mushroom compost from your garden centre). When you have finished harvesting the mushrooms at the end, you can mix the spent compost with 3 parts soil to make a superb compost for your flowers. Approx 25-40 days after sowing before the spawn is ready to start producing mushrooms.