Answer: Let me begin by defining the terms mulch and compost. Sometimes they are used interchangeably, but they are really different things. Mulch refers to organic (some non-organic like plastic) materials that are layered on top of the soil after planting. Mulch is great to help retain soil moisture, reduce weeds, and as it breaks down it provides nutrients to the soil. Any organic matter can be used as mulch, such as bark, wood chips, straw, pine needles, and even compost. You can use freshly chipped bark and wood as a surface mulch.
If you want to mix the organic matter into the soil, then it's best to compost, or age, it first. Compost is decomposed, or aged, organic matter that is incorporated into the soil. It adds organic matter to the soil, improves drainage in clay soils, improves water and fertilizer retention in sandy soils, provides nutrients, increases soil microbial activity, and encourages earthworms. It will also act as a great mulch/top dressing for your existing flowers and over time break down further and improve your soil. As it ages, some of the nutrients in compost will leach away, but compost is generally not extremely high in nutrients anyway. It will still provide many benefits for your soil no matter when you use it.
Compost should be aged before mixing into the soil, particularly if it contains manure, so that it won't burn plant roots, and also because the soil microorganisms that are decomposing the organic matter need nitrogen and carbon. They use up the nitrogen faster than the carbon, and can temporarily "rob" nitrogen away from growing plants so they can continue their work of reducing the carbon, which takes longer. Eventually the nitrogen is returned to the system when the microorganisms die off, but it can create a deficit in the short term.
Here's a few basics on compost making: You need four ingredients: carbon (browns), such as leaves, straw, shredded paper; nitrogen (greens), such as grass clippings, kitchen fruit and veggie scraps, manure; water, and oxygen. An easy way to start out is to mix the carbons and nitrogens in about a 50/50 ratio. As you construct the pile, sprinkle it with water from your hose. The ingredients should be as wet as a damp sponge. Don't try to make the pile and then water it all down from the top. The water finds paths to pour through out the bottom! The pile should be at least 3 feet by 3 feet by 3 feet (1 cubic yard) to have enough mass to insulate and retain heat. As the microorganisms that are doing the decomposing die off, they release heat. When the pile cools, they've probably run out of oxygen, which is where turning the piles frequently comes in. The more turning and reapplying of moisture, the more quickly the materials will decompose. On the other hand, you can construct a good pile and then just let it sit. It will decompose, but take 6-8 months. Hint: the smaller the ingredients, the faster they will decompose.
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