Answer: Grass clippings can be used fresh in light layers. Thicker layers may mat down and impede moisture from reaching the soil surface, a thick layer may also be "hot" enough to burn nearby plants. Often, gardeners will mix clippings with another non-matting mulch material such as chopped leaves and use that.
Using the clippings will not cause grass or weeds to grow in a given area unless it the clippings are full of grass seed (or possibly weed seeds). The mulch serves to impede seed germination by blocking out light from reaching the soil, so mulching with any material cuts down on weeds and grass in the garden.
Often, gardeners find that the grass clippings are a welcome addition to the compost pile and use them for that purpose rather than using them immediately as mulch. (The fresh clippings add needed nitrogen to the pile and help more carbon-rich materials break down.)
Mulching is used all year and for a number of purposes. A mulch layer in summer will help cut down on weeds, help the soil stay cool and retain moisture, and as it breaks down, will "feed" the soil. In winter, the mulch can help insulate the soil from the freezing and thawing action or frost heaving; it can also help insulate the plants' roots. Snow is also an excellent insulation layer.
All natural mulches break down over time, so they need to be renewed. There is no need to dig it into the soil in the fall. Usually, by fall it is time to top up or add to the summer mulch in order to provide that winter protection. By spring, that layer will be deteriorated and in early summer you may need to add more again. You may mix and match mulching materials depending on what you have available. Larger mulches such as bark nuggets will last longer than finer mulches such as the double shredded bark or a layer of straw.
Mulching over the crowns of perennials or up against the bark of shrubs or trees is not recommended as it will cause moisture to collect there and may promote rot or fungal problems. It can also encourage rodents and insect pests to take shelter and/or feed there.
Perennials that are hardy in your zone do not usually need protection over the top. It is enough to mulch the soil surrounding them. However, to protect marginally hardy perennials, some gardeners will layer evergreen boughs or possibly oak leaves (these do not pack down but instead stay fluffy) to help block the wind and add a modicum of insulation. Since these are "airy" they will not trap moisture in the crowns. These extra layers need to be removed from the crowns in early spring so they do not delay the plant's coming out of dormancy.
I hope this helps clarify the mysteries of mulching. As with so many aspects of gardening, the rules are not as strict as we might like, but instead involve a lot of "learn as you go". With a little trial and error you will have it down pat.
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