Mulch is a layer of material that covers the soil, it can be applied at different times of year for different reasons.
Inorganic mulch : plastic, rubber. I am not going to cover these here as they are not appropriate for use in an organic situation.
Organic mulch : well rotted compost, partly rotted compost, manures, grass cuttings, leaves and leaf mould, wood chippings, bark chippings, paper and card, wool, grit.
There are several different reasons for mulching. Applied in autumn and winter, mulch helps to regulate soil temperatures and protect roots. Applied in the spring and summer, mulch helps to regulate water content in the soil and prevent it from drying out. Depending on the type of mulch that you use, it slowly rots down and provides nutrients to the plants. It also will improve soil structure and health to the soil.
One added benefit of using mulch is that it attracts insects and invertebrates into your garden, which in turn helps to feed birds, hedgehogs, frogs. It helps to support a chain of wildlife. Organic mulch can also help hibernationg frogs, toads, lizards, grass snakes, hedgehogs. Having piles of leaves and logs in your garden will also help.
The pros and cons of different mulchs:
Well rotted manures and composts are best used in situations where you want nutrients to be readily available, less slugs and snails, less contamination from seeds. Because they are well rotted they can be dug into the soil as well. For example vegetable gardens and herbaceaous borders, where plants need a lot of nutrients quickly in spring and summer.
Grass cuttings, straw and leaves are excellent in the autumn and winter, under shrubs and more tender plants that need to be protected from cold spells. The leaves/cuttings/straw will gradually break down over winter giving a nutrient rich soil ready for the spring when deciduous shrubs and trees start to awake. This type of mulch attracts many insects, so will promote wildlife in your garden. A word of warning, do not use leaves from Rhododendrons, Laurel or coniferous plants. These contain toxins in varying degress, and are designed to suppress anything other than the host plant.
Wood chippings, bark chippings. Longer lasting, usually applied at least 2" deep and are used to suppress weeds, especially in new planting schemes. They break down very slowly providing nutrients over a long period of time. Best used under trees and shrubs. Be careful where you source your wood chippings as some come from wood that has been treated with chemicals. Chippings from conifers will increase the acidity of your soil.
Wool breaks down very slowly, taking one to two years. It provides many nutrients including nitrogen that are released as it breaks down. Helps to suppress weeds and very effective in retaining moisture in the soil. Either source wool as a fleece or felt trimmings, but use felt that has not been dyed. Second rate wool or felt used to be ploughed into ground to add nitrogen and nutrients, called Shoddy. I use unprocessed fleeces, no thicker than the fleece and be careful around tree/shrub stems as they can rot if swamped.
When choosing the type of mulch that you want to use, assess the needs of your plants and garden, what materials you have easy access to. Household waste and weeds can be composted to give a free mulch. I keep horses so have a supply of rotted or partly rotted manure. In the autumn and winter I topdress my beds with Miscanthus based manure (I find wood shavings produce a manure that is too acidic and takes ages to rot down) fresh from the stables. The dung rots down quickly leaving just the Miscanthus in the spring which lasts until the following autumn. I also put cleared leaves around shrubs and hedge areas. In the spring I use wool around my newly planted trees and hedges and fruit trees. For the vegetable garden I use well rotted manure.