To start with, what do mean by mulching? Well, mulching is the addition of a top layer of material to the beds, borders, and pots in our gardens.
Why would we do it? For many reasons, which often overlap. They can include the following:
to smother weeds. A thick layer of organic matter can suffocate a smattering of casual weeds, although it won't help with perennial weeds such as dock, bindweed, couch grass etc.
to prevent new weeds appearing. A layer of inorganic material such as pebbles, grit, gravel, or slate will cover up the soil, so any weed seeds wafting in on the breeze won't be able to germinate.
for decorative purposes: some people don't like seeing bare earth, particularly in their glazed pots, so a layer of coloured slate, or bright clean pebbles, can make things look more contemporary and “clean”. In addition, some plants really benefit from a light-coloured mulch: I'm thinking of my favourite dark grass, Ophiopogon planiscapus 'Nigrescens' which can be completely lost in a border, flanked by dark earth: but which really comes into its own when growing through shingle.
to retain water. Water evaporates from the surface of the soil, so a thick layer of mulch will keep it in. This is particularly valuable if your soil is thin, sandy, or poor: just make sure you apply the mulch after it's rained - or after you have watered well - otherwise the mulch will act as a roof, keeping the water out.
to enhance the soil structure. Leaf mold or compost, spread on top of the soil, will be taken down into the topsoil by the action of worms: we don't even need to dig it in! Whether the soil is thick, heavy clay, or light starving sandy soil, the addition of organic matter will improve it. And even bark chippings will, in time, break down and become incorporated in the soil.
to add nutrients. Home made compost is rich and can really help a poor soil to improve: the natural action of rain on the top, and worms on the underside, will transport all that goodness down where the roots of our plants can get it.
So when should we apply our mulches? The usual answer is “in spring, and in autumn” but frankly, I shovel the stuff on whenever I have time, whenever the soil seems to need it, and whenever there is plenty of it on the compost pile!
Having said that, it is easiest to apply it at the end of the year, when you have cut back all the tatty old herbaceous material and can see where each plant is. Without all that vertical material, you can fling it from front to back of a border with ease. This has the advantage of also offering frost protection to the crowns of slightly delicate plants, such as dahlias and peonies, who seem to enjoy a bit of cover.
Then again, in early spring, when the plants are starting to grow: this is a good time to get nutrient-rich home made compost out there, so that the nutrients don't get washed out over the winter.
Oh, and I should also say, that another good time to apply a compost or leaf mold mulch is when you are expecting visitors to the garden: there's nothing like a layer of dark, clean mulch to smarten up the beds!