The plan is always to get the mulch in place in early spring, but I am usually mulching gardens all season long.
I once wrote about mulch for a feature story that ran in September, and the landscape designer I interviewed questioned why I didn't do the story in early spring. I didn't have an answer other than my spring stories were all about planting, and while most people cover their landscapes with mulch first thing and then sit back the rest of the season, I am never finished with a garden until the snow drives me out. Mulching isn't just a springtime event at my house. But mulching in spring seems to be encouraged because every garden center has stacks of bags on pallets and piles of shredded bark in parking lots.
People do their mulching in early spring primarily to keep down the weeds and in many landscapes, mulching finishes the gardening job for the rest of the season. It is usually those folks who prefer maintenance-free mulch, particularly inorganic products that will last several years. These types of mulches incorporate heavy landscape fabric beneath a pile of stones or, worse, shredded recycled tires. While I don't think shredded rubber is a good product for any landscape (I hate the thought that 100 years from now we could be constructing our houses on top of giant rubber trash dumps instead of rich topsoil), stones don't seem as offensive to me, even though they aren't conducive to a natural-looking landscape.
I like to plant new things in my landscape each season, so I prefer organic mulch even though it has to be reapplied each year. I don't use landscape fabric, unless it's beneath a permanent walkway, and even then weeds still grow between the spaces of the stones or bricks.
I'm not a fan of dyed mulches. I like my mulch to be double- and even triple-ground and dark in color. Mulch should break down throughout the summer and over the winter so that by the beginning of the next growing season, it has turned to compost adding even more organic material to the soil. To keep weeds under control I use newspapers or cardboard underneath the mulch, laying it between the plants where possible. If I want to plant something in those areas anytime during the season, I can push away the mulch and cut a hole in the newspapers or cardboard.
In my vegetable garden, I also put newspapers between the plants and between the rows, but instead of ground bark mulch, I use seed-free straw. Like the bark mulch, it breaks down by the next growing season and has to be reapplied, but it has turned into compost and helps amend the garden soil. Newspaper, as long as it is not the shiny, advertising paper, uses soy-based ink and is not toxic to the garden.
In cases where the mulch hasn't completely broken down over the year, a simple top dressing of fresh mulch is usually enough to keep the area looking fresh. Mulch should be applied 2 to 3 inches thick.
Weeds and building good soil, however, aren't the only reasons to mulch a garden. In all types of gardens, whether you are enhancing the look of your foundation plants or keeping weeds at bay in the perennial or vegetable gardens, mulch helps retain moisture in the soil after watering or rain. Mulch protects the soil from harsh weather and keeps the soil from becoming hard and compact. Compact soil causes water to run off rather than soaking down to the plants' roots where it is needed.
Several years ago I fell for the cocoa hull trend. Bags of cocoa hulls were sold in most garden centers and may even still be sold there, but it has been a long time since I used them. If you are a fan of chocolate, they smell wonderful and are most often used in herb gardens where the fragrance of the plants mingled with the cocoa hulls can be a pleasant experience in the garden.
But my dogs also loved the fragrance of the hulls and I would catch them sniffing through the mulch, tasting a few and sometimes unpleasantly bringing the shells back up on the carpet later. If having curious pets isn't enough reason to avoid cocoa hulls, they also tend to mildew easily where there is a lot of moisture. When there were a few wet days in a row, it was inevitable I would find patches of white mold on the surface of the gardens.