I can't tell you how many garden consults I did this year where people complained "My plants just don't seem to be doing very well. I want a more lush look." In particular, I had a lot of clients who had their gardens professionally landscaped 2-4 years ago and are not happy with the performance of their plants. When I ask them what they do to fertilize, they say "nothing." Many seem to think that putting bark mulch on top of their soil is enough. It is not. (For the people who had their gardens done 1-2 years ago, I also question the quality of soil that their landscaping companies supplied, but that is a subject for another post.)
Plants need food too! And by far the best way to feed your plants is to feed your soil. In terms of doing minimal work for optimal gain, I suggest putting a topdressing of 1-2" of compost on top of your garden as frequently as once a year if you can get to it, or at least every other year (let's be realistic here, we're all busy!) It just so happens that compost looks nice too - like beautiful, rich, dark soil.
When should I spread compost?
I find the easiest time to do this is in late late May/ early June, when all the plants are up and growing, but there is still enough space between them to make putting down compost an easy task. But really, it can be done anytime. I just did my front Shade Garden (above) this past week-end, because that's when I got around to it. (This area was newly planted this spring and yes I know it is a little bare still - it is a work in progress but at least it is easy to spread compost here!)
What about mulch?
Compost also acts as a fairly good mulch, in terms of helping to retain moisture in the soil, regulating temperatures and keeping down weeds. But bark mulch is better for that. If you have bark mulch, you can rake it off to the side, put down the compost, and rake the bark mulch back. Or, if you prefer to take the path of minimal work (this is what I would do!), there is nothing wrong with just putting the compost on top of the mulch. The compost will gradually decompose and work its way down through the mulch. Alternatively, whenever you are going to top up your mulch (this needs to be done every few years), put some compost down first then new mulch on top (I don't like this method only because it means you need to do 2 big jobs at the same time rather than spreading out the work - pun intended!)
If you have rock/gravel mulch, well then we have a problem. Compost on top of rock mulch is not going to look so nice. This is why I never recommend rock mulch to people unless they really, really like the look. It is just not as low maintenance as people think. If this is your situation, then perhaps pull back the rocks from around each plant and put some compost down around each plant - being careful not to have it touching the stems or leaves - then put the rocks back. Or get a water soluble, organic fertilizer and fertilize your garden that way. It's not optimal, but it's better than nothing.
I don't make enough to cover my garden. Where do I get compost?
Lots of places deliver by the cubic yard. Or you can pick up in bulk. I like Western Canada Compost in particular (they are not paying me to say this!) I talked to lots of people this year who were very unhappy with Burnco's compost, but that's second-hand information.