Someone once said, "A weed is simply a plant growing where you don't want it to be." From the dandelions we used to collect by handfuls, as kids to the nasty quack grass invading our gardens, with its runway system of roots--weeds are everywhere. And they have a knack for emerging quickly, once the soil warms!
A few tips, as you find yourself walking in the garden. Watch out for tree seedlings. If you have large trees, or a wooded area nearby, chances are pretty good you will see them popping up. I make it a habit to walk through my landscape, often, and pull them out. One way to know its a tree seedling is if you leave them too long, the stem will become woody with a few green leaves, resembling their parent tree, on top.
Preen is a great product to help keep weed seeds from sprouting. You can find it at your local garden center or hardware store. Apply in April/May, and again in July, if you desire. This goes right on top of your wood mulch and creates a barrier to the seeds from germinating. I like to wait a little longer to apply so I can see if any annuals, from last season, might be reseeding. A note about mulch: to help keep weeds at bay, its best to have a good 3 inches on top of your soil and around your plants. Preen also makes an organic product, out of corn gluten, for those who want a chemical-free option. You apply the same, but more often, as it breaks down more quickly.
It can be hard to tell a weed from something you may want to keep. Getting to know your plants, over time, will help. In the picture above, you can see what a maple tree seedling looks like. In my garden, I have Gallardias(Blanket Flower) which reseed every year.
Sometimes its easy to mistake them for dandelions, as they look similar. Blanket Flowers are fuzzy, and have a blue-grey green, as opposed to the smooth, bright green of this common weed.
Weeding can be a chore, but if you do it often, they are easier to pull(especially after a rain or watering) and once those plants fill in, you will have less work, come summer!