Article submitted by K. Ruby Blume, The  Institute of Urban Homesteading, http:www.iuhoakland.com

California coastal winters, rain followed by periods of crisp sunshine, result in massive amounts of weeds in our gardens.  I spend days upon days weeding out what is unwanted and it seems there is always more.  Plants are opportunists of the best kind and their seeds will slip into small bits of dirt and widen cracks in sidewalks.  They find their way to distressed soil, where the more pampered horticultural plants would never think of settling.  They are tough survivors with strong constituents and tenacious roots.   They know how to do a lot with few resources.  The type and amount of weeds you find in your yard tell a story about your soil.   Making your soil better doesn’t necessarily cut down on the weeds–probably the opposite, but as your soil health improves the type of weeds that proliferate will also shift.  Plants that thrive in highly compacted, low fertility soils are different than what will grow in your well amended, extra-fertile garden beds.  Of course the dreaded Oxalis pes-carpe will grow anywhere.  

As an example:  In my hard-to-amend front strip, which is sticky, soggy clay in winter and hard-pan in the summer, plantain proliferates as it does nowhere else in my yard.  In my nicely amended vegetable beds I get fields of cress, purslane and scarlet pimpernel.  Whether your soil is clay or sand, boggy or dry is probably pretty obvious to you, but weeds can also indicate soil pH and nutrient levels.  
For a simple list of weeds as soil indicators see here: http://www.co.thurston.wa.us/tcweeds/weeds/fact-sheets/weeds%20as%20soil%20ind.pdf

For a more comprehensive chart which include indications of nutrient presence or dearth, look here: http://oregonbd.org/Class/weeds.htm.   
For help identifying weeds, let one grow out a little and take it to a garden center or experienced gardener.  Don’t try to describe it (“you know it is green, with leaves”), bring the actual plant or a clear close up photo with you. If it is flowering, more the better.  Here is a nice little pdf with some good drawings of the most common Bay Area Weeds–I see all of these from time to time in my garden: http://www.greenbelt.org/downloads/resources/curriculum/chapter2.pdf

And by the way, when you weed try to pull the whole plant, including the root, as many weedy plants have tap roots that will simply spout new growth if left in the ground. 

Of course, not all weeds are bad and some traditional folks even see weeds as important companion plants, “guardians of the soil;” not just opportunists but filling important niches and offering soil reparation.    Check out chapter six, “Weeds as Mother Crops” of this longer article championing weeds: http://journeytoforever.org/farm_library/weeds/WeedsToC.html.  Besides assisting in re-greening distressed areas, where no other plants would, many plants we call weeds are useful, edible, medicinal or both. Here are a few you might want to consider keeping from time to time:

Edible:  Cress, Purslane, Dandelion, Plantain, Chickweed, Mallow.  Young leaves  can be great additions to spring salads.

Medicinal:  Mallow, Dandelion, Plaintain, Dock, Chickweed, Yarrow, mugwort, Cleavers (uses too lengthy to go into here)

The Institute of Ubrban Homesteading offers classes in gardening, urban animal husbandry, food preservation, brewcraft, herbal medicines, and more. Go to www.iuhoakland.com for information and class schedules.