The Missouri Botanical Garden web site http://www.mobot.org/gardeninghelp/ is an excellent source of information for gardeners who need information on garden, tree, or ornamentals culture/pest control. There is also a telephone help line manned by volunteers and staff if you can't get thru to them via the web.
Serious gardeners may want to consider membership in the Fleur de Les Garden Society -- the St. Louis Missouri branch of the National Garden Clubs. The Fleur de Les Garden Society will put you in contact with people like yourself who are familiar with the challenges of gardening in the unforgiving St. Louis Missouri climate.
For grass, trees, shrubs, fruit, vegetables, and ornamentals the Missouri University Extension Service has an extensive list of publications available on their web site : http://muextension.missouri.edu/xplor/agguides/hort/
Just about every other state land grant university in the US has a web site similar to the two above but geared more to their region. Over the years I have found the most useful to be: (in no particular order) Purdue, Cornell, Michigan State, Univ. California at Davis, Ohio State, Penn State, University of Maine, Iowa State, Nebraska, Kansas State, Clemson, Univ. Maryland, Rutgers (NJ), Univ. Mass, Virginia Tech, and the Univ. Wisconsin. Taken together, there is an immense wealth of information available for those who have an interest.
Don't forget that the world wide web is also an excellent source of information (depending upon your searching skills, of course). I never cease to be amazed at what pops up when I do a Goggle search!