The "Wildlife Resources Management Manual", prepared by Chester Martin and others for the Environmental Impact Research Program, contains several chapters that provide guidelines on surveying and sampling plants:
6.2.1 Point Sampling
6.2.2 Distance Sampling Techniques 
6.2.3
Prism Sampling 
6.2.4 Fixed Area Plot Sampling for Forest Inventory 
6.2.5 Line Intercept 
6.2.6 Visual Obstruction 
A document titled "Guidelines for Mapping Vegetation on Army Installations" applies to civilian plants as well: http://el.erdc.usace.army.mil/elpubs/pdf/tr00-8/use.pdf. See especially Section 3G, Remote Sensed Data and Section 3I, "Field Methods."
The National Wetland Inventory (NWI) maintains a helpful page with multiple products: http://enterprise.nwi.fws.gov/index.html. NWI maps and data layers can be ordered on-line, several publications can be downloaded, and NWI Coordinators are listed. Additional pertinent Web links are given.
Additional sources of information are found in the "Literature Cited" section.