To get coverage, multiply the area covered per plant by the density of the plant. Plot points must be randomly determined. The plot is sampled exhaustively to get percent cover, relative cover, or relative frequency. Multiple plots are used. This is a very tedious method, so small frames are advised.
Percent cover A densitometer is used to measure percent cover.
Commercial ones are expensive, but you can make your own using a toilet paper tube. Have one person hold the tube level. Two people are used because the tube must be level and it is difficult for one person to do this. The distance from eye to tube must be consistent as well. The data are binomial.