Can you clarify - are you trying to measure the canopy cover of the cassava plants? There are many ways to measure canopy cover, usually as a percent cover of the area when under a tall canopy (which, most cheaply and basically, is usually done with either a densiometer (old technique) or nowdays often with a fisheye camera and associated software). But cassava usually grows pretty low and has a very thick, full canopy so it might be better to try and get at LAI (leaf area index), which is the average amount of leaf area in the vertical profile (in m2 of leaf area per m2 of ground). A good review of methods to measure LAi is here:

http://jxb.oxfordjournals.org/content/54/392/2403.full

I would think that there are decent values for cassava LAI, possibly even based on height, and you could probably measure height of the cassava, width of the cassava, and use established cassava LAI to estimate total canopy of the different areas or stages of growth that you are interested in.

Thank you Sir.

In case of small or medium plots, specially when the colors of soil background and plant canopy are clearly different, it will be better to use Image Analysis techniques due to it's high accuracy and speed.

Some references:

http://www.ics.uplb.edu.ph/node/419

https://elibrary.asabe.org/abstract.asp?aid=30726&t=2&redir=&redirType=

www.scielo.br/pdf/rbeaa/v15n1/a14v15n01.pdf‎