Can we harvest energy from plants?
It is not possible to do this directly. Indirectly, it is possible, this is actually done by harvesting fruits - they contain the energy of the sunlight conserved in chemical compounds like sugars or starch and their cellular structures. The basic process for this is photosynthesis.

The products from the fields are used technically to produce gas by fermentation, which then can be burned to produce electricity. Read reference 1 for more details.
What is also done is the use of sugar cane (done widely in Brazil) or corn to produce ethanol which is then used in the fuel of cars. See references 2-4.

Besides these technical processes, there is of course still the possibility to simply burn whole plants or the wood of trees, which is also the result of the fixation of sun energy.

References:


Biogas Production from Maize Grains and Maize Silage
Ethanol Production Using Corn, Switchgrass, and Wood; Biodiesel
Production Using Soybean and Sunflower
Ethanol fuel in Brazil
Corn ethanol