There are several different standard methods for this including various mixtures of concentrated acids but the question is what are your heavy metals of interest and if maybe dry ashing or the wet digestion method would be more suitable for your case.

There is a few method for extraction of heavy metals from biomass (not digestion). You coud see following article:

http://www.ncbi.nlm.nih.gov/pubmed/15750871

The best method for extraction of heavy metals namely Zn, Cu, Fe, Mn, Ni, Cd, Pb etc from plant sample is wet digestion with di acid mixture (HNO3: HClO4 ::3:1). Before digestion of plant samples at hot plate (preferably 80oC temp) pre-digestion of samples with HNO3 is recommended for easy and complete digestion of the materials. 

I am attaching an article on metal extraction. In the introductory and materials and method part there is a wide discussion on the better methods of extracting metals from plant. Though none can be said to be  the best method, still there is a comparative discussion on metal extraction methods in this article.

Article Identification of metal tolerant plant species in mangrove e...

Please also have a look at the following ResearchGate link.

https://www.researchgate.net/post/Which_method_should_I_us_for_extracting_heavy_metals_from_plants

Dry-ashing and wet oxidation are the two widely adopted methods for extraction of heavy metals. For details go through this important descussion. https://www.researchgate.net/post/Which_method_should_I_us_for_extracting_heavy_metals_from_plants

An important question is that what will be the fate of plants after being used for phytoextraction of heavy metals? Such plants after burning, can be either disposed as hazardous waste safely in specialized dumps or if economically feasible, processed for bio recovery of precious and semiprecious metals (a practice known as phytomining).