It is generally accepted that phytoremediation takes advantage of the fact that plants have extensive rooting systems which explore large volumes of soil, support larger bacterial populations in the rhizosphere and produce exudates which can directly affect the activity of the rhizobacterial populations (Farrell et al., 1999). Selecting a plant, or community of plants, for a contaminated site can be difficult; and often requires lab or field trials. Research has indicated that plants used in phytoremediation must have extensive root systems in order to contact the maximum amount of contaminants.

A plant must be able to tolerate the contaminated soil in which it will be growing. This must be true at all stages of the life cycle. If the seeds will not germinate under the given conditions establishment becomes more difficult, and more expensive. Germination studies have been conducted at various pollutant concentration levels with many different species. For more information about these studies see pp. 26-27, Frick et al.,1999.

Transplanting is more labor intensive then seeding but can be used in certain instances to establish mature plants on a site. Lin and Mendelssohn (1998) were able to transplant mature Spartina patens into marsh soil with at crude oil content as high as 100mg crude oil per gram of soil (Frick et al., 1999). Research has shown primarily grasses and legume family plants to be most tolerant of organic pollutants.

The ability of a plant to tolerate an organic pollutant and to phytoremediate an organic pollutant is not the same thing. A chosen plant species must be able to support a microbial population which will degrade the contaminant, directly degrade the contaminant, or a combination of both. Plant and microbial interactions are very complex and not highly understood; more research is need in this area. The rhizosphere effect (See How does it work?) seems to be a necessary element in phytoremediation of TPH. Often the best course of action is to do an extensive inventory of plants and microbes currently present at the site, and to use this information in choosing plants, or microbes, for a phytoremediation project.  There are extensive lists of plants which have been found to directly remediate organics via phytodegradation or volatilization.

There are conflicting reports on the effectiveness of inoculating contaminated sites with microorganisms (Frick et al., 1999). Cunningham et al. (1996) state that it is common for soil and plant inoculates to be out competed by native microflora. Yet another reason why a site inventory is essential.

Often there situations where plants and microorganisms that are appropriate for phytoremediation are not native to the contaminated site. The introduction of non-native plants and microbes to an area can have serious consequences. In fact, research shows that 4 to 19% of introduced organisms in the United States have had an adverse effect on that area's environment, economy, or both (Frick et al., 1999, OTA, 1993). Genetically modified organisms (GMOs), while not used extensively for phytoremediation of petroleum hydrocarbons, are treated as non-native species. Research is currently underway to investigate the effect GMOs may have on ecosystems. This is a difficult undertaking since microbial populations evolve relatively quickly to new inputs. Therefore, native plants and microbes are preferred for phytoremediation whenever possible.

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