It should be obvious that the major criteria for plant selection are the particular requirements for the method to be employed and the nature of the contaminants involved. For example, in the case of organic phytotransformation this means species of vegetation which are hardy and fast growing, easy to maintain, have a high transpiration pull and transform the pollutants present to nontoxic or less toxic products. In addition, for many such applications, deep rooting plants are particularly valuable.
On some sites, the planting of grass varieties in conjunction with trees, often in between rows of trees to stabilise and protect the soil, may be the best route since they generate a tremendous amount of fine roots near to the sur-face. This particularly suits them to transforming hydrophobic contaminants such as benzene, toluene, ethylbenzene, xylenes (collectively known as BTEX) and polycyclic aromatic hydrocarbons (PAHs). They can also be very helpful in con-trolling wind-blown dust, wash-off and erosion. The selection of appropriate plant species for bioengineering is not, however, limited solely to their direct ability to treat contaminants, since the enhancement of existing conditions forms as much a part of the potential applications of phytotechnology as bioremediation. For instance, legumes can be of great benefit to naturally nitrogen-deficient soils, since they have the ability, via symbiotic root nodule bacteria, to directly fix nitrogen from the atmosphere. With so much to take into consideration in plant selection, the value of a good botanist or agronomist in any interdisciplinary team is clear.