The search for a crop that can be grown as an energy fuel begins by finding fast-growing plants that develop a significant xylem ring in their stem. Growth capacity can be estimated from several measurable factors. First is the size or weight of the individual dried-down stalk. Second is the field density at which it can be planted (stalks/ft2. Third is the density of the material itself (kg/m3). As each plant species is introduced in the following sections, the physical characteristics of the plant are given to make the growth capacity estimation.

Although one important criterion for an energy crop will be the bulk of burnable mass that can be harvested, per acre, there will be other factors as well. For instance, it may be important for a plant to "dry down" quickly, so it can be taken immediately from field to burner. It may be important that the burnable portion of the plant be easily cut and handled with con­ventional harvest equipment. It may be important that the plant cause no ill effects to the local population where it is grown and to the workers who harvest it. The type of seed and their dormancy period may be an impor­tant consideration. Does it keep for periods of time without rotting? There are some political implications for some of the candidate plants, for they may have been categorized as "invasive plants" or more seriously placed on a list of "noxious plants." Their introduction into certain geographic regions may be forbidden. Therefore as each plant species is introduced next, general information is provided that may be significant in evaluating its potential (7).

New England Aster (Asteraceae novae-angliae) (Fig. 2)

New England Aster is a wildflower that has such a beautiful and hardy purple-violet flower that it has been domesticated and offered to gardeners as an autumn flower. Although it is a perennial, a multitude of seeds are dried from the flower and scattered each year. The stem has extended branches primary at its top and grows to a height of 2 m. Mostly it grows in a single stalk, but certain types grow as bushes and others grow in several stalk clusters.

Seeds are available commercially and there is sufficient experience with the cultivation of New England Aster to suggest the best planting time, germination rate, and compatible herbicides. Suited for full sun to partial shade, it is a vigorous plant when grown in wet to mildly wet soil, either fertile clay or loam and preferably slightly acid. Flowers bloom from early September through early November. The stalk dies in the middle of October and is reasonably dried by the first of December.

Stem construction consists of a waxy epidermis with xylem cylinder and spongy pith with a "pinhole" tube at the center. The xylem/pith radius ratio (XPR) is about 0.46 and typical stem diameter is 0.95 cm. Typical stalk weight is 26 g, height is 130 cm, and cultivation density is 72/m2. The density of the aster woody material (xylem) was determined to be 474 kg/ m3, which is similar to that of a light hardwood (lighter than pine).

New England Aster is probably the latest maturing of the SSPs. This could be an important factor in extending the harvest season.

Kinghead Ambrosia (Asteracea Ambrosia trifida) (Fig 3)

Kinghead Ambrosia is a wildflower, but the flower color is green, and therefore it is not recognizable in the field as such. Definitely considered a "weed" by farmers and woodsmen alike, it lines ditch banks and invades cornfields with equal vigor. It is a floodplain species but does well also in moderately wet fields. The plant height is amazingly impressive, easily exceeding 3 m and sometimes reaching 5 m.

Kinghead Ambrosia is an annual but drops sufficient seed (up to 275/ plant) to ensure its steady existence. Pollination occurs in mid-July. Pollen is very small and easily windborne, making it an allergen for humans, although not as serious as its related Common Ambrosia. Seeds are 0.3 to

0. 5 mm long, with a woody hull bearing blunt ridges that end in several short, thick spines at the tip. They germinate after a cold, moist dormant period. The stem has extended branches primarily at its top and grows thick enough to make a canopy to maximize its absorption of solar radia­tion. The stalk dies in late September or early October and dries down quickly (typically 2 wk).

Stem construction consists of a porous epidermis (not waxy), which probably is the cause of its quick drydown. The xylem cylinder has an XPR of 0.23, and a typical stalk diameter of 1.6 cm. Xylem material has a density of about 240 kg/m3, which makes it one of the lightest materials of all the SSPs introduced here. The pith is a solid spongy inner core that rots after about 2 mo of drydown.

Kinghead Ambrosia was suggested as an energy fuel more than 10 yr ago (6). Yields on the order of 2.5 t/acre were quoted. In the analysis done here, I find this number a gross understatement and project yields of 13.4 t/acre without adaptation or modification of the plant as it currently exists.

Kinghead Ambrosia is the largest plant on the list of SSPs, in both height and stem diameter. Although this corresponds to a great bulk of material from this plant, it does not generate a tremendous weight because of its relatively low density. However, the pollen from this plant is very small and easily airborne, causing allergies in humans. Before this plant could be cultivated commercially, it probably would need some alteration.