Attachment 1: SEEDING MIXTURE
Selection of a seed mixture and planting protocol needs to be site specific. There is no substitute for understanding the local environmental conditions, soil properties, and contaminant characteristics at a site. In addition, there is usually a wealth of local knowledge of what grows well in an area from local sources including but not limited to agricultural extension, the Natural Resource Conservation Service, and local seed suppliers.
For phytoremediation, an important consideration is contaminant composition and concentration, at high TPH levels, phytotoxicity may limit seed germination and plant growth. This can be handled by selection of more petroleum hydrocarbon tolerant species and by increasing seeding rates. At lower contamination levels, many species grow quite well, and this is less of a concern. Depending on the site, it may be useful to run germination trials prior to seeding a site to determine if plants will establish and if seeding rates should be increased to accommodate expected low germination.
Time of Planting:
Time of planting should be determined by local conditions, the availability of moisture for seed germination, and plant species characteristics. Warm-season species will often need to be established in the spring. Cool-season species are often established best in late summer or fall, but these can often be spring seeded. Again, site specific considerations are important. Seed germination requires good seed-soil contact under moist conditions and the appropriate temperature for a given species.
Seeding rates:
In general, planting conditions at petroleum contaminated sites will be more stressful than most agricultural fields and landscaping situations. In that light, seeding rates should usually be quite a bit higher than recommended for other purposes, perhaps doubling the seeding rates recommended for lawn establishment in the area when seeding tall fescue. If contamination levels affect germination and seedling growth, than seeding rates may need to be even higher. Another important influence on seeding rate is water availability. Under irrigation and higher rainfall conditions, plant populations can be more dense than under drier conditions. This suggests higher seeding rates for wetter conditions.
Overseeding:
Prepare two lots of planting seed at the initial time of planting. If several species have been planted in a mixture, do not mix the reserve seed because a limited number of the species may need to be replanted. Be prepared to overseed areas that establish poorly. Overseeding can often be done about six to eight weeks after the initial planting or at the beginning of the next acceptable planting period in the spring or fall. Even after the initial establishment of vegetation, overseeding may be helpful. For example, annual ryegrass can be seeded into bermudagrass in the fall to maintain growth through the winter. Some plant species are short lived and may need to be replaced. Stressful conditions such as drought or flooding may also cause stand loss and require overseeding. Local conditions will determine if any ground preparation or mulching is needed during overseeding. Again, for seeds to germinate they must have good contact with moist soil.
Example seed mixture composed of tall fescue, annual ryegrass, and a legume:
8 lbs/1000 square feet tall fescue
2 lbs/1000 square feet annual ryegrass
1 lb/1000 square feet legume (such as white clover, yellow sweet clover, or
birdsfoot trefoil)
Legumes should inoculated with the appropriate Rhizobium strain to facilitate effective nitrogen fixation. This example is a rather high seeding rate similar to a mixture used under natural rainfall condition. Local conditions need to be considered.
Mixtures involving native plants or trees will require site specific considerations. Seeding rates for native mixtures will often be much less than the example given above.