The cost of seed varies considerably between different species. Seed of agricultural strains of native grasses is the cheapest, followed by field-grown native wildflowers and the seed of wild collected native grasses and forbs. By purchasing the seed of species that are not native to Britain, for example North American Prairie forbs and grasses, from native seed producers in those countries, costs are often very comparable to that of British native wildflowers. For exotic species in general, there are a number of wholesale seed companies whose main market is the nursery industry. The company with the most extensive range of perennial forbs and grasses is Jelitto Seeds.
Comparing species only in terms of the catalogue price of seed is potentially rather misleading. A species with expensive seed may demonstrate very high percentage establishment when sown in the field, whereas species with inexpensive seed might show low percentage establishment. Individual seed weight is also important, the bigger the seed the fewer supplied per gram. Some seed price and seed weight differentials even out when percentage field establishment (the number of plants you expect to establish for every 100 seeds sown) is taken into account. For any given species, actual values for percentage field establishment vary between seed batches and from situation to situation according to soil moisture, temperature and sowing practice. Despite this, the research of the author suggests it is possible to categorise seed into broad categories, as shown in Table 6.12. Most annuals fall into the high to very high field establishment category.
Species that show low field establishment are often (but by no means always) those with very small seed, which are more sensitive to soil-moisture stress during germination and emergence.