When making up seed mixes it is desirable to set a target number of plants per m2, both as a total for all species present and for individual species, and, from this, work out the amount of seed required. The total number of plants required will vary depending upon the size of the plants at maturity, the visual characteristics required and the weediness of the site. A typical total plant target would be between 100 and 200 plants/m2. On weedy sites, higher seedling densities are desirable, as if one starts with a low density of sown species it is very difficult to dominate weedy species in the longer term. Sites that will be viewed from close quarters, plus the edges of sowings, also require higher than average densities. High-target densities are, however, potentially problematic, not only in terms of greater seed cost but also because this may lead to the elimination of the slower growing sown species by more vigorous species. In prairie sowings on fertile soil, for example, Baptisia australis and Echinacea pallida show relatively high establishment, but seedlings grow slowly and few seedlings survive the first year. Despite this, it is often better to go for higher densities, as at low densities on weedy sites, slow growing species may, in any case, be eliminated by weedy species. Low target densities are only sensible on sites where weed competition can be effectively managed, or with species that, in the longer term, will reliably fill in gaps through their own self-seeding.
Within the overall total target density (for example 100 plants/m2), it is necessary to decide on target densities for individual species. This involves decision making analagous to that undertaken in conventional planting design. If, for example, you want more or less even numbers of plants of each species in the vegetation, you might go for a target density of 10 plants per species/m2. In many cases, however, this would be unsatisfactory. It is more likely that you would want to establish one or less plants per m2 of really tall emergent species, for example Silphium terebinthinaceum. Any more and you will lose the rhythmic emergent qualities and will end up with a dense stand. You might want to increase the numbers of key species, for example, those that have a particularly long flowering season, for example Echinacea purpurea, or flower at a specific time when it is desirable to maximise impact. Target numbers allows practitioners to ‘design’ sown vegetation, rather than be passive bystanders. This is contingent, however, on having access to percentage field establishment data, as shown in Table 6.12.
Following the example of a seed mix given in Table 6.10, pasting the following formula into an Excel spreadsheet will automatically calculate the weight of seed per m2, where seed numbers per g, typical percentage field establishment, and target number of plants per species is known. For example:
=SUM(1/(cell reference that contains seed number/g* cell reference that contains typical field % establishment/100)*cell reference that contains desired number of plants/m2)
The total weight of seed sown per g are typically between 0.5 and 2.0 g/m2, depending on the target numbers and the weight of the seed of individual species. Providing that species with extremely expensive seed are excluded, the cost of seed mixes of this type will generally be between £0.50 and £1.50/m2.
