Seedling size is usually described by several morphological and empirical nursery rearing attributes which include:
The table on the right categorizes seedlings into qualitative descriptions of small, medium, and large:
| Typical Seedling Size Class | |||
|---|---|---|---|
| Seedling Characteristic | Small | Medium | Large |
| Average seedling height range (cm) | 16 – 20 | 20 – 24 | 25+ |
| Average seedling RCD range (mm) | 2.6 – 3.0 | 3.0 – 3.4 | 3.5+ |
| Volume range of rooting media (ml) | 40 – 80 | 80 – 125 | 125+ |
| Growing density range (seedlings/m2) | > 553 | 364 – 553 | < 364 |
| Styroblock designation | PSB 211A – 240/40 | PSB 411B – 128/85 | PSB 412A – 77/125 |
| PSB 310A – 160/60 | PSB 410B – 112/80 | PSB 415D – 77/170 | |
| PSB 313B – 160/65 | PSB 412B – 112/95 | PSB 512A – 60/220 | |
| PSB 415B – 112/105 | PSB 615A – 45/340 | ||
| Bareroot designation | P + 1 | ||
The amount of vegetation competition on the site is one of the most important parameters to consider when deciding what size of stock to deploy. Generally large stock will perform better than small stock in competitive environments unless there are other site limiting factors such as low snow levels, warm winters, and high potential for winter desiccation damage. Larger stock is taller, has more foliage, and therefore has a greater photosynthetic capacity with which to compete against other vegetation.
Larger containers have greater volumes of rooting media in the plug, which is positively correlated with seedling growth and survival under a wide range of conditions. However, in dry conditions larger stock tends to be more susceptible to drought stress and may perform worse than smaller stock of the same species. Larger stock can also suffer winter desiccation damage in areas that experience low levels of snow cover and high winds.
Therefore, small sized seedlings should be considered for dry, low competition sites or areas that are at risk of winter desiccation damage. Larger stock should be considered for high-competition sites with a low risk of winter desiccation damage.
