Trees and shrubs must be able to survive the lowest temperature each winter in order to survive for the next growing season. Such plants have a threshold temperature below which they will die. This is signified by the hardiness scale. The scale ranges from one to ten. One will survive arctic winters. Ten is tropical. The scale we use is based on UK hardiness scale.
Genetic diversity often leads to individual plants with higher thresholds of hardiness. Therefore, it is good practice to experiment with growing plants one or two zones above your recommended limit.
Local climate and topography also need to be considered. For example, we are growing in Bulgaria which is generally zone seven or eight. However, our particular gardens are 580 meters above sea level, effectively becoming zone five.
Micro-climate can also provide differing conditions for plants. Plantings near south-facing walls will retain higher temperatures due to sun exposure and thermal mass. A low lying area along a north-facing wall will be colder and vulnerable to deep freeze.
Every plant species requires some set of conditions to break dormancy and trigger seed germination. Some are ready to go once ripe, while others have more specific needs. To accelerate or encourage germination, a seed can be prepared in a variety of ways to initiate its particular enzymatic process.
Not dormant, sow in spring.
Not dormant, but must be sown immediately.
Scarify and sow in spring.
Cold stratify (5°C). # indicates length of stratification in weeks.
Warm stratify (15-21°C). # indicates length of stratification in weeks.
Stratification involves mixing the seed with a moist medium and keeping warm or cold for a certain length of time before sowing. We recommend mixing seeds with a moist seedling mix (sand and loam). Warm stratification calls for 15-21°C (60-70°F). Cold stratification requires 5°C (40°F). Whenever stratifying seed, check every week or two for germination.
Scarification involves breaking through a protective seed coat to allow water into the seed. One method is to place seeds in hot water (about 88°C or 190°F) and allow them to soak for several hours while the water cools. Alternatively, the seeds can be physically rubbed between two sheets of fine sandpaper, taking care not to rub too much to avoid destroying the seed.