Overcoming seed dormancy
In nature, many tree and shrub seeds have a natural dormancy to help them survive. This dormancy allows seeds to survive hostile conditions and only germinate in favourable conditions, such as the warmth of spring, or after a rainy season. As gardeners, we can supply seeds with favourable conditions all year round, so sometimes we need to artificially break dormancy for seed to germinate.
Scarification and chitting
Seeds with hard seed coats need to have their seed coats scraped, cut or soaked to allow in moisture before sowing.
Use sandpaper or a file to scarify (abrade) the seed coat. Chit the seed either by using a knife to nick the seed coat or by soaking the seed in warm water for 24 hours. Care should be taken when soaking seed, as too much can cause rotting.
Examples of seed benefitting from these treatments are plants from the pea family, such as Colutea, Cytisus, Genista, Spartium, Lespedeza, Lupinus and Robinia.
This is used for seeds which respond to either heat or cold. Stratifying will break dormancy by copying the conditions these seeds are naturally subjected to in the ground to trigger germination. Most commonly this is a spell of cold similar to that experienced in the winter.
A few seeds have multiple dormancy and only germinate in their second spring after periods of both cold and warmth. Fraxinus (ash) is a typical example. Subjecting seed to a warm spell followed by cold can increase germination in the first spring after sowing. This is achieved using the techniques of warm and cold moist stratification.
Cold moist stratification
- Place seed in a clear plastic bag filled with moist but not wet coir, composted bark, or a mix of equal parts of the above with coarse sand, perlite or vermiculite, and seal the bag
- Chill seed in a refrigerator, kept below 5°C (40°F) for four to 20 weeks, depending on the species
- Shake the bag periodically, and sow immediately if the seed germinates in the bag
Warm moist stratification
- Place seed in a bag as above and keep in a warm place at 18-24°C (65-75°F) for up to 12 weeks, before giving a period of cold stratification
- Alternatively, sow in pots and place in a heated propagator for the required spell for the seed in question. Following this, place in a cold frame for the winter