Seed is a cheap way to produce large numbers of new shrubs or trees when starting a new garden. It is also fun to try growing trees from seed, although patience is required as it takes many years to grow a large tree.
These techniques apply to all hardy trees and shrubs that have successfully set seed. Bear in mind that named cultivars do not come true from seed, but species plants usually do.
When to sow seed
While many tree and shrub seeds can be sown in spring, after storage over the winter, others must be sown fresh in the autumn. Many tree seeds, in particular, become difficult to germinate if not fresh.
Sowing in autumn allows the natural winter chilling effect to help break seed dormancy.
If seed is bought in spring and cannot be sown fresh in autumn, there are techniques (described below) that help the gardener to artificially break seed dormancy.
How to sow seed
- Fill a pot, seed tray, or modular tray, with proprietary seed-sowing compost (John Innes Seed for example), or use 50% multi-purpose compost mixed with 50% perlite or coarse grit
- Firm the compost with your fingers, or with a pre-cut firming board
- Water the compost and allow the surplus water to drain out the bottom through the drainage holes. Label with the date and plant being sown
- Sow the seeds thinly, with a finger-width between each seed, or sow one or two per module (depending on the size of the modules)
- Large seeds require covering with sieved compost or vermiculite. Small seeds do not need covering and in fact some, such as Paulownia, need light in order to germinate, so are best left uncovered
- Prevent pots from drying out by covering with an inflated polythene bag secured with an elastic band, or with a suitable cloche (half a clear plastic bottle works well). Trays can be covered with a clear plastic lid, or with a sheet of glass or perspex
When large enough to handle, transplant/prick out each seedling in its own pot of multi-purpose compost.
Seedlings in shallow seed trays need transplanting promptly, handling them carefully by holding the seed leaves, rather than the emerging true adult leaves. Seedlings in modules can be left a little longer before transplanting, allowing their roots to fill the module, and then transplanting the whole plug of roots and compost in one go. Tree seeds often do better in deep modular trays such as ‘rootrainers’.
Water regularly, as needed, and feed with liquid fertiliser every month, growing the seedlings on into small plants.
The following spring or summer, when the plants are more robust, start to introduce them to outdoor conditions, a process known as hardening off.
Plant them out into the garden in the autumn, giving them the winter to settle their roots into the soil before coming into active growth the following spring.
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
Damping off (rotting) of seedlings can be a problem, especially for the novice gardener or where the seeds have been too wet.
If tree and shrub seeds fail to germinate in their first season, then it is worth leaving them for another season to see if they perform, as germination can often be erratic and slow, especially for seeds where dormancy is a problem.