Winter is a fantastic time of year on the nursery it may be cold and wet outside but in the potting shed and the glasshouse one of the best tasks of the year is being undertaken – seed sowing.
This year at Sissinghurst over 400 different species and varieties of plants will be propagated from seed. The vast majority of this seed is collected directly from the garden by our team of gardeners who each have responsibility for collecting seed heads, pods and fruits from specific areas of the garden. So many months of work collecting, drying, cleaning, labelling and storing lays down the groundwork for this most rewarding of winter jobs.
The process of sowing seeds is a calm and methodical affair with the majority of seeds having very straightforward and unchallenging needs to ensure successful germination. But there are many which have more specific requirements in terms of temperature and sowing conditions. This should not however be seen as daunting or off putting in anyway, as these more detailed seeds offer a challenge which can prove highly engaging and rewarding.
Seeds come in a stunning variety of shapes and sizes from tiny dust like genus such as Primulas and Meconopsis to large, easy to handle types such as Sweet peas and Nasturtiums. The beauty and diversity of seeds is a worthy and fascinating subject in its own right and perhaps a topic for a future blog.
All of our seeds here at Sissinghurst are sown into peat free compost with added perlite. The compost is an excellent fine grade and the addition of the perlite provides an open, free draining media for the emergence of the seeds young root system. Small, shallow pots are used for each type of seed and each pot is clearly marked with the sowing date and plant name.
The seeds are sown evenly across the surface of the compost, avoiding large clumps of seeds in one area. Spacing the seeds and sowing evenly will help prevent issues with overcrowding and poor growth when the seedlings emerge. There are many different methods of how to sow seeds to achieve an even spacing and everyone will have their own technique, I favour pouring the seeds into the palm of my left hand then taking a pinch of seeds with my right and gently sprinkling them over the compost in a similar fashion in which you would add herbs or spices to a cooking pot.
The size of the seed determines the depth of sowing, very fine dust like seeds are sown directly on to the compost surface and are not covered with any additional compost. All other types of seeds are sown and covered by a fine layer of compost appropriate to the size of the seed. A good rule of thumb is that the sowing depth should be roughly the same as the size of the seed itself. So a seed which measures 5mm in size should be sown with a covering of 5mm of compost.
The pots with their sown seeds are then placed on to a heated bench (set at a temperature of 18 – 20⁰C). The majority of seeds will germinate over a period of 2 – 6 weeks (depending on genus). (It is worthy to note here that some genus such as many types of perennial Primulas prefer a lower heat of 15⁰C and will germinate successfully in a heated glasshouse with no additional bottom heat).
Once the first set of new leaves appears the seed pot is transferred from the heated bed to the growing bench and a light layer of very fine horticultural grit is applied over the compost surface.
The grit has a number of benefits but primarily it is to help protect against damping off diseases which lead to rotting and eventual death of the young seedling. The grit aids drainage and prevents excess moisture around the base of the seedlings stems helping to prevent the conditions favoured by these fatal diseases. This reduction in surface moisture also helps prevent pests such as Sciarid fly.
As mentioned earlier some seeds require a little more work to achieve successful germination and one of the most common requirements is a period of cold.
The process of subjecting a seed to a period of cold is known as stratification and this can take the form of time in a fridge prior to sowing or sowing the seeds then placing them outside over the winter months.
The process of placing the seeds in a fridge for a period of time breaks the seeds natural dormancy. Dormancy is nature’s way of ensuring that the seeds only germinate and emerge when conditions are at their optimum level for the seeds successful growth and development. Seeds treated in this way are sown as described previously and then placed onto the heated bed germination normally takes between 6 and 12 weeks.
The technique of sowing seeds and placing them outside for the winter is used when the fridge method has proved ineffective. These seeds require a natural period of cold to break dormancy and allow germination, this is often a period of some months and it is important to be patient and give them sufficient time, normally seedlings will emerge once the temperatures increase in mid spring and early summer. This method requires a significant investment of time but is well worth the effort when you find the young seedlings poking theirs heads above the compost surface after the long cold winter months.
Finally of huge benefit is the keeping of notes when sowing seeds, this will provide a wealth of information which can be built on each season based on what has been successful in the past, average germination times, preferred temperatures of particular genus and so on. Seeds are a fascinating subject and a wonderful form of propagation which offers gardeners the opportunity to grow a stunning array of diverse and enticing plants, while always offering the chance to try something new.
Emma – senior propagator