The Guru Speaks
March 2010
After the first few days of uninterrupted sun in what feels like an eternity the joyous thought of another gardening year ahead is tempered slightly by the urgency of getting our carefully stored seed into trays or pots as one thing’s for sure they wont germinate in the packet!
We all try to grow new varieties, things we haven’t tried before, things we have admired in other gardens or at shows. There is a bewildering choice available and sometimes knowing whether they are ‘easy’ or ‘difficult’ can be a deciding factor in our selections. A seed contains all the information needed to make a new plant. It just needs sufficient warmth, moisture and oxygen to drive development. Since the seed takes all of this from the soil when sowing it’s worth remembering that overwatering drives out air pockets in the soil,| Germination Inhibition Explained... Germination inhibition prevents seeds from germinating inside the fruit and prevents the seed germinating into a hostile environment. There are 3 primary forms of germination inhibition. - Tough, waterproof, seed coat. Gradual imbibing of water causes the seed to swell until its enlargement cracks the brittle seed coat. Alternatively temperature fluctuations, UV irradiation or mechanical abrasion cause this waterproof layer to crack allowing water and oxygen into the seed to facilitate germination.
- Chemical inhibition. Chemicals in the seed coat, for example a high potassium concentration inhibit metabolism. Often this mechanism is found in seeds from fleshy fruits, where it prevents germination within the fruit. Seeds of this sort should be thoroughly washed and dried before sowing to remove the inhibitor.
- Light sensitivity. Woodland species such as foxgloves require light to germinate. This allows seed to lie dormant until leaf-fall or the opening of a woodland clearing creates a patch of sunlight for the seedling to exploit for maximum growth. Because of this foxgloves should be sown on the surface of the soil to receive light.
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while under-watering means that the water needed in metabolism is not available to the seed, ideally the soil should be just damp evenly over the surface. Soil texture is also of great importance, a good seedling compost will have a loose granular texture optimised for moisture retention and the formation of air pockets. It is not in the plants interest to make germination impossible but plants have evolved several ingenious mechanisms for inhibiting germination until conditions are just right for plant development. With ‘difficult’ seeds sometimes it just requires a clever mimicry of the optimum conditions by the gardener and germination takes off.
Annuals from temperate climates, cornflowers and Orlaya are two of our favourites, naturally tend to germinate easily. Allowing the seedling to grow quickly, develop flowers and produce thousands of seeds to germinate the following spring, completing the cycle. Thus annuals are generally very easy and rewarding. Don’t discard the weaker seedlings in an annual sowing of a mixed colour range as due to artificial selection the dominant colour in a group is usually the stronger seedling. Gardeners are sometimes surprised when their friends, to whom they gave the runts have a much greater range of colours from their seedlings!
Biennials such as the purpurea foxgloves have developed to produce huge quantities of seed. To do this they put all of their first year growth into a basal rosette of foliage to provide the resources to thrust often very tall flower stems skyward in the following year. They do not like much root disturbance so it is best to sow directly into little pots or if the seed is too fine to do this sow into seed trays and prick out as soon as their big enough to handle. Biennials are not usually difficult to germinate but with a few exceptions they have not been selected by growers so are closer to their natural state and germination requirements are closer to natural conditions. Be patient with biennial seeds that don’t seem to be doing anything. High temperatures can trigger a secondary dormancy so sowing in the cool of early spring or very early autumn is often best and no artificial heat is required just a cold frame or a sheltered spot in the garden. Germination is often erratic, again this is a normal adaptation the seedlings have made by altering the thickness of the seed coat to avoid the entire generation being eaten by a wandering goat!
Hardy perennials and shrubs are the biggest category of seed types and so specific advice might be needed-feel free to ask! There is one fundamental truth, those sorts that come from the coldest parts of the world have evolved to require a period of chilling before germination takes place. Plants originating from N. America, N. Europe, the colder parts of China and Japan are particularly likely to require chilling. Like all life a plants metabolism is sensitive to temperature, generally warmth causes the chemical reactions involved to occur faster and growth to accelerate with rising temperatures. Because the growing season therefore starts later in cold climates, after the winter frosts, seed is ‘set’ later, often in autumn which is generally a warm settled time. If the seed were to come up then the baby plants would surely perish in the long winter. So the seed needs a period of warmth corresponding to autumn, a period of cold corresponding to winter and this followed by the warmth of spring. By sensing the cycle of warmth and cold the seed ‘knows’ winter is over and it’s safe to germinate. These conditions can be artificially recreated. For example by putting the sown pots in a propagator at 15-20C (60-70F) for say 4-5 weeks then transfer to the fridge 6-8 weeks followed by warmth to initiate germination. A simpler approach may be to sow in the normal way in autumn and leave the pots outside all winter somewhere they wont be disturbed by rodents, curious children or those pesky wandering goats! The seed then comes up naturally when it is ready or in spring you could bring it into a cold greenhouse. Remember that even early in the year the sun can suddenly be very bright so don’t let the seedlings scorch. The period of chilling required is proportional to the severity of the winter the seed would experience in its native climate so researching a plants country of origin can be highly beneficial, this information is available from any serious gardening book and from the RHS.
Like Flora you can now ‘dispense your favours on the Earth’!
Good Gardening
Terry
Seeds now available to purchase from our website www.botanicguru.co.uk
If there is a topic you would like the guru to meditate upon for the next newsletter feel free to get in touch via celestial email botanicnursery@botanicguru.co.uk