Читать книгу Home Gardener's Propagation (UK Only) - David Squire - Страница 47

Seeds often appear to germinate in the most inhospitable of places, perhaps in cracks between concrete paving slabs and on the tops of walls; seeds of native plants often show this tenacity and adaptability.

Each seed contains an embryo plant, surrounded and protected by a seedcoat and with a source of stored food. Germination is triggered by the presence of moisture, air and warmth, and when these are present germination occurs. Most seeds also need darkness; a few require light.

• Moisture: Essential to soften the coats of seeds to enable roots to develop and shoots to push their way upwards through the soil or to the compost’s surface. At the onset of germination, a seed absorbs large amounts of water that stimulates growth; roots develop first, then shoots.

• Air: All life processes, including germination, need oxygen. To enable oxygen to be available, compost must be well drained to allow air to reach the seeds. There is a desired balance in compost and soil between retaining sufficient moisture and allowing the presence of air; this leads to the practical advice for ‘moisture-retentive but well-drained soil or compost’.

• Warmth: A suitable temperature is vital to initiate and encourage chemical activity within seeds when moisture and air are present. Seeds vary widely in the optimum temperatures they require. After seeds are liberated from a parent plant, they usually undergo a resting or dormant period that, for many plants, coincides with cold weather, and chilling seeds of some plants will encourage germination (see ‘Stratifying seeds’, right).

• Light: Its influence on germination is variable and many seeds are able to germinate in both light and darkness. However, rather than just leaving seeds on the surface of compost or soil, it is best to sow such seeds at a depth that enables good root development and keeps them moist. The compost or soil also gives the subsequent seedlings a secure base. However, there are some seeds that will only germinate when exposed to light.

VIABILITY OF SEEDS

The ability of a seed to germinate when given suitable conditions (see left) gradually diminishes, and the question many gardeners ask is ‘how long will seeds remain viable?’ This depends on several factors:

• Storage: The best ways to store seeds are described below.

• Type of plant: This varies widely; whereas seeds of willows retain viability for only a few days, and those of Cocos nucifera (Coconut Palm) only slightly longer, seeds of Nelumbo nucifera (Sacred Lotus) are claimed to be viable for more than 100 years.

Usually, it is the viability of vegetable seeds that interests gardeners. Here are indications for a few popular vegetables:

VEGETABLE	VIABILITY
Asparagus	3 years
Beans	3 years
Beetroot	4 years
Broccoli	5 years
Cabbages	5 years
Carrots	3 years
Cauliflowers	5 years
Cucumbers	5 years
Lettuce	5 years
Onions	1 year
Peas	3 years
Peppers	2 years
Pumpkins	4 years
Radishes	5 years
Spinach	5 years
Squashes	4 years
Sweetcorn	2 years
Tomatoes	4 years