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Horticulture 1000
HT1091 - Plant propogation 1 - Seed dormancy
Seed germination Sowing seeds in open ground
4

Seed dormancy

Most  seeds will germinate if they are given the right conditions of moisture, oxygen  and warmth. Other seeds go through a resting stage known as dormancy. This prevents seeds germinating at the wrong time when conditions may not be present for successful germination, for example, cold, frosty or dry conditions. When the seed is dormant it is alive or viable but germination is delayed.

Dormancy  in seeds can be caused by:

  • a hard coat that water can’t penetrate
  • a need for cold chilling below a certain temperature for a certain period of time
  • immature embryo that may need more time to develop to be able to germinate
  • a chemical inhibitor (a substance that prevents the seed from growing).

Dormancy is an advantage because the seed will only germinate when the environmental conditions are suitable. The seed can last a very long time in a dormant state. Native broom seed has germinated after being stored for 140 years.

To get some seed to germinate you need to break dormancy. The method used will depend on the type of plant and the different mechanisms that control dormancy.

In  nature these things help break dormancy:

  • physical  rubbing of the seed coat to make it thinner
  • seeds  passing through the gut of an animal
  • long  wet and/or frosty conditions
  • fire.

An apple seed from an apple in your garden for example, will not germinate straight away. Apple seeds need chilling before they will germinate. In nature they rest through the winter then germinate in the spring.

Artificially, apple seeds are chilled between layers of moist peat, or sphagnum moss and vermiculite. When put in warmth again, a false spring is created and the seed germinates. This process of moist chilling is called stratification. Stratification means something is arranged in layers.

Breaking dormancy

Hard seed coats

The following are some methods that can be used to break hard seed coat dormancy. Several methods can be suitable for some seeds. Examples of seed that have a hard seed coat are kowhai, lupins and many other legume plants. The problem is that water can’t enter the seed if it has a hard seed coat. Scarification, chipping and soaking are all methods that can be used to solve this problem.

Scarification

Scarification is when the seed coat is rubbed with something such as sandpaper. This makes  the seed coat thinner so water can enter more easily. This method is best for smaller seeds that are hard to cut for example kaka beak, boronia. To do this easily you can shake the seeds forcefully in a container lined with sandpaper.

Chipping seeds

Chipping is cutting a small piece out of seed coat with a sharp knife. Water can then enter the testa through the cut. This is useful on large seeds for example, lupin, kowhai or sweet pea.

Soaking seeds

Soaking seeds in water overnight will soften the testa. This can be useful for seeds such as wattle, lupin, sweet pea and kowhai. Commercial growers use diluted acids on some seeds.

Breaking other types of dormancy

The  table below shows methods used to break other types of dormancy.

                                                                   
Cause of dormancy
 An immature embryoChemical inhibitorsA need for chilling
The problem The embryo is not fully developed when the seed is dispersed. Some seeds contain chemicals that prevent germination. This is common in fleshy fruit but it depends on the seed type as to what will get rid of the inhibitor. Plants living naturally in cold climates may have seeds requiring a period of    chilling (stratification).
What to do Store seeds before planting. Waiting for a period of time will allow the embryo to    mature. Rinse or soak seed in clean water, expose the seed to light, or just wait for a    period of time. Chill the seeds for 4 to 8 weeks either outdoors in winter or in a fridge. The seeds must not be frozen as this could kill the embryo.
Plant example Lettuce
      Ranunculus
      Holly
Tomato
      Feijoa
      Karaka
Flax
      Water    fuchsia (impatiens)
      Apple

Complete Activity 4A in your workbook.

Key points   Key points

Dormancy is a resting stage. Seeds will only germinate when the conditions are suitable.  Some causes of seed dormancy are:

  • a hard coat
  • a need for chilling
  • immature embryo
  • a chemical inhibitor.

What's next?

Go to: 5 Sowing seeds in open ground.

 
Seed germination Sowing seeds in open ground