Methods of propagation

Sexual propagation involves the fusion of male and female reproductive cells.

This results in the creation of a genetically diverse offspring. In plants, the male reproductive cell is the pollen, which is produced by the male part of the flower, the stamen. The female reproductive cell is the egg, which is produced by the female part of the flower, the pistil. When a pollen grain lands on the stigma of the pistil, it germinates and grows a tube down to the ovary where it can fertilize the egg. This process results in the creation of a seed, which can grow into a new plant.

Put simply, sexual propagation is growing plants from seed.

Plants commonly propagated this way are:

• bean
• broccoli
• carrot
• celery
• cucumber
• eggplant
• leek
• lettuce
• lima bean
• onion
• parsley
• pea
• pepper
• pumpkin
• radish
• spinach
• sweet corn (maize)
• squash
• tomato
• turnip
• watermelon.

There are several advantages to producing new plants through sexual propagation, including:

• Genetic diversity: Sexual propagation creates offspring that are genetically diverse, which can be advantageous for the survival of the species. Genetic diversity enables plants to adapt to changing environmental conditions and to resist pests and diseases.
• Evolution: Sexual propagation promotes evolution by creating new combinations of genes in each generation. This enables plants to evolve and adapt to changing environmental conditions, which is critical for long-term survival.
• Seed production: Sexual propagation produces seeds, which can be stored and planted later. This makes it easy to propagate large numbers of plants quickly and efficiently.
• Breeding programmes: Sexual propagation is the primary method used in plant breeding programmes to develop new varieties of crops. By selecting and crossing plants with desirable traits, breeders can create new varieties that are more productive, disease-resistant, and better adapted to specific growing conditions.
• Hybrid vigor: When two different plant varieties are crossed through sexual propagation, the resulting offspring can exhibit hybrid vigor, which means they are stronger, more vigorous, and more productive than either parent. This is because the offspring have inherited the best traits from both parents.

But there are some disadvantages too:

• Time-consuming: Sexual propagation can be a time-consuming process. Plants need to flower and produce viable seeds, which can take months or even years in some cases. This can slow down the propagation process and limit the number of plants that can be produced.
• Variability and unpredictability: Because sexual propagation produces genetically diverse offspring, the quality of the resulting plants can be inconsistent. This inconsistency can make it difficult to predict the quality of the resulting plants and to maintain a consistent level of quality in a crop.

Growing from seed

As we've seen earlier, the two main approaches to growing from seed are:

• Direct seeding
• Transplanting.

If you want to refresh your memory, review Module 1 and Module 2.

Seed saving

You may or may not have given much thought to where the seeds you use come from. Buying seeds from well established companies should mean you're getting excellent quality seed. But there is another way and that's to save the seeds from the plants growing in your own garden.

In this short video by Gardening Australia, Tino Carnevale shows us the basics of seed saving.

Remember that when growing from seed, the plants you grow will be genetically different to the plant you harvested them from. While they will, in most cases, be the same species, they may have features that differ from the parent plant that you saved the seeds from.

It can be fun to see what grows from your own saved seeds, but bear in mind that the resulting fruit might not be at all desirable.

For quick growing plants, like most vegetables, you might still want to give it a go; it'll only take a few months until you know if the edible parts are any good to eat. For plants that take a long time before they fruit, such as fruit trees, growing from seed may not be the best approach. In fact, asexual propagation is often preferred for commercial production to ensure consistency and quality in the fruit produced.

Plant breeding

And if you're left wondering how it is that the seeds you buy from a seed supplier grow plants that all have very similar characteristics, well that is because they carefully control the pollination of the breeding plants. This video by The Oregonian shows how plant breeders control variability in plants they breed.

Asexual propagation, also known as vegetative propagation, does not involve the fusion of male and female reproductive cells.

Plants commonly propagated this way are: 

• artichoke 
• garlic
• potato
• rhubarb
• sweet potato (kumera).

Instead, a new plant is created from a part of the parent plant, such as a stem, leaf, or root. This process results in a genetically identical offspring, known as a clone. Some common methods of asexual propagation include cutting, grafting, and layering.

Some of the key advantages of asexual propagation include:

• Uniformity: Since new plants are grown from a single parent plant, asexual propagation results in genetically identical plants. This ensures that all of the resulting plants have the same desirable characteristics as the parent plant, such as flower colour, fruit size, or disease resistance.
• Rapid multiplication: Asexual propagation allows for the rapid multiplication of plants, as it does not require the time and energy needed to produce seeds. This can be particularly useful in agriculture or horticulture, where large numbers of plants with desirable traits need to be produced quickly.
• Maintenance of hybrid vigor: Asexual propagation is often used to maintain hybrid plants, which are the result of crossbreeding two different species or varieties. Hybrids often exhibit hybrid vigor, which means they have improved growth, yield, or other desirable traits compared to their parent plants. Asexual propagation ensures that the hybrid plant maintains its desirable characteristics, as any offspring produced through sexual reproduction would likely exhibit reduced hybrid vigor.
• Preservation of rare or endangered species: Asexual propagation can be used to preserve rare or endangered plant species that are difficult to propagate through seed. By propagating these plants vegetatively, their genetic material can be preserved and propagated, ensuring their survival.

Disadvantages of asexual propagation include:

• Lack of genetic diversity: One of the main disadvantages of asexual propagation is that it results in genetically identical plants, which means that there is no genetic diversity among the offspring. This can make the plant population, as a whole, more susceptible to diseases, pests, or environmental stress, as there is no variation in the genes that control resistance or adaptation.
• Loss of adaptability: Asexual propagation can also lead to a loss of adaptability in plants. Because the offspring are genetically identical to the parent plant, they may not be able to adapt to changing environmental conditions as well as plants that are produced through sexual reproduction, which can lead to a loss of fitness over time.
• Accumulation of mutations: Asexual propagation can lead to the accumulation of mutations in the genetic material of the offspring, which can result in reduced growth, yield, or other undesirable traits. Over time, this can lead to a decline in the overall quality of the propagated plants.
• Requires more expertise: Asexual propagation generally requires more specialised skills than growing from seed, although this will depend on the type of asexual propagation being used.

There are numerous different ways to propagate plants asexually, broadly the main categories are:

• dividing established plants
• taking cuttings
• grafting the shoot of one type of plant onto the roots of another
• layering plants.

In the following video by Epic Gardening (16:13) Chris Chung provides a great overview these approaches.

The following topics build on these approaches and walk you through some specific techniques and tools that relate to each.