Lesson 1 - Introducing plant propagation
Growing your own plants from either seed or cuttings is incredibly satisfying and an economical way of trying many different species. We encourage you to give it a go. It is also kind of addictive. Gardeners are a bunch of generous souls who once you have expressed interest in a plant will often share seed or a cutting. So let us teach you how to propagate to make the most of those seeds or cuttings from your aunt or neighbour. It is a bit of a trial-and-error process and not all that hard. But first, a bit of botany.
Many plants reproduce from seed that develop through the process of flowering - these plants are called angiosperms.
There is another group of plants that are known as the ‘naked seed’ plants and they don’t produce flowers and are called gymnosperms. (Think gym bros who never see the light of day). Gymnosperms are plants like conifers, pines and cycads and the seeds are what make up their cones.
For the purposes of this Growing Food course, we are focusing on angiosperms. To make things a bit more complicated angiosperms are split into two groups which are identified by the number of leaves (which are known as cotyledons) that emerge when the seed first sends out its first shoot. Plants with one cotyledon are known as monocots. Plants with two cotyledons are known as dicots.
Dicots that many of us grow in our vegetable gardens include: tomatoes, peppers, potatoes, cauliflower, broccoli, turnips, beans, peas, apples, pears, celery and parsley.
Monocots include garlic, onion, corn and asparagus, all the grasses, lilies and irises, orchids, ginger and palms. You can identify them by their herbaceous growth habit.
Each seed is a plant embryo that is the result of a sexual means of reproduction. Like us, a seed combines both male and female genes that is genetically different from the parent plant. This ensures plants are constantly evolving and adapting to the environment. Seed is produced by both angiosperms and gymnosperms but in very different ways.
In flowering plants or angiosperms, the process of sexual propagation starts in the flower. To make it a little bit more complicated, the flowers will be a mix of female and male flowers. A female flower must be pollinated for it to produce seed. Pollination occurs when pollen produced by the male flower on its anther connects with the female flower’s stigma. The stigma is attached to the ovary by the style. Together these three structures form a carpel. There might be just one or several carpels at the centre or apex of a flower.
The wonderful thing is the methods that nature has engaged to get the pollen to the stigma. There are several ways this can occur, including attracting pollinators; insects, birds and bats for example. To attract help with pollination, nature has employed many ingenious ways to lure pollinators. The colour of flowers has evolved to attract insects. Flowers on the blue, purple, yellow and white spectrum are very attractive to bees and butterflies. The structure of some flowers provides the perfect landing pad for insects and small birds to reach into flowers and drink nectar and pick up pollen while there. As they move onto the next flower, they are cross pollinating as their bodies brush against the anther and then stigma in the next flower.
Smell is another method flowers attract pollinators, the complete smell spectrum is employed - from a sweet perfume, the mimicked odour of a female insect to attract male insects or the smell of rotting flesh to attract flies. Mother Nature uses every method she can to encourage reproduction.
Plants also use water and wind. For example, corn has the male pollen producing flowers at the top of the plant that is then blown down to the female flower’s stigma lower down the stem. Other wind pollinated vegetables include beetroot, silver beet and spinach. Pollen is also carried on water down a plant and travels from one plant to another, this primarily occurs with aquatic plants.
Some angiosperm species have male flowers and female flowers on separate plants which means you need one of each species to propagate new plants, these plants are called dioecious species, examples are asparagus and kiwi fruit. Approximately 5% of all plants are dioecious which comes from Ancient Greek and very roughly translated means two houses. Implying the sexes live in two different houses or plants.
Back to the Gym Bros or Gymnosperms. They also use wind for reproducing. However, the female cones are located at the top of the tree with the smaller male cones located below and this is an adaptation in the plant to prevent self-pollination. Pollen (the male gametophyte is released from the male cone, and it floats into the female cone and finds its way via a pollen tube into the female gametophyte and then up to three years later a seed develops. It’s complicated stuff, a detailed explanation can be found in this excellent LibreTexts Biology article, Plant Reproductive Development and Structure – Sexual Reproduction in Gymnosperms.
Some plants have adapted to reproduce without the sexual process, and some (the very vigorous ones) have adapted to do both. Plants like ginger and onions develop new plants from buds on their stems. Plants also naturally reproduce asexually by sending out adventitious roots or runners known as stolons. Sweet potatoes and strawberries are two plant examples that do this. Which means more plants to share.
The other methods asexual propagation occurs when a gardener is involved and they include cutting, layering, grafting and micropropagation. We will cover cutting and layering in more depth later in this module. Grafting and micropropagation are more advanced gardening practices. You can see evidence of grafting on citrus trees, look for the collar at the bottom of the trunk, this is known as the graft. In grafting, two plant species are used: part of the stem of the desirable plant is grafted onto a rooted plant called the stock. The part that is grafted or attached is called the scion. Joining the plants by creating a clean, same sized cut on both plants ensures that the vascular systems of both plants fuse together and form a strong attachment. Unless you fall in love with roses, get passionate about citrus or plant a vineyard you won’t need to know how to graft just yet. Put it on your bucket list for later.
Micropropagation involves labs and white coats and petri-dishes. Plant tissue is fed hormones and nutrients to encourage it to reproduce new plants. It’s a bit Frankenstein.
Now that we know how plants reproduce and that you can propagate from a cutting, let’s consider the quality of our seeds and plant material. Be very aware of the origin of your seed or plant cuttings which are sometimes called vegetative material. Buy from a reputable seed company or nursery. Check with the gardener or community group that may have shared seeds with you to make sure that they are supplying you with viable healthy material. When you store your seed make sure you keep it in a cool, dry place in either glass jars or paper bags free from moisture. Cuttings from a healthy plant should be propagated as soon as possible after collecting.
Prepare your seed or cutting trays outdoors or in a well-ventilated shed. If you are making your own seed raising mix or when using a commercial preparation, always wear a mask to avoid inhaling fine particles and wash your hands before touching your face.
Many experienced gardeners grow their seedlings on north facing decks or balconies, on windowsills, under old windows beside the north facing brick wall of a house and some even have fancy greenhouses. For the purposes of this course, we are going to encourage you to grow plants inside or in a sheltered area close to the house where you can check on your propagation trays at least a couple of times a day. This is to ensure that water and temperature is easy to control, and your plants are protected from pests (think possums)
Diggers tip
Heated propagation trays are an excellent way to keep your soil at a regular temperature. Just make sure you don’t let your seed propagation mix dry out! Diggers have and Easy Heat Propagation Kit to get you going.
We don’t mean to scare you, but it is really important to practice good plant hygiene. There are several viruses that are transmitted by mechanical means (contact) which will affect the growth of a plant, damage leaves and alter the forming of fruit on plants. For example, tomatoes, capsicum and chilies are at risk from one group of viruses called the Tobamoviruses.
These viruses can survive on implements, trellis wires, stakes, containers and contaminated clothing for many months in the absence of any plant material. The viruses can also survive in crop (leaves, stems) and root debris on the soil surface for at least several months and can infect a new crop planted into a contaminated site.
Tobamoviruses are also carried on seed, leading to the infection of germinating seedlings. Tobamoviruses are easily spread by contact; this includes touching infected and healthy plants during operations such as transplanting, pruning, tying, cultivation, spraying and harvesting.
The best way to keep your garden and propagation area free from viruses is to make sure that you know where your seed has originated from, that your propagation area is clean, ideally disinfected with either dilute chlorine bleach or disinfectant, and that your seed raising/cutting mix is healthy. All seed raising trays or cutting trays must be scrupulously clean. Make sure you clean your bench where you mix the media is sterile and use it within six months. Discard any old or unused compost.
There are many recipes available in good gardening books and on the internet. Look for a recipe with organic ingredients. Diggers sell a Seed Raising mix which has been made to ensure good water holding capacity without compromising drainage. If you want to make your own seed raising mix, we have a DIY recipe, Diggers DIY Seed Raising Mix Recipe
When making a mix you aim is to make sure the mix:
- Has a fine texture to support and surround seeds of all sizes.
- Allows ideal drainage while still maintaining adequate moisture.
- Provides gentle nutrition to support the developing seedlings after germination.
- Is free from weed seed.
Growing your own plants is both a bit of science and an art, however, if you manage to get the environmental conditions right, you will be surprised by how much you can propagate.
Water
This incredible resource truly is the bringer of life. Water is the catalyst that starts the germination process and signals to seeds that it is time to reproduce. Water levels must be kept at a constant level and never be allowed to dry out. Like us, seedlings will die without water and will drown with too much water. Too much water reduces the amount of oxygen available in the soil for the roots of the seedling. Oxygen is needed by the plants to help take up both water and nutrients. You are aiming for a moist but not saturated conditions.
Temperature
The optimal temperature for seed germination varies according to plant species and we will cover the requirements for several different vegetable seedlings in lesson 5. The optimum temperature range for many seeds is 18°C to 24°C. When we talk about temperature here, we are talking about the temperature of the soil. Providing a consistently warm temperature when seeds are germinating will ensure that you get good germination results (all or many of your seeds will grow into seedlings). To provide a constant temperature, many gardeners use heat mats or place seed trays on warm windowsills. Be careful if you are using a glass house because temperatures can fluctuate quite a lot in colder climates and drop significantly overnight.
Humidity
Heat and moisture combined create humidity. High humidity levels during germination provide the perfect environment for seeds to kick on and start sprouting. To maintain high humidity levels during germination, cover your seed trays with a plastic bag or piece of glass or a seed raising tray cover. Keep an eye out for condensation and remove the cover to let the environment dry out a bit, otherwise you risk the bane of every propagator – damping off. This condition makes your baby seedlings look like they are drying out but in fact there is a soil borne fungi present.
Light
Interestingly some seeds need light to germinate, and some seeds germinate in darkness. Seeds that have thin casings and are small need to be at or just below the surface of the soil to germinate, for example lettuce and carrots. These seeds are spread on the top of seed raising mix either mixed in with a bit of sand or covered in vermiculite or perlite (more about these later). Seeds with thicker casings prefer to start in darkness and these are planted into seed raising mix, examples are corn and beans.
Further learning
- Name that flower: the identification of flowering plants Ian Clarke and Helen Lee (Melbourne University Press, 2019)
- Lifecycle of an angiosperm - Course Hero
- Flies like yellow, bees like blue: How flower colours cater to the taste of pollinating insects - The Conversation
- What flower colours do birds and bees prefer? - ABC News