An Introduction to Propagation
Over the coming weeks and months Dish, our Head Farmer, is going to share his experiences of moving our urban farm to his home and provide tips and guidance on how you can also grow at home to help you learn, clear your mind, and grow your own fresh produce.
In this blog we’ll be looking at the different ways that plants are propagated. Propagation is important for home growers to understand because you can utilise these methods to multiply how many plants you are growing, including by using the plants you already have!
What is propagation?
Propagation is the action of increasing in number or spreading plants. There are different types of propagation, which indicate how plants reproduce and pass on their genetic information. I’ll outline the conditions required to grow from seed and explain how you can grow herbs from stem cuttings.
There are two main types of propagation: sexual and asexual. Sexual propagation occurs when plants reproduce by seeds, the genetics of two parent plants are combined by pollination and fertilisation to create offspring that are genetically different to each parent.
Asexual propagation (or vegetative propagation) occurs when plants are grown by taking pieces of vegetation (stem, leaves and/or roots) and regenerating them into new plants. The resulting plants will have identical genetics to the parent plant used. Advantages for each type of propagation
It can result in new cultivars and vigorous hybrids.
It can be quicker than asexual propagation.
When seeds are sown in sterile conditions, it provides a way to avoid the transmission of particular plant diseases.
Some plants can only be grown from seed.
It can be faster than growing from seeds for some species (e.g. mint or rosemary).
You can maintain desirable characteristics by growing genetically identical clones.
It may be the only way to grow particular cultivars.
One parent plant can produce multiple cuttings.
Growing from seed
Seeds have three major parts, the embryo, endosperm and seed coat. The embryo contains the plant’s genetic material and is in a dormant state until germination. The endosperm is an internal food supply for the seed’s first couple of weeks of life. The seed coat is for protection against mechanical damage, disease and pests. It also acts as a barrier until enough moisture is present to begin germinating.
It is advised to buy the seeds you will need for one year, this is because the likelihood of germination goes down over time. To keep seeds viable it is best to store them in an airtight container in a cool, dry and dark place.
What conditions will affect the germination of my seeds? Seeds will germinate when certain environmental conditions are met. Germination is the process of seed embryos coming out of dormancy and resuming growth.
The first step in this process is the absorption of water, this is why many recommend keeping conditions very moist in the first few weeks after sowing seeds. Once a seed has germinated, a dry period can kill the embryo.
Light is a factor which can aid or inhibit germination. We find that when planting vegetable or herb seeds in rockwool for our hydroponic systems, depending on what species, you should keep them in the dark for between three days and a week before exposing to light. Alternatively if you plant seeds into soil, a soil layer as big as the seed should be used to cover them while germinating. This means planting small seeds like kale and chives near the surface and bigger seeds like beans and pumpkin seeds approximately 1cm below the soil. The seedlings will soon reach for the light
The temperature where seeds are germinating is also very important. Crops like broccoli, kale and radish can germinate in very low temperatures, whereas some like eggplants or oregano require high temperatures to germinate. We recommend using seedling heat mats to encourage germination. You can find germination temperatures for most vegetables and herbs here.
Respiration rates increase during germination so ensuring the young plant has access to oxygen is important, without it plant growth can be stunted. Having loose soil mixes is recommended to ensure the germinating seeds can access oxygen in the soil’s air pore space.
Check out my blog on how to sow seeds for soilless hydroponic systems and in soil.
Growing from vegetation
Some edible plants naturally reproduce asexually, by growing from buds on their stems (e.g. ginger and onion) and others extend adventitious roots under the ground, which give rise to plants (e.g. sweet potato). There are also many ways for us to artificially propagate plants, including grafting, division, layering and cutting.
Grafting is commonly used to propagate fruit trees; a stem of a desired fruit tree is grafted onto a compatible rootstock. This maintains the exact genetics of the parent tree so identical fruits are produced. The vigour of the rootstock can also be chosen so a gardener can choose how tall and fast the tree will grow.
Propagating by division involves separating a larger plant into multiple parts which will then grow as individual plants. The crown and root must be kept intact for it to be successful. The less damage to roots in this process, the better for the plant. This works really well for woody perennial herbs like rosemary.
Layering allows shoots of the parent plant to form roots while still attached. The simplest way to achieve this is to choose a stem of the new growth on a plant that is flexible and can be bent to ground level. Secure the shoot tip to a bamboo cane so it faces upwards and then cover the rest of the stem with soil. The roots will develop over the next 6-12 months, after which the stem can be cut and planted into a growing medium
How to take a stem cutting
The best way to propagate edible herbs is by stem cuttings. This is done on softer herbaceous plants because they have a higher chance of rooting. Some woody plants can be grown from cuttings but it is significantly harder.
It involves taking a 3-5 inch cutting of herbs like rosemary, mint or basil.
The cut must be made below where leaves grow, called a node, this is where the plant hormone auxin will accumulate and begin to form roots. Cutting above a node is not advised because the stem will decay and can cause root diseases.
The leaves on the lower one-third to one-half of the stem are removed and the cutting is placed into a growing medium (or water). The cutting will need a few leaves to continue photosynthesising, but having too many will create a higher risk of drying out from transpiration.
Using rooting powder can speed up this process, it contains hormones that kick start the growth of roots. Dip the cutting in water and then the powder before placing into your growing medium.
When rooting a few stem cuttings at home, you can place them into plant pots or in wider flat trays, as long as they have holes in the bottom for drainage.
The growing medium used should be low in fertility, free draining and moisture retentive. It ideally stays moist but never waterlogged. A 1:1 mix of peat/coco coir and perlite should work fine. When initially rooting, the cutting relies on energy inside itself not nutrients in the soil.
You should place a clear plastic bag over the pots to maintain a high humidity, however make sure you prop it up with skewers or wire so the leaves do not touch the plastic. You can also use a bottomless water or milk bottle like I did. These can be easily removed to allow air circulation to the cutting.
A good sign that your cutting is ready to transplant is when it begins to produce new leaf growth. This indicates that the roots are strong enough to successfully grow new foliage.
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