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Horticulture 1000
H1092 - Plant propogation 2 - Encouraging root growth
Asexual propagation Choosing plant material for stem cuttings
2

Encouraging root growth

To successfully encourage a piece of stem or leaf to form roots, ideal conditions need to be provided. These can include:

  • using plant rooting hormones
  • using a growing media that is free draining and holds some water
  • providing the cuttings with a warm humid environment
  • using bottom heat where appropriate
  • providing protection from disease.

Hormones

Plants produce hormones. These are chemicals inside the plant that regulate its growth. They are mostly produced at the growing points in the plant. Some of these hormones promote the development of roots.

When you take a cutting, the plant sends the hormones called auxins to the cut stem to form new roots. Chemists have manufactured hormones, which have the same effect as the plant's natural hormones. These artificial hormones come in powder, gel and solution form. Some have a fungicide in them as well to help prevent fungal infection.

Using artificial hormones improves the chances of the cuttings forming roots. They come in different concentrations, called 1, 2 and 3. Each concentration is suitable for a different type of cutting.

The recommendations are:
  1 for softwood cuttings
  2 for semi-hardwood
  3 for hardwood cuttings.

How to use hormone rooting powder or gel

How to use hormone rooting powder or gel
  • Dip the base of the cutting in water.
  • Dip the base of the cutting into the powder and shake off any excess, or
  • Dip the base of the cutting into rooting gel.

Environmental conditions

If a cutting is to survive it must grow its own roots, otherwise it cannot take up water and will die. To encourage root development on cuttings you need to provide moisture, high humidity, warmth and light.

Growing  media for cuttings

You plant cuttings into growing material called media. Cuttings will root in a variety of media.

Characteristics of a good media for cuttings:

  • has good drainage
  • has air spaces
  • retains water
  • is clean and free of pests and diseases.

A number of different materials can be used in a propagation media such as peat, coarse sand or pumice.

These materials are either used by themselves or more commonly as mixtures. A common mix is 50% pumice and 50% peat. The peat will hold water to prevent wilting and the sand will provide drainage and aeration.

The media needs to be well aerated so a good supply of oxygen is available at the base of the cutting so the developing roots can carry out respiration. Good aeration can also help to prevent rotting at the base of the cutting.

Water needs to be available in the media. Cuttings don't have roots so it is more difficult for them to take up water easily. If they lose water faster than they take it up, then the cuttings could wilt and die.

Fertiliser is not needed in growing media for cuttings to root, because the cuttings have no roots therefore cannot take up nutrients.

Water and high humidity

Because cuttings have no roots they must be kept moist to prevent wilting. Commercial growers use mist units to provide high humidity. These spray a fine mist over the leaves, keeping the air around them moist.

Fogging is a form of misting system. A fogging nozzle breaks up the water into fine droplets that remain in the air for much longer periods than mist droplets. It keeps a very thin film of water over all surfaces of the cuttings and keeps a high humidity around them. The air around the cutting is humid so water is not lost from the leaves through transpiration.

A plastic bag over the cuttings creates a humid environment where water loss through transpiration from the cuttings is reduced

A plastic bag over, but not touching, a few cuttings will keep the air around the cuttings humid.

A plastic bag over the cuttings creates a humid environment where water loss through transpiration from the cuttings is reduced.

A misting system

A misting system.

Bottom heat

Where possible the base of a cutting should be kept warm while the top of the cutting around the leaves is kept cool. Bottom heat encourages cell division at the base of the cuttings.

Keeping the top of the cutting cool helps to prevent disease and slows down transpiration (water loss) from the leaves.

Commercial growers use bottom heat to provide a warm temperature at the base of the cuttings. The warm temperatures encourage cell division in the node at the base of the cutting.

Heating cables in sand Heating cables in sand.

Methods of bottom heating used include:

  • electric heating cables in a bed of sand
  • concrete benches heated with either hot air or water travelling through pipes under the bench
  • heating cables embedded in benches
  • electric heat pads.

Containers of plants are placed on top of these heat sources.

Light

Cuttings need enough light for their leaves to continue to carry out the process of photosynthesis to make food. The food is then broken down during respiration to provide energy for root growth.

The cuttings need plenty of light to carry out photosynthesis, but not direct sunlight as that could cause high water loss.

Shelter

Cuttings need to be placed in a sheltered position. Strong winds can move them around as they have no roots to anchor them in the media. They may also need shelter from strong light to prevent excess water loss and wilting.

Protection from diseases

The conditions that help cuttings form roots also help fungi to grow causing diseases like ‘damping off' and botrytis. These diseases attack stems and cause wilting.

This is how you prevent fungal disease on cuttings:

  • Take cuttings from a healthy parent plant.
  • Use clean, dry tools.
  • Make a clean cut, not a jagged or crushed cut.
  • Use a clean, aerated growing media.
  • Use clean containers.
  • Remove dead leaves or cuttings that die.
  • Spray or dip the cuttings in fungicide if you are a commercial grower.

Complete Activity 2A in your workbook

Key points   Key points

  • Cuttings have no roots so they must not dry out.
  • Most cuttings require humidity, warmth, oxygen, light, shelter and water.
  • Covering cuttings with a plastic bag can provide high humidity. Commercial growers use misting or fogging in a greenhouse.
  • Fungi, which cause diseases, also thrive in high humidity and warm temperatures, so commercial growers use fungicides on their cuttings.

To strike cuttings successfully:

  • treat the cutting with rooting hormone
  • use a growing media that has good drainage and retains some water. It must be clean.

What's next?

Go to: 3 Choosing plant material for stem cuttings.

 
Asexual propagation Choosing plant material for stem cuttings