Plantconnection

How adventitious roots form

This is where the science jargon starts to kick in, so we’ll try and keep this as simple as possible! Basically, adventitious roots tend to form from cambium cells. These are the cells that surround the water and nutrient carrying vessels in the stem and cause the stem to thicken during growth. As a result, the new roots are perfectly positioned for accessing these nutrients as they develop.

The actual growth of the new roots is triggered by auxins (growth hormones). All gardeners should be familiar with auxins as they are the cause of root and shoot growth in all plants. Natural auxins accumulate at the site of the cutting and cause the development of new roots from the cambium cells.

For the majority of cuttings we recommend that you use a rooting powder to help stimulate the development of adventitious roots. These powders are inexpensive and available from all garden centres. They contain synthetic auxins that work alongside the natural auxins in the plant to trigger root development.

Stem cuttings

Stem cuttings can be taken all year round however the timing does determine how the cuttings should be treated.

In spring it is possible to take cuttings from new growth, either from the tips of new shoots or from basal growth near the foot of the plant. Cuttings taken from new growth at the start of the season are normally referred to as softwood cuttings and usually develop roots more easily than cuttings taken later in the season.

However, although it is normally easier to start softwood cuttings off, they do require a lot of care to prevent them from wilting. The cuttings will need to be propagated in either a greenhouse or a propagator. You will need to provide some form of shading to ensure that the plants are protected from extremes of temperature and you will also need to ensure that there is adequate ventilation. However, bear in mind that as the cuttings don’t yet have their own root systems it is very difficult for them to take up water and you’ll need to ensure that their environment is kept humid to help reduce water loss. The key is to maintain a balance between adequate air movement and humidity, while trying to keep the environment as constant as possible.

Greenwood cuttings are taken from early to mid summer as growth starts to slow. They don’t produce roots as easily as softwood cuttings but do tend to survive better. You will still need to propagate them within the controlled environment of a greenhouse or propagator though!

In late summer it’s possible to take semi-ripe cuttings with firmer stems. While it is usually harder to persuade semi-ripe cuttings to produce roots, they are easier to look after once the roots are formed as they don’t wilt as easily as softwood or greenwood cuttings.

From autumn through to spring, stem cuttings tend to fall into the bracket of hardwood cuttings. These can take time to develop roots, but are the easiest to maintain.

The best time of year for taking cuttings varies dramatically depending upon the nature of the plant. Success is often a matter of ‘try and see’ and if your cuttings fail at one time of the year it is definitely worth trying again later on. Some varieties for instance are reluctant to produce new roots, and these are the ones you should propagate early in the season. Cuttings from thick leaved evergreens such as holly are more difficult to propagate from new growth as the thick waxy coating on the leaves has not yet developed and they wilt more easily. These varieties should be propagated from hardwood cuttings. Experimenting to see which plants you should try and when is all part of the fun!

How to take and prepare stem cuttings

Stem cuttings are normally taken just underneath a node (leaf joint) on the plant stem. This is because the nodes are the sites where the cambium cells are most active and will produce the best adventitious roots. Where leaves are densely crowded on the stem you can take the cutting between two nodes (internodal cutting).

Ideally you should leave five or six nodes along each cutting. The actual length will therefore vary according to the variety, but generally this will be around 5 to 12cm (2 to 5 inches). The lower leaves of the cutting should be stripped to leave a length of stem that can be inserted into the growing medium. Remember to add rooting powder to the exposed stem before planting.

If you are planting semi-ripe or hardwood stems, it is worth removing a sliver of bark from the bottom of the cutting to expose more cambium cells. Make a diagonal cut around 2.5cm (1 inch) long. More of the cambium helps the formation of adventitious roots. A similar effect can be achieved by pulling away side shoots from ornamental shrubs, leaving a ‘heel’ with exposed cambium cells.

Hardwood cuttings have sufficient food reserves from previous season’s growth to sustain the plant until roots are formed. However younger cuttings will still need to produce energy via photosynthesis. For this reason, you will need to keep some leaves on your cuttings, however the drawback of this is that water is lost mainly through the leaves by transpiration. To help reduce this water loss, you will need to remove as many leaves as possible, leaving a minimum of three full sized leaves. If these leaves are themselves large, you can trim them down to around half their size. This ensures that there is still enough photosynthesis taking place to supplement the lack of food.