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RHS Journals

The Garden
April 2005

At the root of it all

Encouraging severed stems to form roots, aptly known as taking cuttings, is a powerful propagation technique that allows large numbers of identical plants to be raised. In the second part of his propagation series, David Hide explains the processes and factors involved

Images: Tim Sandall & David Hide

Tools of the trade

Equipment for taking and establishing cuttings includes: secateurs (for collecting cuttings), an extremely sharp knife (for trimming cuttings and wounding their bases), plastic bags (to prevent them drying out), plugs of cutting compost (dry and flat plus wetted and expanded), black plastic module or cell trays (to hold plugs), synthetic rooting hormone (powder in white plastic lid and liquid formulation in plastic jug), notebook and labels (for keeping records), together with several examples of rooted cuttings

In the March issue I examined how to unlock the secrets of dormancy to germinate a range of seeds successfully. This month I explore the many factors that contribute to rooting a range of exciting plants from cuttings, the basis of much plant propagation. For success both with easy-to-root species such as Fuchsia and more difficult subjects like deciduous Rhododendron (‘azaleas’), understanding the interacting factors that control the process of producing roots will help propagators realise their goals.

Vegetative propagation

In most instances plants grown from seed will not be identical to the parent plant. The advantage of vegetative propagation methods such as taking cuttings is that by careful selection of material, the resultant plant will be identical to the parent from which it came (a clone). Most popular plants purchased from garden centres are named cultivars, and many are raised from cuttings, making this a vital technique for the nursery-stock industry.

Select cuttings from parent plants that are healthy and true to type (as far as possible resembling the original cultivar). Always ignore non-typical shoots, for example those showing variegation or different leaf forms (unless you observe an attractive ‘sport’ that seems worthwhile propagating).

In the past two decades much work has been done on identifying the right type of cutting for successful propagation. Cuttings taken from lower down a plant, closer to the root zone, appear to root more easily than those taken from higher up. Likewise, shoots that have grown in partial shade and have been subject to etiolation (growing tall and thin in the search for light) and are naturally thinner again appear to root more readily. In most circumstances, using thinner cuttings is preferred. Providing that they are rooted in optimum conditions, thinner cuttings will root more quickly than thicker ones. This is probably because while both have the same amount of food-producing leaf surface area and manufacture food reserves at the same rate, thin cuttings require less energy to maintain themselves and so any excess food can be directed to the rooting zone.

Many plants reproduce vegetatively in the wild, from suckers, layers and even natural grafts. In making cuttings we are adding an element of control to the process. All plants produce chemicals, types of hormones, that facilitate rooting, the most common of which are auxins. By understanding how these hormones work, the propagator can improve the proportion of cuttings that take root, the ‘strike rate’.

Cutting types

Untrimmed (left) and trimmed (right) heel cuttings of ceanothus 1.

Preparing a nodal-tip cutting of holly 2.

Applying liquid rooting hormone to a camellia cutting 3.

Semi-ripe cuttings of rosemary and santolina in a cell tray root more quickly with regular misting 4

Descriptions found in textbooks of different types of cuttings can be confusing. These range from softwood through to hardwood and include nodal-tip, inter-nodal, stem-sections or leaf-bud. Cuttings can also be taken with or without a ‘heel’.

Softwood cuttings are taken from the first flush of growth appearing on both deciduous and evergreen plants. Deciduous trees and shrubs such as Deutzia, some Acer palmatum cultivars and Hydrangea root relatively easily when the material is ‘soft’, but these cuttings need a humid atmosphere as they can wilt and quickly die in dry conditions.

Evergreen plants are usually rooted later in the season from stems that are beginning to go woody; this firmer material is described as ‘semi-ripe’. It is also possible to take hardwood (of older, woody stems) cuttings of a range of deciduous and evergreen plants. As you progress from soft through to hardwood, the length of time taken to root increases, but the level of sophistication needed to maintain the rooting environment reduces.

Ease of rooting varies markedly between and within genera. Acer palmatum (Japanese maple) requires a warm, humid environment out of direct sunlight, in a closed case, or using mist or fog systems to have a chance of rooting. Some cultivars are so difficult to root they have to be grafted. By contrast, hardwood cuttings of willow pushed into the ground in autumn will root over winter.

Nodal-tip cuttings are the ‘Rolls Royce’ of plant propagation and are the type of cutting recommended for the best chance of success with a wide range of material. They consist of a growing tip with 6-10cm (2.5-4in) of stem, trimmed just below a pair of leaves or node (2). Such cuttings retain both the factory that produces rooting hormone, the growing tip, and the point at which such hormones concentrate, the node. These two factors make nodal-tip cuttings the best option for rooting difficult subjects such as Rhododendron, Pittosporum, Garrya, Rhamnus, Fremontodendron and Escallonia bifida.

At Wisley we have discovered an interesting variation on nodal-tip cuttings. With a sharp knife we split the bottom 2cm (0.75in) of cuttings of Edgeworthia chrysantha (paper bush), a plant we had hitherto failed to root, in two. They rooted in a fortnight.

A wide range of plants root much more easily and so the type of cutting is less important. For further guidance on ease of propagating a range of plants refer to a guide such as RHS Propagating Plants (see further reading). In species where the nodes are either close together, such as heather or rosemary 4, or far apart, as in Verbena and Lavatera, nodal-tip cuttings are unneccessary. Internodal or leaf-bud cuttings can increase the number of plants that can be propagated from available material, and often produce better-quality, bushier young plants.

For plants that are considered more tricky to root, retain a small piece of older wood from last year’s growth at the base of the cutting. Such ‘heel cuttings’ are often used to root Camellia and Ceanothus, but can be used for any plant you are finding difficult. Trim the heel to no more than 3mm (1/8in) of last year’s growth (1, untrimmed left, trimmed right). This harder wood is less likely to rot and should have higher levels of naturally present rooting hormone. Used correctly, artificial rooting hormones 3 can greatly improve success rates in all forms of cutting.

Propagation environments

The care given to cuttings during the critical phase of forming roots is just as important as choosing the type of cutting and preparing it well. Understanding the requirements of particular species is all-important and should determine the propagation environment. Cuttings of cacti placed in a moist, humid environment will fail: the cell structures will rupture and the cuttings will collapse. Cacti root best in a drier environment more like the conditions to which they are adapted.

The key to creating the best environment for rooting is to understand the relationships between temperature, light, moisture, oxygen and carbon dioxide. By providing cuttings with a warmer soil than air temperature (‘cool tops and warm bottoms’), the biochemical processes related to rooting speed up. Too warm an air temperature however, often caused by high light levels, reduces humidity and increases the rate of transpiration (moisture loss) of cuttings. In extreme cases, this can lead to wilting and then death. Shading can help, but light levels still need to be high enough to allow photosynthesis to replenish the limited food reserves of the cutting and fuel the rooting process.

Water maintains the turgidity of a cutting and transports nutrients, oxygen and carbon dioxide, all vital for plant growth. The availability of carbon dioxide often limits photosynthesis, so a degree of air movement from ventilation can help, but too great a flow of air will also reduce humidity.

Roots need oxygen too, so success rates can also be improved by adding perlite or bark to seed-and-cutting composts, giving a more open, better-aerated medium. Cuttings in waterlogged compost will rot even if other factors are optimal.

In summer the primary consideration is usually to keep humidity high, particularly if propagating large-leaved deciduous trees and shrubs. The best rooting environments for many plants combines high humidity and light levels with intermittent misting of foliage. This prevents wilting, encourages food production and keeps cuttings cool as moisture evaporates from their foliage.

Equipment such as heated propagators, cold frames, soil-warming cables and misting or fogging units can all be useful, particularly with difficult-to-root species. However, many easier subjects can be successfully rooted in an unheated propagator or even a polythene bag stretched over a pot, and regular hand misting 4 can be just as effective as an expensive automatic mist unit.

Rooting cuttings can be easy or frustrating depending on the subjects. With difficult species, it can be a matter of trial and error to find the best propagation environment, but this makes success more satisfying.

Deciduous rhododendron cuttings

For best results with deciduous rhododendrons take cuttings in April or May, when the new flush of growth reaches 10cm (4in) in length. Cut at the point when the softwood has just begun to firm 5.

Collect cuttings early in the morning when plants are fully turgid and place in polythene bags to reduce wilting. Store in a fridge if you are unable to make the cuttings immediately. Using a sharp knife make 6–8cm (2.5-3in) nodal-tip cuttings, wound the bottom 2cm (0.75in) 6 and apply a rooting hormone to the bottom centimetre. Retain the tip and reduce the number of leaves (7, right).

Insert into an open, low-nutrient, ericaceous cuttings compost 8 and place into a humid environment. The foliage scorches easily so regularly misting improves rooting, or shade the cuttings with opaque polythene.

Maintain bottom heat at 15–20°C (59–68°F) and place under milky polythene or ideally on a mist bench 9. If
you have hit the ‘propagation window’ and managed your environment well, the cutting should root in eight to 10 weeks.

However, it is important to obtain a small amount of growth from rhododendron cuttings and a well-developed root system 10 ahead of winter, otherwise the rooted cuttings often fail to grow away the following spring. Extending the season using artificial lighting can give the cuttings vital extra maturation time before they shut down for winter.

Rooting hormones explained

Indoleacetic acid (IAA), an auxin, is probably the most widespread hormone that controls rooting in plants. It is manufactured in the growing tips of shoots, which is why it is important with difficult-to-root plants to retain the tip. Often, plants only produce IAA for limited periods in the growing season, giving a narrow ‘window’ for propagation by cuttings. Deciduous rhododendrons are a classic example, only producing IAA in sufficient quantities to facilitate rooting for a few weeks in spring.

Cuttings have limited food reserves, and in protected rooting environments are prone to attack from disease. Applying a synthetic rooting hormone should make cuttings root faster, producing more, larger roots. The most popular formulation is indolebutryic acid, which closely resembles IAA and can also extend the window for successful propagation.

Rooting hormone actually inhibits plant growth in tissues outside the root, and naturally moves away from the growing tip to the base of plants. Most rooting hormone is applied to the base of cuttings, but some propagators also apply it to the growing tips, replicating what happens naturally and forcing cuttings to develop roots while holding shoot growth in check.

Flowering stems attract rooting hormones to them, acting as a ‘sump’ so it is always best to select non-flowering stems. If this is not possible remove flowers and buds when preparing cuttings. Wounding the base of a cutting by removing a sliver of bark (no more than 2cm/0.75in in length) enables it to absorb more moisture and rooting hormone, and increases the surface area on which roots can form. It is easy to apply the wrong rate of rooting hormone (which has a limited shelf life - store in a cool, dry environment away from direct sunlight): too much and the cutting will die, too little and it may fail to root.

David Hide is Senior Supervisor within the Glass Department at RHS Garden Wisley

 

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