The Propagation of Banksia
There are about 75 species of Banksia in Australia1. They are all woody evergreen plants, ranging in habit from prostrate shrubs to trees up to 30 metres tall. About 80% occur in Western Australia only. One species, B.dentata, extends from northern Australia into Papua New Guinea, Irian Jaya and the Aru Islands.
Tree forms usually have a single trunk, while shrub forms have one or more stems at ground level. A number of species can regenerate from a woody stock (lignotuber) situated at the base of the stem at or just below ground level. Growth from this region is stimulated by destruction of the above-ground parts of the plant, usually by fire. Other species regenerate from seed released spontaneously after death through fire or other causes.
Some others (eg. B.marginata, B.integrifolia and B.ornata) have swollen nodes containing dormant buds as much as 10 metres from the parent plant. These occur on long, thin surface roots and remain dormant as long as the parent plant is growing vigorously. Damage such as fire or drought will trigger these nodes to shoot, and what appear to be seedlings around burnt or damaged plants are in fact root node growths.
Methods of Propagation
The above are “natural” methods by which banksias are propagated – ie. where there is no human involvement. There are, however, a number of other methods by which they can be grown. These are:
- Seed – The most common method for both the professional nurseryman and home gardener. It is the simplest and cheapest way of producing new plants. Seed of most species is generally available, and remains viable for some years if stored properly. However, it has the disadvantage of not always producing plants with identical characteristics of the parent plant, due to hybridisation. Generally, however, banksias are much more stable in this regard than, say, callistemons, grevilleas or eucalypts.
- Semi-hardwood Stem Cuttings – Also a fairly simple process, but not as common a process as propagation by seed, and not as widely used, but has the advantage of replicating all the characteristics of the parent plants. This may be important if consistency is required in particular features of plants, such as flower colour or leaf form. Also has an advantage in that cutting grown plants flower earlier in their life than seed grown ones. This is especially important in the flower growing, fruit production (eg macadamia nuts) and retail nursery industries.
- Grafting – The least common method, but often the only way in which some difficult-to-grow western species can survive in conditions in the eastern states. Also is more time-consuming and requires more specialised techniques. Banksias are renowned for the magnificent flowers they produce, and are popular as ornamental plants and for cut flowers. This is the major reason why efforts are being made in eastern Australia to perfect the art of grafting western species onto hardy eastern rootstock. The inconsistent results achieved so far mean, however, there is still plenty of work to be done.
Propagation from seed and cuttings are the more reliable methods provided the correct techniques are applied and proper aftercare given.
One interesting point that came out of discussions I had with one experienced banksia propagator during my research was his very limited use of fungicides and insecticides at all stages of the propagation process. His results have still been very successful, so perhaps the books which advocate virtually throwing chemicals at the plants need some fairly hefty revision, and give more emphasis to good cultural practices.
Descriptions of each of the techniques used for the above-mentioned methods are provided in the following sections.
Propagation from Seed
A “seed production” area can be established if desired. This is an area containing a group of shrubs or trees that have been identified or set aside specifically as seed sources. The value of the area can be improved by removing those plants that don’t meet minimum quality standards, eg. unwanted flower colour or leaf form.
Choose the best individual specimens. Just as human characteristics such as hair colour can be inherited by offspring, some characteristics of a plant can also be passed on. Highly inheritable characteristics of a plant include stem form, branching and, to a lesser extent, growth rate. However, selection of suitable plants in natural stands is not always reliable.
Some, although genetically sound, may be of poor appearance due to factors such as fire or flood damage and competition between plants for water and nutrients. Some useful guidelines are:
- Collect seeds only from healthy, vigorous specimens of desirable form.
- If possible, don’t collect from isolated specimens, as self-pollination often yields low quality seeds.
- To encourage genetic diversity, collect similar quantities of seeds from several well-spaced specimens about 100 metres apart. This encourages genetic diversity, and the more specimens sampled the better. This spacing may be reduced where specimen numbers are small.
- For most purposes seeds obtained from different trees or shrubs of the same species from the same geographic region can be mixed. However, if you are uncertain about tree/shrub identity or seed quality, keep the seeds from individual specimens separate until identity and viability are checked.
- For those species that retain their seed indefinitely, it is essential cones are not collected too early after development of the follicles, otherwise the seed inside will be immature and therefore unviable. It is recommended the cone be at least 12 months old. A good indication of maturity is to scratch the surface of the follicles. If they are brown and hard the cones are ready for collection; if they are soft and green the seeds inside are immature.
- For those species that release their seed upon maturity, collection can be difficult, but can be achieved by tying old stockings or muslin over the cones while they are still on the plant and leaving them there until the seeds are released. The cones will not be damaged by this method as air can flow freely through the material.
Several methods can be used:
- Place the cones on a metal tray in an oven at 120 – 130oC for one hour, or 200oC for 10 minutes, until the follicles begin to open. When the tray is cool the seed may be tapped out or extracted with tweezers (being careful not to mistake the separator, which is woody, for the seeds, which are usually black with a brown papery wing).Note: Microwaving the cones is not a good idea, as the only thing that will be achieved by this method is “cooking” of the cones, rendering them useless.
- Place the cones on an open fire and remove the seeds when the follicles open, taking care not to spill the seeds into the fire in the process.
- Alternately soak the cones in water then heat in an oven until the follicles open, then extract with tweezers.
If not sowing them immediately, the seeds should be dusted with an insecticide such as carbaryl, and a fungicide such as Captan or Thiram, and then placed in sealed, labelled, glass, plastic or metal containers for storage away from direct sunlight and away from vermin, at room temperatures of between 10 and 25oC and relative humidity of between 40% and 60%.
Generally Banksia seeds require no special treatments prior to sowing in order to overcome any inhibitors to germination, such as a hard coat (testa) or chemicals occurring in the testa or endosperm that have to be leached out. However, several species that occur in mountain areas where there are many frosts require stratification (ie wet, cold treatment) before germination can take place. These species are B.canei, B.saxicola and some sub-alpine forms of B.marginata.
Stratification can be artificially achieved by placing the extracted seeds on damp sand in a container in a refrigerator at about 5oC for 60 to 120 days prior to sowing.
There are several factors to consider:
- The propagating media
- needs to be slightly acid (pH about 6.0), open, free draining but also retain moisture. A suggested mix is the one used at Burnley Horticulture College in Melbourne, which is 2.5 parts of a blend of fine and medium grade pine bark, 2.5 parts of medium grade pine bark, 0.5 parts of coarse sand, 0.5 parts of sieved peat moss and 1110 gms of dolomite per 0.5 cubic metre of mix.
- needs to be free of pathogens. Ideally the mix should be sterilised. This can be done by pumping steam heated to 60oC through the mix for 30 minutes.
- should be maintained at between 20 and 25oC, which is the optimum range for germinating banksia seed.
- The propagating area should be well ventilated, as an adequate oxygen supply is important in triggering reactions within the seeds to stimulate germination.
- The mix must be kept moist but not wet at all times during the germination process.
The sowing method itself is straightforward:
- Select suitable cleaned or brand new container/s, which should be relatively deep, so that water can drain through the media and not sit at the top to waterlog (ie kill) the seeds. The containers can be narrow, deep tubes into which single seeds can be sown (eg to be subsequently used in reforestation programs) or larger pots or trays into which a number of seeds can be sown.
- Fill the container/s with the media and firm down, leaving room at the top for water catchment.
- Water the media to field capacity.
- Sow the seed (not too thickly if the container is to house a number of seeds), firm down and just cover with a layer of fine media or vemiculite, and then thoroughly water them in with a fine spray, so as not to dislodge them.
- Place container/s in a greenhouse, where optimum temperature and moisture conditions can be provided in a controlled environment. This can comprise a misting system and thermostat-controlled bottom heat through a bed of coarse sand heated by resistance wires. Alternatively, they can be placed in a warm, sheltered position such as in a cold frame, and kept moist.
- Germination times after sowing vary with species and germinating conditions, but expect results to show within three to six weeks.
Damping-off of the seeds can be a major problem if not controlled. The fungi responsible are Rhizoctonia and Pythium, and they cause the stems to rot at the base and the consequent collapse of the seedlings. An application of furalaxyl (Fongarid) or Captan fungicide sprays at germination time should give adequate control.
The fungal spores are dispersed by splashed water, so an alternative or complementary method of control would be to water the seedlings from below instead of above by means of a capillary watering system. One suitable method is to place the seed trays or pots on a bed of sand and apply water to the sand only. The water is then taken up by the media through the drain holes of the trays/pots by capillary action.
The germinated seedlings rapidly develop a taproot and a fibrous root system. Plants germinated in community trays or pots should be transferred (ie “pricked out”) into individual tubes or pots no later than a few weeks after germination, otherwise the roots are prone to damage when pricked out. The pricking out medium can be of similar formulation to the germination mix.
If the seedlings have been grown in a sheltered position such as a propagation house or frame, they must be hardened off for 3 to 5 days before transplanting, otherwise they could suffer damage (fatal) through a too rapid change in growing conditions. Hardening off could merely consist of shutting off the bottom heat and misting system, slightly opening the lid of he cold frame, or moving to a slightly less sheltered.
Following pricking out, place the seedlings in a mild, sheltered position or under shade cloth for up to 2 weeks to continue the hardening-off process to the stage where they will able to withstand open conditions. During this time they should be kept moist and given half-strength fertiliser very low in phosphorus (banksias are susceptible to phosphorus (P) toxicity, as they are very efficient at extracting P from normally-deficient soils through root structures called proteoid roots. They can easily take in too much P if more than a small amount (> 4.0) is applied to the growing media).
After this period they can be placed in a sunny position where they will grow into strong young plants provided an adequate watering and fertilising regime is maintained. A slow release native plant fertiliser such as nine month Osmocote applied in spring will provide all the plants’ nutritional needs during the growing season.
Semi-hardwood Stem Cuttings
Selection of Material
Many of the fine-leafed banksias and some of the coarse-leafed species are propagated from cuttings. Species commonly grown this way are B.ericifolia, B.marginata and B.spinulosa. However, some species do not lend themselves very well to cutting propagation because they can form poor root systems, eg. B.quercifolia, B.speciosa and B.victoriae. Seed propagation should be employed for these species to ensure development of a strong root system.
Cuttings from leafy semi-hardwood stems are taken, and glabrous species are generally considered to be more amenable than species with hairy stems and foliage. “Semi-hardwood” could be defined as growth that is supple but not too soft. This occurs around February/March when the spring/summer flush of growth has not fully hardened.
Select material that is healthy and free from obvious disease or distortion, otherwise there could be disastrous consequences. Firstly, infected cuttings are more difficult to strike, and secondly the infection could easily spread to other healthy material in the propagating area.
In general, the source of cutting material should be known, managed plants in the juvenile stage of growth. Material from these form roots more easily than those from the adult phase.
Material taken from plants in bushland is not recommended because:
- It is often illegal to take cuttings from such plants.
- They may carry more pathogens and viruses than managed plants because they are not under the same care as nursery or garden-grown stock.
- In many cases “bush” material is much harder (because of the conditions under which it grows) and consequently more difficult to strike.
- Because growth of garden or nursery plants is easier to observe, it is easier to judge the best time for collecting material.
Ideally, collect material early in the morning before the sun has been on the plants. The plant is at its best at this time because sap flow to stems is at its peak, having been replenished during the night.
Avoid taking cuttings from branches that are near the ground. They could be contaminated by spores of the fungal disease Phytophthora cinnamomi, which is a soil borne disease that causes root rot in a wide range of native plants, and can be devastating to banksias, especially those from Western Australia.
Use clean, sharp good-quality secateurs for the task. Sharp implements make a smooth cut. Blunt ones can tear and bruise tissue on the cutting material and parent plant, and provide point of entry for pathogens. They can be cleaned with a germicide or disinfectant such as 2% sodium hypochlorite, methylated spirits or “Dettol”.
Take cuttings from between 8cm and 16cm long, just below a node (which is where the adventitious roots on the cutting will form). The cuttings must not be allowed to dry out, and should be processed as soon as possible for propagation, otherwise the strike percentage will be greatly diminished. Clean plastic bags can be used to store material for short trips; for longer trips, cuttings can be wrapped in moist newspaper and placed in plastic bags then stored in a car fridge or similar.
Preparation for Processing of Cuttings
To minimise the risk of contamination by pathogens, everything must be clean:
- Propagation benches must be wiped down with disinfectants such as “Dettol”, “Biogram” or sodium hypochlorite.
- Secateurs must be regularly dipped in Dettol, methylated spirits or sodium hypochlorite.
- The containers into which cuttings are to be placed must, if they are not brand new, be washed with hot water and detergent and then sterilised with heat (60oC for 30 minutes) or a chemical such as “Dettol”, sodium hypochlorite or methyl bromide.
- The propagating media should be sterilised as for the propagation by seed.
- Wash hands regularly with hot water and soap.
The mix itself should be open (about 25-30% air-filled porosity), slightly acid (6.0 pH), free draining yet also retain some moisture. An aerated mix is vital, as oxygen plays a major role in the process of cuttings forming callus and roots. A suitable mix is the one that Burnley uses, which consists of a blend of 100 litres of perlite, 40 litres of sieved peat moss, 360 litres of medium grade pine bark and 375 gms of dolomite per half cubic metre. There are many other mixes of various materials that could be used, as long as they have the above properties.
A bath of 2% sodium hypochlorite, “Dettol” or “Hibitane” should be prepared into which cuttings can be immersed prior to preparation for insertion into the media. This is used as a general disinfectant.
Prepare a container of rooting hormone, which promotes the development of adventitious roots on the cutting,into which cuttings can be dipped prior to placing in the propagating mix. A suitable substance is IBA (indole butyric acid) at 3,000 parts per million. IBA is a synthetic auxin manufactured by several companies. (Note: Use of a rooting hormone on Banksia cuttings has caused some debate, and there is a school of thought which says they are unnecessary and could even be lethal. So use such substances with caution. Perhaps treat half with and half without and observe the results.)
Use relatively deep pots or trays (better for drainage) and place the propagating mix into them to level with or just above the top.
Processing of Cuttings
- Immerse the cutting material in the prepared disinfectant bath for two or three seconds, then rinse in a bath of clean water.
- Remove the leaves on the lower half to two thirds of the cutting, taking care not to damage the bark. If large leaved species are being propagated (eg B.robur), remove half of each of the remaining leaves to reduce water loss through transpiration.
- Dip the cutting in the prepared rooting hormone (if you are using it) and shake off the excess.
- Use a dibbler to make a hole in the propagating medium, and insert the cutting half to two thirds of the way into the medium and firm it down.
- Repeat the process for each cutting. A reasonable number of cuttings can be placed in the one container, but don’t overcrowd, as it could increase the risk of infection by pathogens due to decreased aeration and increased humidity. Cuttings touching one another could also assist the spread of pathogens.
- Thoroughly water the cuttings in.
Environmental Conditions for The Successful Rooting of Cuttings
The cuttings will strike successfully if the following environmental conditions are provided:
- Adequate watering, either through an automatic or manual watering system, or through the agency of an automatic misting system. Misting not only keeps the propagating mix moist, but places a film of water on the leaves of the cuttings, creating a humid atmosphere and reducing the temperature of the cuttings.This lowers transpiration and promotes root development, as ideally the tops of cuttings should be maintained 5 – 10oC cooler than the base.
- Correct Temperature; which should be maintained in the range of 20 to 27oC during the day and 15oC at night for best results. As mentioned earlier, the base of the cuttings should be 5 to 10oC warmer than the top, so the application of bottom heat by heated cables/pipes through sand would be beneficial. This promotes root development before bud growth. The growth of buds before the development of roots can have a detrimental effect on the success of cuttings.
- Aeration; not only within the mix as explained earlier but to allow respiration to occur in the leafy parts of the cuttings. Air movement around the cuttings also inhibits the development of fungal and mould growth, so adequate ventilation of the propagating area/structure is very important.
- Light; a balance must be achieved between having sufficient light available for photosynthesis to occur in the cuttings, and having too much, which can lead to an undesirable rise in temperature to the point where cuttings may desiccate and cause leaves to become pale and bleached through greater nitrogen use. Careful observation will be necessary to maintain the correct ratio.
The length of time taken for cuttings to strike will depend on the species, but generally if the proper treatment and environmental conditions have been provided roots should develop successfully within 8 to 10 weeks.
I can verify this was the case for B.marginata cuttings I collected in late April this year and placed in a foam box under a clear plastic sheet in my backyard receiving morning sun. One of those cuttings is now growing away happily in my garden (I gave the others away) after striking successfully within the above time frame.
It is essential for struck cuttings to be gradually exposed to the full range of outdoor conditions. Too sudden a change from the protected warm, humid conditions under which they have been growing will result in damage to the soft, tender new growth.
If the cuttings have been propagated under mist and bottom heat, turn off the systems a few days prior to removing the cuttings from the propagating structure, watering only to keep them moist. Move them after this time to a sheltered location, such as under shade cloth.
If neither mist or bottom heat has been used, move the cuttings to a sheltered location such as a cold frame or shade house and immediately pot them on into individual pots.
Where cuttings have been struck in individual tubes, move them to a cold frame or shade house, where they should be watered and potted on if desired.
Following the hardening off period, the cuttings not already potted on can now be done so. It is recommended they be treated with a fungal drench such as “Terrazole” and a root stimulant 7 to 10 days prior to this process.
There is a wide range of potting mixes that can be used at this stage, but whatever mix is used it must have the following properties:
- Good aeration (oxygen is important for good root growth).
- Good drainage, yet holds sufficient water for plants to utilise.
- Holds adequate nutrients for plant growth.
- Free of organisms detrimental to plants.
A suitable mix is one consisting of fine pine bark, coarse sand and peat moss.
Cuttings rooted in community pots or trays are best potted on as soon as possible to avoid damage from their roots becoming entangled with other cuttings. A prior thorough watering will make the job of disentangling any roots easier, and ensure the roots and leaves are in good condition for the potting process.
Prune the roots if they are too long for the new pots, and follow up with a good watering, which can include a fungal drench and root stimulant if desired. Place the pots in a sheltered area until the plants have recovered from the disturbance and new root growth has commenced, which may be within 4 to 7 days. Shadecloth can be used to provide some sun protection during this time.
Following this, they are ready to be placed in the desired area for growing on to mature plants. Maintain a good watering and fertilising regime, but remember to go easy on the phosphorus, as banksias don’t like high P (which to them is anything over 4.0. A nine month pellitised fertiliser suitable for native plants would be OK to apply at half recommended rates, as would “hoof and horn”.
Generally, Banksia species native to south-western areas of Western Australia will not grow well, or at all, in those eastern parts of Australia subject to high summer humidity and heavy summer rain – conditions foreign to banksias which enjoy the Mediterranean climate of Western Australia. These species dislike high humidity and are susceptible to attack by Phytophthora cinnamomi, a fungus which causes root-rot, and conditions in eastern areas unfortunately favour its growth.
Over the last 15 years or so, members of the Society For Growing Australian Plants and the Australian National Botanic Gardens in Canberra have been experimenting with various grafting techniques, using hardy eastern Banksia rootstock, in order to overcome this problem.
However, further work is needed, as success to date has been patchy. If perfected there will be the opportunity for commercial stands of banksias to be established to service the cut flower market. Several hundreds of thousands of western Banksia flowers, especially B.coccinea, B.prionotes and B.hookerana are imported to eastern Australia annually from Western Australia, so I would expect suppliers over there would be watching grafting trials with some anxiety.
Some of the scion/rootstock combinations that have been tried to date include:
|Rootstock||Scion (On any of the rootstocks)|
|B. integrifolia||B. coccinea, B. baxteri|
|B. serrata||B. brownii, B. grandis|
|B. spinulosa||B. prionotes, B. hookerana|
|B. marginata||B. sceptrum, B. pilostylis|
|B. ericifolia||B. baueri, B. lehmanii|
Grafting techniques that have been tried (with varying success) are:
- Approach – Stock and scion plants are placed close together. A thin slice of tissue is taken out of each plant in the area where they are to be joined. The cut areas are then tied together with grafting tape and the plants left united until callus forms, usually within 3 to 6 months. After callus formation the stock is cut back gradually before the scion is severed below the graft.A variation of this technique, called the “irrigated approach graft”, has also been tried with some success. This involves preparing the stock and scion as for the approach graft, but then severing the scion below the graft and placing it in a container of water until the graft takes. The use of mist is not required for this method.
- Wedge – Used where the stock is of a larger diameter than the scion. Remove a vee section from the top of the stock and continue down in a vertical cut for 2 to 3 cm. Cut shoulders on the scion to match the cut in the stock. Insert the scion into the stock, ensuring cambium layer of the scion is in contact with cambium layer of the stock on at least one side.Bind the graft with grafting tape and place under mist if possible, or loosely drape with a plastic bag to increase humidity.
- Cutting – The wedge grafting technique is used as per above. The bound stock/scion combination, totalling about 15 to 20 cm in length is started as a cutting under mist until roots have formed, and is then potted on.
- Cotyledon – Seedlings of the stock and scion plants are germinated so that they reach the grafting stage at the same time. When the stock seedling has developed 2 or 3 true leaves the stem is cut off above the cotyledons and a split made vertically between them.A young scion seedling is cut off at the growing tip (usually about 5mm long) and the base shaped into a narrow wedge and inserted into the split in the rootstock. The graft is held together with self-adhesive medical tape (eg. stericrepe) and kept under mist or bell jar until a union has formed. This could be in as little as one or two weeks.
- T-Budding – Make a vertical cut about 3 cm long through the bark in the stock. Make a horizontal cut above this cut about one third around the stock. Twist the cutting implement to slightly open the two bark flaps.Starting from about 1.5 cm below the bud to be propagated, make a slicing cut under and about 2.5 cm beyond the bud. Then make a horizontal cut to connect with the sliced section, and remove the bud. Push the bud into the flaps of the bark on the stock plant and position evenly, then bind with grafting tape.
(Note: I would have thought the Whip and Tongue graft, which is a very common grafting method, would also have been used. However, none of the publications I researched mentioned it. Perhaps Banksia wood is too soft for it to be practical.)
The main points relating to establishing a good graft union are:
- Collect scion wood as close a time as possible to the grafting operation, preferably within 1 hour, and keep moist.
- Ensure the scion and rootstock are disease and pest free.
- Maintain a high standard of hygiene around the plants and work area, including all tools used.
- The closest fit possible made between stock and scion.
- Matching of cambium layers of stock and scion, otherwise the graft will fail.
- The grafting operation must be performed at a time when the stock and scion plants are in the proper physiological state, ie growing actively. Temperatures should be within the range of 15 to 30oC. Outdoor grafting operations should therefore take place in the summer months. Where grafting is performed in greenhouses, the temperature can be more readily controlled.
- Proper care of the plants following grafting, ie mist in most cases for a short time (no longer than 14 days according to a study performed by Macquarie University in 1986), and/or application of grafting wax to the graft area to minimise desiccation.
- Compatibility of rootstock and scion.
Compatibility has been the cause of much frustration. Certain combinations will grow normally for a few weeks, months or even years, but then the scion will suddenly die or break off at the graft point.
Some combinations unite, but abnormal symptoms such as dwarfed or stunted growth, yellow foliage or tissue breakdown at the graft union soon become apparent. Such combinations usually last for a time, then die. Surprisingly, some closely related species which would be expected to graft readily either fail to form a union or unite only in very low percentages. As there is no short cut to predicting the success of a graft combination, the answer can only be found by more exhaustive trials.
Grafting attempts themselves are not new, as I read during my research that a grafted specimen of B.solandri was growing very well on B.integrifolia rootstock in Kew, England during the 1880s.
It would appear B.integrifolia rootstock shows the most potential. This is not surprising, as this species thrives right along the eastern seaboard, but as yet no-one can say with any certainty they have perfected the art of grafting banksias.
Elliott, W. Rodger, and Jones, David L. , Encyclopaedia of Australian Plants, Introductory Volume (Volume 1) (1980) and Volume 2 (1982) – Lothian Publishing Co.
George, Alex S. (1987) – The Banksia Book (Second Edition), Kangaroo Press P/L
Wrigley, John W. and Fagg, Murray (1989) – Banksias, Waratahs and Grevilleas, William Collins Publishers
Banksia Study Group – Reports no. 5,6,7,8 – The Society For Growing Australian Plants
Greening Australia – How to Collect Native Tree Seed Easily (pamphlet)
Hartmann, Hudson T., and Kester, Dale E., (1983) – Plant Propagation: Principles and Practices (Fourth Edition), Prentice/Hall International Inc.
- This article was written prior to the transfer of species in the genus Dryandra to Banksia. This has raised the number of Banksia species to about 170 (although the transfer remains controversial).
- This article is based on an assignment Russell undertook as part of studies for the Advanced Certificate in Horticulture at Burnley Horticulture College in Melbourne, 1991 – 1993.