Seed sowing and seedling transplantation
A simple approach to propagation or reproduction of Magnolia is from seed. For example, summer-flowering M. sinensis and other members of the section Oyama are usually prolific seed bearers. In China, seed sowing is a popular method for commercial cultivation of medicinal M. lilifiora, as Biond Magnolia Flower (Flos Magnoliae).
Many Magnolia species are prolific seed producers, and good seed crops are usually produced every year. For example, trees of Magnolia grandiflora as young as 10 years can produce seed, but optimum seed production does not occur until age 25. Cleaned seeds range from 12 800 to 15 000/kg. Seed viability averages about 50%. The relatively heavy seeds are disseminated by birds and mammals, but some may be spread by heavy rains (). Magnolia grandiflora is pollinated by insects ().
In order to harvest Magnolia seed, it is important to know the seed-ripening date of each species. For example, the seeds of Magnolia officinalis and Magnolia hypoleuca can be collected in late September, as soon as the carpels on the fruit cones begin to split longitudinally to reveal the bright orange, scarlet, or crimson seeds. Delay in harvesting may result in rapid losses to squirrels and other seed-eating rodents and birds. After windrow, the collected seeds can be plunged in cold water and rubbed to take off the arillus of the seeds. The seeds obtained must be indoor seasoned, not sun dried. The seasoned seeds should then be deposited and stored with clean sand in a humid and low-temperature location.
The seeds may be stored in a refrigerator without removing the fleshy coating until February and then sown immediately after cleaning, if it is not possible to clean and treat the seeds soon after harvesting. The cleaned or uncleaned seeds may also be stored immediately in small polythene bags and kept in the lower section of a domestic refrigerator for about 100 days. Magnolia seeds tend to lose their viability after six months of storage in normal conditions. Sometimes germination is delayed for over a year from the date of sowing, but rapid germination is usually assured by vernalizing the seed for about 100 days at 2–5 °C ().
In practice, the seeds of Magnolia grandiflora usually germinate the first or second spring following seedfall; germination is epigeal. The best natural seedbed is a rich, moist soil protected by litter. Even though viable, seeds rarely germinate under the parent tree because of reported inhibitory effects ().
The most economic and effective approach to sowing and cultivation is forced germination in greenhouse and seedling transplantation in the field, rather than direct seeding in the field. The four steps of the approach are as follows: (1) sow the deposited seeds densely into a sandy bed in greenhouse, with a 2 cm thickness of cover sand over the seeds; (2) water the sandy bed daily to keep it wet; (3) transplant each seedling in full-bud stage into a paper cap with soil, and keep the seedling in normal rooting for a month; and (4) transplant seedlings with soil attached into field. Partial shade is beneficial for the first two years of seedling growth. The advantages of this approach over the direct-seeding method are that seed is saved (about 50%) and the emergence of seedlings is accelerated (about one month in advance). For commercial cultivation in China, the approach has proved successful and it is reported that the cultivated trees can reach to 3 m in height and 6 cm in thickness in three years ().
It has been reported that only a small percentage of fruit cones of Magnolia develop perfectly in English gardens. This is particularly so with the precocious-flowering species and hybrids, and may be associated with the effect of weather on early insect activity and the susceptibility of Magnolia blossoms to frost damage. On the other hand, plants raised from seed normally take longer to attain flowering age than those raised vegetatively as described below, and there is often a considerable variation even between seedlings raised from seed borne within the same carpel ().
Cutting is another type of propagation that can be used for many Magnolia species, e.g., Magnolia liliflora, which fails in attempts to cultivate it commercially with the seed sowing and seedling transplanting approach described above. Magnolia does not normally root from dormant hardwood cuttings, but they can be rooted from soft or half-ripe cuttings in the summer. The most suitable periods for cutting propagation for a particular species vary in different locations; for example, the period for Magnolia liliflora is May to June in Shanghai and June to July in Beijing, China. The earlier cuttings can be rooted better, since cutting propagated late tend to perish during the first winter after having formed roots.
The rooting medium may be pure sharp sand or a mixture of moss peat and sand. Vermiculite and perlite can be used instead of sand and have the great advantage over sand of being relatively light weight, thus saving time and energy where large-scale propagation is entailed.
The propagation bench should have bottom heat supplied by under-bench hot-water pipes, hot air ducts, or electric soil-heating cables embedded in the drainage layer, to provide a constant temperature of 21–24 °C within the rooting medium. Root initiation can be ascertained by periodic testing of the cuttings. As soon as good clusters of roots have emerged from their bases, they may be lifted and potted. The size of pot selected is only just large enough to accommodate the young roots ().
Layering is the oldest method of vegetative propagation for woody plants, which do not root spontaneously from branches that have been inserted or driven into the ground. The layering of Magnolia has been extensively practiced in nurseries, especially in the Netherlands and Belgium, since the introduction of the Soulangiana hybrids in the 1830s. In the Boskoop nurseries the mother plants or stool bushes (on their own roots) are set out in plots of very heavily manured peaty soil. They are kept meticulously clear of weeds and are mounded in January with cow manure overlaid with leaf-mold.
The long, whippy young shoots that develop are arched outward and downward in August or September, so that portions of them may be buried to induce root formation while the tips are bent upward, supported with bamboo canes. Sometimes it is desirable to layer a Magnolia that has no branches close to the ground. In this case, it is necessary to introduce the rooting medium wherever a suitable branch happens to be growing.
The budding of Magnolia is used extensively in Japan and South Korea. The early successful propagation of Magnolia species (Magnolia kobus and Magnolia salicifolia) by budding was reported by Mundey in 1952.
The seeds of Magnolia kobus are drilled about 8 cm apart in rows 15–23 cm apart. Dormant growth buds are cut from half-ripened shoots off the mother trees and the leaf blades are removed before the buds are inserted into vertical “T” cuts on the north sides of the stems of two-year seedlings during warm weather at the end of September. The buds are wrapped with polythene strips and banked up with soil to prevent sun scorch. In the following March the stocks are headed back to a point close to the successful unions before the “buds” begin to swell. Growths of 90–120 cm or more during the first season are not unusual ().
Mundey also discussed the considerable time taken to reach flowering maturity, and suggested that a required carbon/nitrogen ratio had to be reached before flower bud initiation could occur.
In practice, grafting has been a routine and effective method of propagation of Magnolia. It may be carried out from about mid-August up to the time of growth commencement in May. Better results can be obtained from stocks that have developed hardwood. This contains a thick layer of cambium, those actively dividing cells responsible for callusing and healing.
Grafting should be done with dormant cions. The tree in which the cions are set need not be dormant; in fact, grafts set up to the time of blossoming should grow, provided the cions are dormant. The best time for top working is just before the bark slips.
Cions should be cut from wood of the past season’s growth. Better results are from short woody spurs, which are closely ringed with petiolar scars, than from strong young shoots of the current season’s growth. With late summer grafting it is usual to reduce the leaves on the cions to half their length to check transpiration.
It has been observed that the two Japanese species Magnolia kobus and Magnolia hypoleuca provide good understocks of Asian Magnolia, while Magnolia acuminata is favored in both American and Asian species. Magnolia sargentiana also provides a reliable source of understocks. However, only seedlings and rooted cuttings of Magnolia gradiflora can be used as understocks for grafting the cultivars of Magnolia gradiflora ().
The understocks must be kept well watered during dry weather and, when the union has been accomplished, all unwanted portions may be cut away gradually. The point of severance from the mother plant must be several inches below the union so that a sizeable snag remains to be pruned away some time after the scion has started into growth, nurtured by the root system of the understock. The final severence is usually best delayed until the end of the following growing season, about the time of normal leaf fall ().
Bud grafting or chip-budding is an adaptation of budding that is more akin to grafting. It has the advantage over budding and stem-grafting that it may be practiced at almost any time of the year, as and when propagating material becomes available. This technique has been developed in America over several decades by fruit- and nut-tree propagators. It is particularly useful for propagating hardwood material, when the bark is tight and when growth conditions are unsuitable for orthodox “T”-budding ().
Table Comparison of graftings of Magnolia (as cions)
|Cion||Understock||Grafting time||Grafting method||Survival (%)|
|M. sprengeri||Michelia champaca||February 1983||Cut||100|
|Magnolia acuminata var. subcordata||M. denudata||February 1983||Cut||66.7|
|M. cylindrica||Michelia champaca||February 1983||Cut||100|
|Magnolia biondii||M. cylindrica||February 1983||Cut||80|
|M. X soulangeana||Michelia macclurei||February 1985||Cut||85|
|M. X soulangeana||Michelia champaca||January 1985||Cut||77.5|
|M. X soulangeana||Michelia platypetala||January 1985||Cut||85.7|
|M. tripetala||M. cylindrica||February 1985||Cut||50|
|M. X soulangeana||Michelia champaca||April 1985||Cut||1|
|M. X soulangeana||Michelia champaca||September 1985||Cut||87.5|
|M. tripetala||Magnolia officinalis||March 1986||Cut||0|
|M. tripetala||Manglietia megaphylla||March 1986||Cut||25|
|M. sieboldii||Michelia champaca||February 1990||Side||50|
|Magnolia biondii||M. denudata||February 1990||Side||62.5|
|M. sprengeri||Michelia platypetala||February 1990||Side||58.8|
|M. tripetala||Michelia champaca||March 1990||Side||10|
|M. sieboldii||Michelia champaca||February 1991||Bud||36.4|
|M. zenii||Michelia champaca||February 1991||Bud||50|
|M. amoena||Michelia champaca||February 1991||Bud||40|
|M. cylindrica||Michelia champaca||February 1991||Bud||70.8|
|M. tripetala||Michelia champaca||February 1991||Bud||66.7|
|Magnolia biondii||Michelia champaca||February 1991||Bud||55|
|Magnolia grandiflora||Michelia champaca||March 1991||Bud||90|
Experiments with a number of combinations of three grafting methods (cut-grafting, side-grafting, and bud-grafting), different species and varieties of Magnoliaceae as cions and understocks, and different grafting times have been undertaken in South China Botanical Garden, Academia Sinica, Guangzhou, China (). The performances of 23 Magnolia species (as cions) and 7 Magnoliaceae species (as understocks) are given in Table Comparison of graftings of Magnolia (as cions). The results show that both intrageneric and intergeneric grafting of Magnolia species can be done if the grafting method and time are suitable. Two fully successful examples (100% survival) are intergeneric, i.e. Magnolia sprengeri–Michelia champaca and Magnolia cylindria–Michelia champaca, by means of cut-grafting. Other combinations also indicate that intergeneric grafting within the family Magnoliaceae is efficient. The grafting time, however, is a limiting factor. For example, the grafting survivals of Magnolia tripetala in February 1985 (50%) and February 1991 (66.7%) were higher than those in March 1986 (0%) and March 1986 (25%), no matter whether the understock was Magnolia or not. Interestingly, the grafting of Magnolia × soulangeana with Michelia champaca was favorable when done in late September 1985 (87.5% survival).