Environmental factors for growing Magnolia

2015

Siting

Many Magnolia species with large-sized leaves seem to be wind-tolerant, but their branches are somewhat brittle. Trees growing in sheltered situations are more prone to damage by freak wind gusts than those growing in open situations, where adaptation to environment brings about the development of shorter and stouter branches and sometimes smaller leaves than usual.

In a part-shaded situation, however, colored flowers of Magnolia are darker and retain their color better. For example, M. × loebneri “Leonard Messel” is at its best when it flowers during a sunless period or in a shaded situation. When grown in the open it loses much of its color when warm sunny weather coincides with its flowering season (). Magnolia grandiflora is also moderately tolerant of shade. It can endure considerable shade in early life but needs more light as it becomes older (). It will invade pine or hardwood stands and is able to reproduce under a closed canopy. It will not reproduce under its own shade. Once established, it can maintain or increase its presence in stands by sprouts and seedlings that grow up through openings that occur sporadically in the canopy (). Magnolia grandiflora has been migrating onto mesic upland sites and establishing itself, along with associated hardwoods, as part of the climax forest ().

Soil

Magnolia likes sour, muck-rich, and well-drained soils, but does not like stagnant water or too high a water table, i.e., waterlogged conditions. Most Magnolia species are very tolerant of heavy clay or chalky soils. However, an excess of lime in the soil can cause chlorosis of Magnolia leaves and accelerate the depletion of soil humus, because lime aids bacterial activity and breaks down the complex carbon compounds in humus. Similarly, very low potash levels in the soil can induce chlorosis in Magnolia. Conversely, an excess of potash may result in salinity and also aggravate magnesium deficiency (). In the United States, for example, Magnolia grandiflora grows best on rich, loamy, moist soils along streams and near swamps in the Coastal Plain (). It is found on a number of different soils including those in the orders Spodosols, Alfisols, Vertisols, and Ultisols.

Climate

Most Magnolia species are thermophilous and cold-tolerant plants. Lape () reported that Magnolia acuminata, Magnolia kobus, and M. stellate as well as the Loebner hybrids (Magnolia kobus X M. stellate) were completely hardy in the winter lows down to -28.9 °C in New York. Leach () also reported a comparison of Magnolia species in winter hardiness, at Brookville, Pennsylvania, in the foothills of the Allegheny Mountains in early 1963 (the thermometer went down to -37.2 °C): among 30 species, cultivars, and hybrids investigated, eight were “no injury”, twelve “slight injury”, two “moderate injury”, six “severe injury”, and two “killed”.

It is known that Magnolia grandiflora grows in warm temperate to semitropical climates in the United States (). The frost-free period is at least 210 days and is more than 240 days for much of the range. Average January temperatures along the coast are 9–12 °C in South Carolina and Georgia and 11–21 °C in Florida. Coastal temperatures average 27 °C during July. Temperatures below -9 °C or above 38 °C are rare within the species natural range. Annual rainfall averages 1020–1270 mm in the northeastern portion of the range and 1270–1520 mm in other areas ().

Planting

Magnolia may be planted from November to May. Container-grown plants can be planted out during the summer months if reasonable care is taken to saturate thoroughly and avoid any disturbance of their root balls, and to keep them adequately irrigated during dry weather until fully established. In order to retain soil moisture and feed the surface roots of Magnolia, a surface-mulch of similar organic materials may also be added after planting.

Deep planting should be avoided for most species because they are surface rooting (). However, some species, e.g., Magnolia grandiflora, are deep-rooted trees, except on sites with a high water table. Seedlings quickly develop one major taproot. As trees grow, the root structure changes. Trees of sapling stage and beyond have a rather extensive heart root system. Older trees may develop a fluted base with the ridges corresponding to the attachment of major lateral roots.

Factor comparisons between wild and cultivated Magnolia: an example

More than 120 Magnoliaceae species, varieties, and hybrids have been introduced from southern and eastern China, Japan, Vietnam, and the United States, and cultivated in the Magnolias Garden of South China Botanical Garden, Academia Sinica, by Professor Y.-H. Liu (Y.W. Law) and his colleagues for about 30 years. South China Botanical Garden is located in northeastern Guangzhou, Guandong Province (23°10 N, 113°21 E) and has an area of 15 ha. Differences in main environmental factors between the original places and the Magnolias Garden have produced different performances of these cultivated plants of the family Magnoliaceae ().

Table Cultivated Magnolia in the South China Botanical Garden, Academia Sinica show the differences of some climate and soil factors between 10 original places and South China Botanical Garden (SCBG), and the performances of 32 cultivated taxa of the genus Magnolia in the garden (). In general, although the environmental conditions between SCBG and the original places are different, the cultivation of Magnolia in SCBG has been relatively successful (for example, 62.5% of cultivated species and varieties as well as hybrids have flowered).

Table Cultivated Magnolia in the South China Botanical Garden, Academia Sinica (). The classification of Magnolia follows Law (1996)

Taxon Year of introduction Original place Performance
Magnolia acuminata var. subcordata 1982 Southeastern USA Good, flowered
M. albosericea 1974 Hainan, China Good, flowered
M. amoena 1981 Zhejiang, China Fair
Magnolia biondii 1981 Hunan, China Fair, flowered
M. championii 1983 Guangdong, China Good
M. coco 1981 Guangxi, China Good, flowered
M. cylindrica 1981 Anhui, China Poor
M. delavayi 1982 Yunnan, China Poor, flowered
M. denudata 1981 Zhejiang, China Fair, flowered
M. denadata var. pyriformis 1993 Shaanxi, China Good
M. globosa 1981 Hunan, China Fair
Magnolia grandiflora 1979 Southeastern USA Good, flowered
Magnolia grandiflora var. lanceolata 1982 Southeastern USA Good, flowered
M. hainanensis 1986 Hainan, China Good
M. henryi 1983 Yunnan, China Good, flowered
Magnolia kobus 1981 Tokyo, Japan Fair
M. liliiflora 1982 Zhejiang, China Fair, flowered
M. liliiflora var. multiplex 1986 Fujian, China Good, flowered
Magnolia obovata 1981 Tokyo, Japan Fair
M. odoratissima 1983 Yunnan, China Good, flowered
M. oerissima 1991 Yunnan, China Good
M. ojflcinalis 1981 Guangxi, China Fair, flowered
M. ojflcinalis subsp. biloba 1977 Jiangxi, China Fair, flowered
M. paenetatauma 1958 Hainan, China Good, flowered
M. purpurella 1981 Hunan, China Good, flowered
M. shangsiensis 1981 Guangxi, China Good, flowered
M. seiboldii 1984 Liaoning, China Poor, flowered
M. soulangeana 1977 Zhejiang, China Poor, flowered
M. sprongeri 1981 Hunan, China Poor, flowered
M. stellata 1988 Tokyo, Japan Poor
M. tripetala 1982 Southeastern USA Poor
M. zenii 1982 Jiangsu, China Poor