Spring in Sydney means many of our favourite trees, like jacaranda and flame tree are losing their leaves and others like bauhinia, poinciana and silky oak are also starting their major spring leaf drop as they become semi-deciduous. What is it about ex-Gondwanan continents that makes our plants adapt by becoming deciduous in spring, rather than autumn, like most European and North American plants? And why do we persist with growing winter-deciduous spring flowering trees that struggle in our east-coast warm, dry springs?
I love the red, orange and yellow of autumn, but there’s also something about that unusual yellow bronze of jacarandas in September-October in Sydney that I just adore. They only colour for a couple of weeks and already their leaves are falling, just as many of the winter deciduous trees like liquidamber, elm and gleditsia are being covered with a fuzz of delicate and fresh, spring-green leaves. Illawarra flame trees are losing their foliage too and beginning to show their thick, slightly stumpy branches.
Plants usually have a dormant period in response to some form of environmental stress. In the northern hemisphere, that’s the freezing cold of snow and ice that blankets the ground for many months, as well as the very short days and cloud cover, making photosynthesis difficult. No wonder many plants, apart from the conifers, shut up shop for several months.
In the southern hemisphere, there’s not much land mass that’s affected by bitter winter cold, except for the highest altitudes of South America and New Zealand. But many of these countries have intense spring drought. In Australia’s north, and along the eastern seaboard, August to November are very dry months, often added to by gusty, drying winds from the hotter interior.
In 2013 we’ve had one of the warmest winters, and driest, hottest springs in many years. And given the recent new report from the Intergovernmental Panel on Climate Change confirming human-made climate change and the predictions of future temperature rises and more drought in SE Australia, we need to think very carefully about the trees we’re planting for the future. In 2013, fed by some late winter rain, our northern hemisphere winter-deciduous trees that flowered early put on a good show, but through August and September, Prunus, Malus and Magnolia species have suffered badly, as has the colourful new growth on maples.
Many southern hemisphere trees have responded to spring drought stress with a mid-spring rather than winter deciduous adaptation, to prevent critical moisture loss from leaves during drought and to coincide flowering with the arrival of pollinating insects and animals in late spring.
Planting these trees in east-coast Australia makes so much more sense than investing time and dollars in pretty ‘blossom’ trees. Spring deciduous trees are losing, rather than getting their leaves when those hot dry spring winds arrive and they are also adapted to our intense summer rainfall events and high humidity, unlike many winter deciduous trees, which are very prone to summer fungal diseases.
And the intensity of the richness of flower colour without leaves is an arresting sight. Purple jacaranda canopies and the carpets of fallen flowers beneath them become a dominant colour through the towns and cities of Australia’s east coast – in fact many spring visitors to Australia are surprised to find that this much-loved tree is in fact native to South America. Combining it with the rich red of Australian Brachychiton natives and brassy-orange silky oak creates a real zing. There is also a rare grafted white jacaranda. Although you most often see these spring deciduous trees as single specimens, I’d love to see some other colour combinations.
Of course an obvious benefit of a dry deciduous tree is its ability to survive long-term drought. They are all very drought-hardy and, although they may lose some leaves out of their regular season when it’s scorching hot, they are very likely to survive years of below average rainfall, compared to evergreens from cooler and wetter climates. In cooler and wetter springs, trees may be only partly deciduous and flowering will not be as intense or long-lasting.
Many of these trees require a maturity of 5-7 years from seed germination to begin flowering, which probably ensures that only strong, drought-resistant trees reproduce successfully. Buying grafted trees, where a branch from a mature specimen is grafted onto more juvenile root stock will give you earlier flowering.
When you’re planting a spring-deciduous tree, note that although it will give you good shade from Christmas on, it will be bare during those warm spring months of October-November, with the new leaf canopy arriving with early summer rains in late November-December. But you will be planting a tree that will last into our uncertain climate future.
Spring deciduous and semi-deciduous trees native to Australia (unless indicated otherwise)
Barringtonia neo-caledonica – (New Caledonia) white flowers, rich-red foliage in spring
Bauhinua variegata – (India, SE Asia) Orchid tree, flowers pink or white
Bombax ceiba var. leiocarpum – (Australia and SE Asia, India) Silk cotton tree, red kapok; red flowers
Brachychiton acerifolius – Illawarra Flame tree; red flowers
Brachychiton bidwillii – Dwarf kurrajong; rosy-red flowers
Brachychiton discolor – Lacebark; pink flowers
Cassia fistula – (India to SE Asia) Golden shower tree; yellow flowers (only slightly deciduous in cooler climates like Sydney)
Ceiba petandra – (South America) Kapok; white-pink flowers
Cochlospermum religiosum – (India SE Asia) Kolkata; yellow flowers
Cochlospermum fraseri – Native kapok tree; yellow flowers
Delonix regia – (Madagascar) Poinciana; orange flowers
Erythrina species and hybrids – (South America) Coral tree; coral-red flowers
Gmelina arborea – (Asia) Gamhar; yellow-brown flowers
Grevillea robusta – Silky oak; orange flowers
Jacaranda mimosifolia – (Brazil) Jacaranda; purple-blue or white flowers
Planchonia careya – Cocky apple; pink-white fluffy flowers
Schotia brachypetala – (South Africa) Tree Fuchsia; red flowers
Sophora species – NZ Kowhai; yellow flowers
Sterculia quadrifida – Peanut tree; greenish-yellow flowers then large orange fruit
Tabebuia aurea (South America) – Tree of Gold, Caribbean Trumpet tree; yellow flowers
Tabebuia rosea (Central America) – Pink Trumpet Tree; pink flowers
Terminalia seriocarpa – Damson plum; whitish-green flowers with strong honey scent
Turraea pubescens – white perfumed flowers
Ziziphus mauritiana – Chinee Apple Tree (environmental weed in northern Australia)