To the indigenous plant enthusiast a frequent refrain from gardeners and farmers is, “why bother trying to try and find and use indigenous trees and shrubs when there are so many wonderful exotic species that are already well established in Zambia?”
The answer requires a short digression into plant ecology.
The more we discover about plants and the way they function, the more intriguing they become. One key finding of recent years is that trees have very sophisticated communication and reproductive systems – linking them with others of the same species, with other plants and with soil flora and fauna. These links also extend to the large animals, birds and insects that eat them, fertilize them, or distribute and help germinate their seeds.The complexities of root-soil relationships (including fungal mycorrhyza and bacterial nodules) are now also better understood. These mutual associations assist plants by accessing and contributing nutrients, but they also distribute valuable nutrients back to the soil and help to maintain steady moisture conditions (and therefore life), in the soil profile. Moving in another direction, we are now also much more interested in the medicinal properties that many plants have that are of huge value to humans (and presumably to plants themselves). And all this is quite apart from the often-overlooked day-to-day benefits we gain from attractive flowers, fruit, shade, timber and fuel wood.
Most of the garden shrubs and herbaceous plants that we enjoy have been collected and domesticated over the last few hundred years. Many of them come from a relatively few areas of high plant diversity (western China, the Cape Province, South Asia), that were the favoured plant hunting areas of western botanists and garden designers. Generally these plants were collected with the intention of filling temperate gardens. A similar process has been going on for even longer – finding, domesticating and using plant and tree crops, drawing from centres of crop plant diversity, in place as widely spaced as central Asia (wheat) south Asia (spices), China (tea), south America (potatoes) and Ethiopia (coffee).
Exotic trees, unless they produced edible fruit or drinking substances (tea, coffee, cocoa), were less important to gardeners. In part this was because there were often attractive indigenous trees already in temperate gardens. In addition, tropical and sub-tropical trees were frequently not adapted to long freezing winters and were often difficult to germinate. It was only in recent years when tropical gardens because more widespread that gardeners started looking again for attractive and interesting garden trees in the tropics. Apart from a few “specials” that were widely distributed by traders and colonial administrations, these efforts have generally been very localised. A good example is the gardens of the “new city” Brazilian capital of Brazilia that were designed by Burle Marx, using many indigenous plants he collected from the Amazonian forests. In southern Africa, shade trees for urban avenues have been one of the few areas where local trees have been widely used – particularly Khaya (Red Mahogany) and Trichelia (Natal Mahogany).
And thus our gardens in Zambia contain many exotic bed plants and shrubs, but fewer exotic trees. The latter have chiefly been introduced from other tropical and temperate areas because they have attractive flowers (Jacaranda [tropical south America], Flamboyant [Madagascar], Spathodia/Tulip Tree [equatorial Africa]), leaves, (Japanese Maple), or shade.
These exotic, colourful benefits cannot be denied. But have you ever noticed how difficult it is to grow anything under them? The reason is probably meshed in the fact that local, indigenous trees have well-established ecological relationships with their environment, while exotic, introduced, plants are less well integrated. Apart from challenging gardeners, this has two consequences – either these trees have few natural controls and have a propensity to spread rapidly and become pests (such as Lantana, Guava and Jacaranda), or they actively exclude other plants (Flamboyant).
These consequences offer two immediate reasons why choosing to use indigenous trees and plants is beneficial. But the wider plant ecology considerations are much more important and more enjoyable – and provide the more than 1,000 reasons for using them! Let’s start with the bigger picture. Zambia has at least seven different habitats: rocky, grassland, savannah, riverine, wet forest, dry forest, wetland; each with at least thirty trees or other key plants that are well adapted to the local conditions. This habitat array offers an immediate diversity of garden design and planting options – with the added interest of combining plant selection to soils type and local landscape conditions.
The next layer of opportunities is that each of these habitats and plant communities has a well-established relationship with a constellation of hundreds of animals and other plants. A plethora of insects (including butterflies, ants, beetles, spiders and mantids), feed on, reproduce in and often provide mutual benefits to the plants they associate with. More reasons accumulate via the streams of value addition from the use of plant flowers, fruit, nesting and refuge sites by many larger and small animals (kudu, duiker and bushbuck, bush babies, squirrels, bats, door mice), reptiles (chameleons, lizards and geckos, snakes), and birds of all varieties (insect, seed, fruit and nectar eaters, and birds of prey).
A frequently quoted biological mantra is that in ecological diversity lies stability. This is now increasingly being reflected in the term ‘ecosystem services’ as realisation widens that maintaining diverse ecosystems and natural processes is fundamental to our survival as a species. More and more we are also noting that these services frequently bring direct economic benefits through functions as diverse as natural pest control and sustaining hydrological cycles.
More prosaically, most people wishing to use indigenous plants have a specific purpose in mind – gardeners want an attractive garden and farmers may want a windbreak, shade, or a nitrogen-fixing tree. So looking at practicalities are there indigenous trees and shrubs that meet all these requirements and how easy are they to grow? In spite of a relative paucity of easily accessible information there are an increasing number good books, and of attractive and useful indigenous trees and shrubs available from nurseries to meet these needs. That is quite apart from the trees and shrubs that are already growing on our properties. But like most gardening, for those wanting to give indigenous plants a try one needs to remember four important factors: purpose, space, soil type and water availability.
Indigenous trees and shrubs come in a wide variety of habitat groupings, shapes, degrees of evergreen-ness, textures, forms, flower colours, seed shapes and insect, large animal and bird associations. Therefore, choosing what to plant depends largely on what you want from it. Are you in a corporate situation where specimen trees and iconic shapes are important, or in your own garden where you wish to have a mixture of shapes, colours and diverse wildlife?
Secondly, available space is critical. Many trees grow to enormous sizes (for example, the Khaya trees along Haile Selassie Avenue, Longacres), and others (especially the figs), have highly intrusive root systems, so planting either in a small townhouse garden will lead to future headaches!
Thirdly, most indigenous plants are fairly soil tolerant, but some are particularly adapted to deep, well-drained, fairly acidic sandy soils and will not do as well on the red, calcareous soils around Lusaka. Knowing your soils and the preferred growing medium of your plants is important.
Lastly, water is a rather key issue! Riverine trees are adapted to constantly high levels of soil water. Conversely, because of our sub-tropical, seasonally dry location most indigenous plants are adapted to a long dry season when their growth slows or stops. But it is a misconception that indigenous trees are slow growing. True, many of our seasonally-adapted hardwoods are (unfortunately) not exactly world beaters in the tree growth stakes, but most trees suitable for gardens and shade grow remarkably quickly with steady watering and a little tender loving care.
So where does one start? Here are just a few suggestions as an introduction: Erythrina – thorny, red flowered trees, attractive to many birds; Dombeya – white flowered trees, attractive to bees and many birds, Acacia and Albizia – shapely trees with white or yellow flowers, loved by bees and a wide variety of birds, Cassias – shrubs and trees with masses of yellow flowers attracting insects and birds, Stereospermum (Pink Jacaranda) –elegant, pink flowered trees and a wide variety of users, Bolusanthus (Tree Wisteria) – small trees with cascades of purple flower, Sterculia – iconic, white barked specimen trees, Trema and Croton – attractive trees with gentle shade, Cordia and Craibia – shady trees with attractive white flowers, Khaya and Trichelia – large trees with deep shade, Cordyla, ficus species, MarulaandSyzygium – attractive trees with edible fruit attracting many animal and bird species, Combretum – often sweet smelling flowers and seeds that feed many birds,Bridelia,- fast growing tree attracting many species to its small, purple fruit, Grewia and Oncoba – attractive flowering bushes.
Our threatened hardwoods: Split Banana (Entandrophragma), Mukwa (Pterocarpus), Rosewood (Guibourtia) and Zambezi Teak(Baikiaea) are generally difficult to grow and represent one of our most urgent indigenous tree conservation issues as they continue to be removed for timber at an alarming rate. Some of our indigenous fruits (Musuku[Uapaca], Mpundu[Parinari] Mfungo[Anisophyllea], are also challenging to propagate and offer interesting areas for research as many have high levels of vitamin C and other valuable constituents.
So there is plenty to experiment with out there. Have fun – and enjoy your indigenous plants!