TREES AND SHRUBS FOR WARM PLACES
A book by our principal and founder, John Mason
Extract from this book:
Many of our most magnificent gardens are found in hot places. The heat and humidity in sub-tropical and tropical areas provides moister areas that encourage diversity of plant life; the range of plants available for such situations provide a palette of unique textures, often brilliant colours and wonderful shapes. With the right approach and careful selection of plants, a once hot and unwelcoming place can be turned into a very beautiful and liveable garden.
Plants can help modify the temperature in a hot place. Large plants not only shade the ground below (keeping it cooler), they also keep the temperature lower by not trapping and radiating heat in the same way as other surfaces such as rock, paving, metal or glass do. Plants also lower the temperature in their immediate vicinity through the release of water vapour from their foliage during photosynthesis.
This book is relevant to gardens in tropical and subtropical climates, and also to hot places in cooler climates such as greenhouses, "heat trap" courtyards and arid sites.
What Causes An Area To Be Warm?
There are a number of things which contribute to overly hot garden spaces:
1. Sun is the most obvious - most heat will come directly from the sun.
2. The latitude - generally the closer to the equator the warmer it is.
3. Inland places - the average maximum daily temperature will generally increase the further you move away from coastal areas.
4. Closeness to the sea - the sea or ocean has a temperature moderating effect. However some coastal areas may also remain warmer than other areas at the same latitude (distance from the equator), due to the presence of warm ocean currents.
5. Altitude - temperature decreases as altitude increases, so even in tropical regions high altitude sites will normally be significantly cooler than adjacent lowland sites.
6. Clouds - cloud cover has an effect on air temperature near the earth's surface by keeping it cooler in the day, but warmer at night - as warmth cannot escape the atmosphere.
7. Trees and shrubs - existing vegetation can dramatically modify temperature over a large land area; vegetation will buffer the temperatures by keeping air slightly warmer in winter and cooler in summer.
8. Hard materials - some materials will absorb and/or store, and radiate heat more than others. Materials such as metal, stone, glass, and paved areas can contribute to increased heat in a garden. The more these materials are used, the more likely the garden is to warm up, and stay warm.
9. Human influence and activity - heat can also be generated (to a lesser extent) by human activity such as burning off, factories, motor cars, etc. Factors such as these often mean that temperatures in a city are likely to be higher than in the surrounding countryside.
· Plants and animals have biological mechanisms which help cool their tissues. This biological cooling can in fact contribute to cooling a garden. By increasing the biological mass within a garden, you will tend to decrease high temperatures. Lawns, for example, will keep an area much cooler than if the area was paved.
· An increase in altitude (height above sea level) will usually result in a decrease in average maximum temperatures (approximately a decrease of 0.6° Celsius per100 metre increase in altitude).
· Any insulating material will also tend to reduce temperatures (e.g. wood furniture or paving will keep a garden space cooler than masonry or metal).
· Good ventilation helps take heat from a garden (i.e. if air can move freely through a garden, cool air will move in to replace hot air. Larger more open spaces allow better air flow.
· Water is always useful in cooling an area. If the water can be splashed through the air (e.g. a waterfall or fountain), the cooling affect is increased.
· Anything which blocks the radiation from the sun will create a cooling affect. This includes shade from trees, shrubs, hedges, pergolas, shade-cloth or structures.
COMMON HOT GARDEN AREAS
When we think of hot gardens, our thoughts usually turn to the tropics or sub-tropics. Gardens in these areas are commonly hot, but not necessarily so. There are exceptions. Tall mountains in New Guinea can be covered with snow. Shaded rainforests in sub-tropical south-east Queensland in Australia can suffer at times from temperatures approaching zero degrees Celsius. Desert areas, such as those found in central Australia, are generally regarded as hot/dry climates, however night temperatures can easily reach zero degrees Celsius or less in winter. And in cooler areas it is possible that a small paved courtyard garden may suffer higher temperatures than a shaded rainforest garden in the sub-tropics will ever face.
Tropical areas are those parts of the world that fall between the Tropics of Cancer and Capricorn, which are lines of latitude lying 23.5 degrees north and south, respectively, of the equator. The most highly populated parts of these areas are typically hot and humid most of the year, with a mainly dry period during the winter and a pronounced wet season during the summer period. Humidity increases during the rainy season. Many of these areas may also be subject to cyclones and severe storms. Some parts of the tropics however are drier, some are even deserts. These areas may also be subject to severe windstorms, but rainfall and humidity will be much lower.
The sub-tropics are generally warm like the tropics, but conditions may be more seasonal. Temperature fluctuations may also be greater than for tropical areas. Sub-tropical areas can suffer from frosts, particularly inland areas. They can also have very hot days. Subtropical climates can be generally described as areas outside the tropics which exhibit a few features similar to those found in the tropics. Areas outside the tropics can be described as zones south of the Tropic of Capricorn; and zones north of the Tropic of Cancer.
A greenhouse is simply a structure or building used to provide suitable growing conditions for particular plants that could not normally be grown, or could only be grown with difficulty, in the outside environment. Tropical plants are most commonly grown as greenhouse or indoor plants in cooler climates.
Hot spaces are sometimes created intentionally (as well as unintentionally), in cooler climates.
· Poorly ventilated areas (e.g. a walled or fenced in garden space such as a courtyard), will not cool down as readily as more exposed sites, and can become a heat trap. Small, enclosed areas are more likely to act as a heat trap. Walls which face the sun (e.g. north facing in the southern hemisphere), tend to heat up faster than walls not exposed to as much sunlight; and they then radiate heat back over the day into the adjacent garden.
· Paving or concrete heats up much more than grass, and can also produce a lot of glare it can also reflect light and therefore heat onto windows and buildings. Unlike grass, paving absorbs heat during the day and releases it at night, so the area will stay warmer or hotter for longer than grassed areas. These situations can cause hot areas that are intolerable to use during the day. At night however they can be pleasantly warm; such a spot can also be used to advantage in order to grow heat loving plants.
· Large ponds or pools can have a moderating effect on the local temperature. As the water heats during the day from the effect of sunlight some will evaporate lowering the temperature in the immediate area. In addition some of the incoming heat will remain in the water to be radiated back into the atmosphere at night, once the sun has gone, helping to keep temperatures slightly warmer in the immediate area.
Arid Area (Deserts and Semi-Deserts)
Arid areas can become very hot during the day - but also very cold at night. Many inland parts of Australia, Asia, America and Africa have low rainfall, and can be very hot - with temperatures in the high 40s (degrees Celsius) during the day, to near zero at night. Plants here must be hardy to extremes. Gardens in such areas should be designed to buffer extreme temperature fluctuations. Use of drought tolerant plants and efficient water management can help in creating an attractive garden in such areas.
WARMING THE GARDEN
There may be reasons to raise the temperature within the garden. One reason may be a preference for warmer winter temperatures, or to give the gardener the opportunity for growing more tropical plants in their location.
The gardener must look carefully at their site and assess which places are the warmest for cold sensitive plants. These positions may be against a north facing wall (in the southern hemisphere), in a garden protected from winter winds, or even in a glasshouse.