Su Fang Bo
Three Directors of Design Institute of Shandong Tsinghua Kangli City Lighting Research and Design Institute Limited
Firstly, the paper analyses the situation and trend of urban night lighting in China, expounds the function of urban vegetation and the scientific explanation of plant sleep, and points out the current lighting situation that the combination of urban night lighting and urban greening plants is relatively deficient. Specific improvement methods are put forward: through the scientific and rational design of lighting designers, together with the cooperation of manufacturers, engineering companies and other parties, plant lighting will be attached importance to and hope to change the existing lighting mode, hope that later plant lighting will be considered in combination with plant growth lights for scientific design. Finally, the prospect of reasonable selection of light color and reconstructing harmonious light environment at night is put forward.
Key words: plant lighting; plant sleep; overlighting; light quality; light intensity; plant growth lamp
In recent years, the domestic city night lighting market is in full swing. The comprehensive construction of night scenery in tourist cities brought about by international conferences is well-known. The improvement of night scenery brought by important commemorative activities in China has also achieved good results. The major provincial capitals and well-known cities have also increased the landscape lighting in key areas one after another. Looking at this momentum, urban lighting will have fruitful results before building a well-off society in an all-round way in 2020. With the full armed landscape lighting in urban buildings, roads, squares and other areas where human activities are intensive, urban parks, embankments, landscape belts and even mountain bodies are also full of landscape lighting.
As a result, urban plants have been successfully affected by landscape lighting designed at will, and innocence has become a victim of ecological balance at night.
Fig. 1: Mountain lighting
Role of Urban Vegetation and Plant Sleep
Urban vegetation refers to all plant species covering the urban surface, including natural and cultivated species. It is the only primary producer in the urban ecosystem. It forms the basis of food chain. Photosynthesis is the primary source of energy and organic matter in almost all ecosystems. It plays an important role in self-purification, beautifying the urban landscape, enriching the outline of urban buildings, providing leisure and entertainment, and providing disaster prevention and shelter. 
Botanists believe that plants also need sleep. Initially, the most widely used theory to explain plant sleep movement was the Moonlight Theory. Scientists who put forward this argument believe that leaf sleep exercise can make plants suffer as little damage as possible from moonlight. Because too much moonlight may interfere with the normal photoperiodic sensory mechanism of plants and damage the adaptation of plants to day and night changes. However, what puzzles people is why many tropical plants without photoperiod also have sleep movement, which can not be explained by "moonlight theory". 
Figure 2: Plant Sleep
Since the "moonlight theory" can not explain, the problem of plant sleep is also pending, but we can be sure that plant sleep is real, but how long is the sleep time and cycle to be further studied by botanists. In the process of evolution, plants have formed the ability to adapt to the environment. The extension of night illumination for about 3 hours per day breaks the original adaptability of plants to the light in the light climate region, resulting in the disturbance of physiological and biochemical indexes of garden plants, affecting their normal growth, and may even cause them to wither and die.
Light quality is an important factor affecting the normal growth and development of plants. From the light source point of view, different light sources emit different wavelengths of light at night, which have different physiological effects on plants. From the plant point of view, photomorphogenesis and Photosynthesis of plants depend on certain specific wavelengths. That is to say, in addition to controlling photosynthesis as an energy source, light quality also acts as a trigger signal affecting plant growth. The illumination colour of unsuitable wavelength can disturb the plant's life activities and cause its illness or even death. 
Present Situation and Lighting Mode of 03 Plant Lighting
Illumination tree lamp is a kind of illumination lamps specially designed for urban trees. Some are round, some are square, some are semi-circular with radian, and some are custom hoops. But without exception, they are either installed next to the bottom trunk or at the intersection of the trunk. Lighting fixtures installed at the bottom avoid direct contact with trees, but have a greater dazzling effect on pedestrians. Lighting fixtures installed at the intersection of tree trunks, whether conventional or bionic or nest-shaped, need to be fixed by hoop-type installation structure. This method needs to be relaxed and adjusted according to the growth cycle of trees. If they are neglected in maintenance and management, they will cause disastrous damage to trees. However, in reality, due to the unclear division of responsibilities, this kind of tragedy caused by poor maintenance in the later period is not rare. In addition to tree lights, landscape lights, street lights, buried lights, high-pole projectors, etc. also have direct or indirect effects on plant sleep at night.
Fig. 3: Tree lamp
Impact of excessive plant lighting on ecology and its harm
It is obvious that unreasonable lighting mode will bring harm to urban vegetation. Firstly, it affects the normal growth cycle of plants. Plants use sunlight for photosynthesis during the day, but they also need rest at night. The change of photoperiod plays a key role in regulating plant seed germination, seedling growth, stem elongation, cotyledon extension, flowering control and dormancy. By measuring the length of night, plant leaves can predict seasonal changes, which is also a signal triggering deciduous and hibernating plants. Trees are exposed to strong light at night, which interferes with dormancy, and causes the abnormality of deciduous morphology and the formation of winter buds. For example, under artificial irradiation, trees or shrubs will continue to grow into the frost season in autumn, and the leaves will contain chlorophyll, which delays the deciduous period. At the same time, over-illuminated plants will be dry, water-deficient and overgrowth, and due to the destruction of chlorophyll, plants will also show other adverse symptoms such as color fading. Of course, the dormancy of trees and the formation of winter buds, in addition to the role of light, are also related to nutritional climate, air pollution and maintenance and management. Therefore, the different effects of light pollution on the growth of different species of plants need to be further studied.
Secondly, it affects the night rest and activities of birds and insects. Studies have shown that, except for a few nocturnal animals, most animals are quiet at night and do not like to be exposed to strong light. However, night outdoor lighting, especially plant lighting, produces sky light, overflow light, interference light and reflected light, which often illuminates the living and resting environment of animals very brightly, disturbs the rhythm of the biological clock of animals'day and night life, and makes them unable to sleep and rest. For wildlife and fish animals, besides visible light, the radiation energy emitted by lighting devices also has an impact on the life and growth of animals. For example, when animals absorb the energy of illumination radiation, not only the temperature changes, but also the electric field and physiology of animal cells will change. When goldfish are placed in a magnetic field, the stronger the magnetic field, the less fish want to eat bait. For example, the influence of night lighting on pests, because of the phototropism of insects, outdoor night lighting can attract a large number of insects, especially during the spawning period. A large number of insects are concentrated in the lighting area, and the eggs will soon become larvae and adults, which will cause insect pests. For beneficial insects and birds, it is worth noting that the overbright outdoor lighting at night will cause many beneficial insects and birds to jump directly at the lights and die. For example, the neon lights of Frankfurt amusement park in Germany will bake tens of thousands of beneficial insects every night. If a city, especially the night city, has tens of thousands or even hundreds of thousands of outdoor lighting lights, the ecological balance of the region will be seriously damaged if it goes on. One of the reasons for the high number of mosquitoes on summer nights in Dussen City is that thousands of neon lights in the city "kill" countless beneficial birds and insects that eat mosquitoes.
Fig. 4: Moths flutter lights
In summary, there are irreconcilable contradictions between plant lighting, animal and plant itself and ecosystem. However, we should not take a totally negative attitude towards plant lighting. Appropriate and reasonable plant lighting is what our lighting practitioners want to pursue.
05 Ways to Change the Status Quo
Firstly, lighting designers should scientifically and reasonably design the illumination lamps of plants, and put forward reasonable values for the wavelength, illumination and illumination time of the lamps. When setting landscape plant lighting, it is necessary to understand the relationship between light source spectrum and plant growth. Firstly, the spectral reflectance of different landscape plants is different because of their different surface materials and different angles of illumination. Some studies have pointed out that the reflectance spectra of plants under natural light are comparatively similar, and the night color of plants under visual observation is closer to the natural state. It is more suitable for night lighting sources of garden plants, such as fluorescent lamp and LED light source. When the spectrum is more discontinuous, the reflection spectrum is partly absent, which makes the color of garden plants deviate from the natural state at night, or makes the color of plants more deviate from the light source color of the light source itself, such as high-pressure sodium lamp, metal halide lamp, etc. Therefore, it is necessary to choose carefully when designing night scenery. Secondly, spectroscopy is an inducing signal affecting plant growth and development, regulating plant photosynthesis. The spectrum of high-pressure sodium lamp and metal halide lamp is discontinuous spectrum, which only provides limited effective energy for plant growth and has little influence on plant growth. The spectrum energy distribution of white LED can provide blue light spectrum and a small amount of red light spectrum for plant growth, which has great influence on plant growth.
Secondly, lighting engineers should put forward more appropriate installation methods according to the growth law of trees and the installation mode of tree lamps, in order to minimize the damage of trees. For example, the combination of landscape lighting and tree lighting, as shown in the figure.
Fig. 5: Light Tree Lamp Combined with Landscape Lamp
The light control ability of lamps is the beam angle, which is also a main factor to consider when choosing plant lighting lamps. Generally, the appropriate beam angle should be chosen according to the height, density and shape of the plant. According to the beam angle, the lamps can be divided into three types: wide beam, medium beam and narrow beam. Wide beam is suitable for illumination that needs to be emphasized on plant morphology: for trees with dense branches and leaves, 40-45 degrees of light control can be used to illuminate trees, which can achieve a better lighting effect; on the contrary, when a narrow and tall tree needs to be illuminated, it is better to choose a lamp with beam angle between 6 degrees and 20 degrees to illuminate.
Thirdly, lighting manufacturers should not only improve the style and appearance of tree lamp, but also give different options for different trees in the wavelength of tree lamp. When necessary, we also need to design and produce plant growth lamps which can promote plant growth and development for different tree species according to the plant growth environment. Plant growth lamps are artificial light sources, usually electric light sources, designed to stimulate plant growth by emitting electromagnetic spectra suitable for photosynthesis. Plant lamps are used in applications where there is no natural light or need to be supplemented. For example, in winter, lights are used to extend the time a plant receives light when the possible sunlight time may not be sufficient to meet the required plant growth. If plants don't get enough light, they will grow. LED growth lamps vary according to their use. Green, red, far red and blue light spectra have been known to have effects on root formation, plant growth and flowering in light morphogenesis studies, but there is not enough scientific research or in-situ testing of specific color ratios recommended by experiments using LED growth lamps. It has been shown that many plants will grow normally if given red and blue light. However, many studies have shown that red and blue light provide only the most cost-effective growth methods, and plant growth is still better when light supplements green. LED growth lights should keep plants at least 12 inches (30 centimeters) away from plants to prevent leaf burns.
Fig. 6: Plant growth lamp
Fourthly, lighting construction units should strengthen regular maintenance of control tree lamps to ensure that the growth and development of trees are not affected by or as little as possible by lamps. Buried lamp and projection lamp are often used in greening lighting, which can easily produce glare. Therefore, if trees are illuminated by buried and projection, it is necessary to choose lamp with grille or shade to reduce glare. Special attention should also be paid to the location of lamps and lanterns in order to ensure safety. Wire laying should be hidden and pedestrians, especially children, should not be directly exposed to exposed electrical parts. In places where buried lamps are used, attention should be paid to the surface temperature of the exit not to be too high to avoid scalding pedestrians. It is strictly forbidden to install the projecting lamps directly on the tree poles. Overweight lamps will have adverse effects on the growth of trees, and winding wires around the tree trunks is not only dangerous for leakage of electricity, but also hindering daytime viewing.
06 How to Create Harmonious Ecological Night Environment
Lighting designers should be particularly cautious in choosing lighting colors for tree lighting. White light can show the original color of plants relatively naturally and purely, make the outline of plants clearer and more sober, and the sense of hierarchy will increase. Usually the yellow light emitted by high-pressure sodium lamp can increase the warmth of the environment, but it should not be used to irradiate green plants, because it will make the surface of green plants look gray, yellow, lack of vitality, yellow plants need to highlight the sense of color will have a better effect. Red light and orange light make plants appear completely different in people's original impression. They are more charming and flexible, and make the environment more warm and beautiful.
Urban night lighting has become an indispensable part of urban development. The existing urban lighting system may not take into account too much of its impact on landscape plants, so it is difficult to change the impact of urban night lighting on landscape plants only from lighting equipment or plant species.  It not only requires planning departments to incorporate the research into the design at the early stage of planning and design, but also requires lighting design practitioners to design professionally and reasonably. Better cities need not only to enable night lighting to continue to play its role, but also to reduce its impact on the growth and development of landscape plants themselves.
Nightscape lighting is not the reproduction of the daytime landscape, but the use of lighting to remodel the night landscape, will be beautiful, orderly landscape at night, will be disorderly, broken landscape hidden at night. Use the light and shadow effect to combine plant and light reasonably and organically, make the city night around you more beautiful, and make the city's ecosystem more harmonious.
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 Plant Sleep Baidu Encyclopedia
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