A1:

The Greening, Landscape and Tree Management Section, Development Bureau has been advocating the "Right Species at the Right Place" principle for planting. Factors such as design objectives, concept/theme, landscape character, biodiversity, environmental factors including soil condition, microclimate (degree of wind and/or salt spray), visibility splays, traffic, spatial factors and characteristics of plants should be considered by the designer to determine the appropriate species for planting. Consideration should also be given to cost effectiveness, life cycle performance of the planting, and balance between design intention and maintenance concern. Moreover, native species generally have a higher ecological value and can enhance biodiversity. Suitable native species plants can be used at location with favourable site conditions. The use of native species and exotic species should be appropriately considered on a case to case basis bearing in mind of their benefits and limitations.

A2:

In general, departments select the species based on the following considerations:-

1. Purpose of greening and design Intention –
   Depending on the purpose of the greening works and circumstances, for instance, for landslip prevention, afforestation, roadside tree planting, greening of open area, etc., designers will select appropriate species to suit.
2. Limitation of the planting site –
   The conditions of the planting site and its surroundings, such as soil volume and condition, above and below ground space for growth, drainage, prevailing wind, microclimate and sunlight, will limit the choice of tree species to be planted. Departments will evaluate the conditions of the planting site and match with the characteristics and growing requirements of plants to decide the species.

To learn more about the government's greening projects, please visit the website here

A3:

A tree should be planted at proper depth with root flare visible. If a tree is planted too deep with the trunk flare buried, there will not be adequate oxygen for new roots to establish and tree growth may be retarded. This may also lead to decay at the trunk base which will render the tree unstable. Further information on "Proper Planting Practices" is available here.

A4:

Adequate growing space is essential for roots to anchor and obtain water, oxygen and nutrients, and also for the crown to develop proper structure. In general, trees planted too close will result in their poor health and structure. When allocating space for trees, the mature form, height and crown spread of the trees as well as the purpose of the planting should be taken into consideration. Further information on "Proper Planting Practices" is available here.

A5:

Proper pruning can improve tree health and appearance, provide adequate clearance for adjacent vehicles and pedestrians and reduce the risk of tree failure. Young and medium-aged trees should be pruned to promote good structure. Pruning to remove dead, diseased, broken or decayed branches can be performed on trees of any age at any time once defective branches are identified. Further information and details on pruning is available here.

A6:

Topping is a harmful and unacceptable pruning practice to trees which involves the cutting of branches to stubs or lateral branches that are not large enough to assume the terminal role.

A7:

Topping removes leaf-bearing branches that provide food by photosynthesis. This temporarily starves a tree and dormant buds are activated for rapid growth of multiple new shoots below each topping cut. Topping will seriously weaken a tree if it does not have adequate reserve to develop new shoots. Furthermore, these new shoots grow rapidly but have weak attachment to the parent branches. They are prone to breakage, especially during windy conditions, and this increases the risk of branch failure.

The topping cuts along a limb between branches create stubs with wounds which may not be able to close. The wounds provide openings for entry of decay organisms. The stressed tree with large open pruning wounds will be more susceptible to decay and pest attacks. Topping also creates ugly stubs and destroys the natural tree form. This adversely affects its amenity value.

A8:

Improper tree pruning is detrimental to the appearance and healthy growth of trees and would result in irrecoverable damage to trees. It is important to develop clear pruning objectives, adopt appropriate pruning types and follow correct pruning methods in carrying out pruning operations. Guidelines on how to prune trees properly are available here.

A9:

Mulches are materials placed on soil to maintain soil moisture and improve soil conditions for plant establishment. Proper application of mulch is beneficial to plant growth. The generally recommended mulching depth is around 5 cm. If the mulch layer is too deep, it can create excessive moisture in the root zone causing stress to plants and leading to root rot. Piling of mulch against the trunk is also inappropriate because it will increase the incidence of root collar diseases and damage by rodents. A clearance of 10 to 20 cm from the trunk is desirable.

A10:

Keeping sufficient space clear of vegetation or excessive soil / mulch fill around the base of tree trunk is important to the health of trees. Dense vegetation cover obstructs thorough tree inspection at the lower trunk and root zone and frequent replacement of plants disturbs tree roots. Regular watering of plants creates moist environment that promotes fungal diseases. Annuals and shrubs planted under trees compete with trees for space, air, water and nutrients and reduce oxygen supply.

A11:

As a general rule, the edge of the clearance zone is recommended to be around 150 mm – 300 mm from the tree trunk, depending on the size of the tree and its root flare as well as the actual site condition. That said, professional judgment is essential in deciding the actual size of the clearance zone and how clearance should be carried out.

A12:

The watering requirements vary by species, age and environmental factors. Established mature trees require less frequent irrigation. Frequent and shallow watering encourages surface root development and this makes trees more vulnerable to desiccation in periods with inadequate irrigation. On the other hand, infrequent and deep soaking encourages development of deeper root system to improve the tolerance of trees to drought. Excessive watering at root flare should be avoided as it encourages fungal growth which will adversely affect the health and stability of trees.

A13:

Tree wounds do not "heal" by replacing the dead and damaged tissue with new tissue. A tree reacts to form barriers around the wounded area to limit the spread of decay. New tissues may be developed at the margins of the wounds to form wound wood to cover the wound if the damaged area is not large. This process is known as "Compartmentalisation of Decay In Trees" (CODIT).

Although the exterior can have a normal appearance upon wound closure, wood may still decay inside. So minimizing the extent of wounds, particularly during pruning operation, is important.

A14:

At times, asphalt, polyurethane and shellac are used as wound dressings. However, international researches have shown that wound dressings do not prevent decay or disease infestations from the wounds. They cannot facilitate wound closure and may encourage accumulation of moisture and cause decay at the wounds. Some dressings may have fungal contamination which will infect fresh pruning wounds. Most tree experts do not recommend the application of dressings on wounds.

A15:

There is no scientific evidence to support cavity treatment. On the contrary, there is more evidence to show that cavity treatment is harmful. Removal of decayed wood may damage the compartmentalised barriers to decay and open sound wood to infection. The recommended course of action is simply to leave cavities undisturbed and improve the tree vigor to encourage the growth of new wood.

A16:

Most insects are either beneficial or are neutral in that they do not have direct impact on the health of plants or people. Cases where plant problems caused solely by insects are relatively small. They are usually secondary agents which attack trees and shrubs weakened or injured by other agents.

A17:

For trees growing in urban areas, proper maintenance is required for healthy establishment of trees with good structure. The "Practical Tips on Tree Care" and "Pictorial Guide for Tree Maintenance" are available here.

A18:

Striking a tree with a broad-headed mallet made of wood, rubber, leather or resin at different locations of a tree trunk to listen for tone change is a recognized practice adopted in carrying out tree risk assessment to detect the presence of internal hollows. The method is simple, direct and non-invasive.

A19:

Tree health and structural conditions are closely related but separate aspects in risk assessment. Tree health condition is relating to foliage density, size, colour, annual shoot growth and wound wood development. Tree structural condition is related to the presence of defects (e.g. large cavity or codominant branches with included bark) that can lead to tree failure causing collapse of tree parts or the whole tree. It is important to note that a tree which is structurally unsound with a large cavity or major decay can still be lush if the cambium tissue at the trunk is living and functioning.

A20:

We need to inspect a tree systematically and thoroughly to cover the crown, branches, trunk and roots to identify the signs and symptoms of poor tree health and defects affecting tree structure. Photos showing examples of tree problems are available here.

A21:

Trees growing within construction sites are often susceptible to damage caused by construction activities. It is essential that adequate tree protection measures be formulated, implemented and monitored for trees to be retained on site to ensure proper care during construction and to minimize construction impact. The key points on tree care during construction are available here.

A22:

Members of the public may report problematic trees to the "1823" Call Centre. Operating round-the-clock and with the capability to receive public complaints / enquiries via telephone, email, fax or letter, the "1823" Call Centre serves as an effective, centralised platform for the public to lodge tree complaints and enquiries. Reports may also be made using the "Tell me@1823" app for smart-phones. Members of the public may submit photographs along with voice descriptions or text when reporting problematic trees. Equipped with geospatial functionality, "Tell me@1823" also captures the physical location of the tree under complaint. This app can be downloaded here.

A23:

Proper tree maintenance contributes to the healthy growth and good form of trees. Many tree failures are triggered by poor tree maintenance such as improper pruning, topping, damage to the root system and other malpractices in tree care. The key points in minimising tree risk are available here.

For more information about tree maintenance for private properties, please visit the website here.