by R. L. Hathaway
Soil Conservation Centre,
Ministry of Works and Development
One of the major decisions to be made in developing a horticultural property is that of which tree species or species combination to plant for wind protection. For many growers, this is a difficult decision, despite the fact that considerable experience with trees for windbreak establishment has been gained over recent years in New Zealand.
The main reason for this difficulty is that there is no single species or species combination that is suitable for all of the environments now being developed in horticulture.
Major factors that affect species performance are:
- Susceptibility to frost damage
- Soil texture and drainage
- Droughtiness of the site
- Presence of salt laden winds
Additional factors which can influence the choice of species are:
- How soon the crop is to be planted i.e., the growth rate required of the shelter trees
- The degree of wind reduction required
- The period during the year when wind protection is required
- The degree of cold air drainage required to limit frost damage
- Host to insect pests which affect the crop
- Competition for bees for crop pollination
- Whether invasion of tile drains by roots is likely to be a problem
- The effect of the root system on crop performance by competition for water and nutrients
- The availability of irrigation.
Despite all these variables, a relatively limited range of species is planted for horticulture shelter. This article describes the major species used, and also discusses some new options.
MAJOR SPECIES are listed in Table 1.
Willows are widely used for horticultural shelter in many areas, largely because of their fast growth rate and ease of establishment. The species most commonly planted are Salix matsudana, and the recently developed Salix matsudana x alba hybrids.
Salix matsudana has been planted very widely since 1973, when the arrival of the poplar rusts in New Zealand severely limited the usefulness of the Lombardy poplar Populus nigra “Italica”).
It is fast growing, with somewhat pendulous lower branches, and develops a wide-spreading crown if now trimmed. It is not affected by any serious pest or disease in New Zealand.
Salix matsudana x alba hybrids. These new hybrids are faster growing and have less pendulous branches than 5. matsudana, and have other desirable characteristics such as longer leaf retention and very good low branch retention. The first released and most commonly planted clone is “Aokautere” (NZ 1002). This was not selected specifically for shelterbelt planting, but its rapid growth rate and narrower crown than S. matsudana were attractive to horticulturalists. Its major disadvantages are a shorter leafing period and lower drought tolerance than matsudana.
The Soil Conservation Centre of the Ministry of Works and Development at Aokautere released in 1980 a further three new S. matsudana x alba clones specifically selected for horticultural shelter planting, and these are now becoming available commercially. All three clones are faster growing than the commonly planted S. matsudana, and are most useful in situations where shelter is required as quickly as possible.
Although similar in many respects to clone “Aokautere” they have generally straighter stem form and each clone has some specific characteristics which make it useful for shelterbelt planting. They are all completely resistant, as is S. matsudana, to the weeping willow rust which recently arrived in New Zealand, and do not harbour any pests or diseases which can affect horticultural crops.
Salix matsudana x alba cl. “Tangoio” (NZ 1040).
“Tangoio” has attractive light grey foliage similar to that of S. matsudana, and a dense moderately spreading crown, which would eventually need side trimming in an orchard. Its main feature is its excellent low branch retention which makes it unnecessary to interplant with any other species.
Tree form is very straight, with good apical dominance and little forking of leaders. “Tangoio” responds well to side trimming and can be kept quite narrow yet reasonably dense. Sex is female.
Salix matsudana x alba cl. “Makara” (NZ 1179).
The very fast growth of “Makara” makes it suitable for tall shelter, although the maximum height is not known as yet. The crown is lighter than “Tangoio” but considerably more narrow, with blue-green foliage. Stem form is straight. The lower side branches do not persist well in dense plantings, but are retained adequately in single row shelterbelts.
It is best interplanted with a shrubby species, or can be planted alternately in the row with “Tangoio”. Trees of “Makara” are generally in full leaf until early June, 3-4 weeks later than the clone Aokautere (“NZ 1002”), making it especially suitable for sheltering kiwifruit and subtropical fruit crops. Sex is female.
Salix matsudana x alba cl. “Moutere” (NZ 1184).
“Moutere” is probably the best all round willow for horticultural shelter, because it has a reasonably narrow crown, with slightly broader leaves than the above two clones, and forms a fairly dense but permeable windbreak under most conditions. The foliage is darkish blue-green and the stem form is straight. Like “Makara” this clone is generally in full leaf until early June. Growth rate is almost as fast as “Makara” and because lower side branches are retained well, ‘Moutere’ can be used in single row shelterbelts without underplanting. Sex is male.
Both S. matsudana and its hybrids are susceptible to damage from salt winds, and cannot be grown successfully in areas exposed to strong salt-laden winds. They will grow readily on a wide range of soil types, but under dry conditions will require irrigation.
Since 1973, repeated epidemics of poplar rust have caused either death or severe reductions in growth of Populus nigra ‘Italica’ (Lombardy poplar) and P. nigra ‘Sempervirens’ (semi-evergreen Lombardy poplar). These trees can no longer be recommended for shelter planting except in areas of low humidity in the South Island, or where regular spraying with copper based fungicides is proposed.
Rust resistant species and clones selected by the Soil Conservation Centre, include Populus x euramericana ‘Flevo’, P. x euramericana “Tasman” and P. trichocarpa.
Populus x euramericana “Flevo”. This has been the only rust resistant poplar available until recently, and has been widely planted for horticultural shelter. It is also resistant to poplar leaf spot (Marssonina brunnea). However, it has a much more spreading crown than Lombardy type poplars and is suitable more for boundary shelter than for internal shelter. It rapidly grows into a large tree, and requires heavy side trimming.
It grows best on friable, well drained soils with adequate soil moisture. It is less drought sensitive than willows, and will retain its foliage through quite dry periods. “Flevo” comes into leaf in mid to late September, and loses its leaves in mid-April. It is slightly more tolerant than willows to salt laden winds.
Populus x euramericana “Tasman”. This clone together with P. trichocarpa was recently released by the Soil Conservation Centre, Aokautere, and is rapidly becoming popular for horticultural shelter. “Tasman” was bred in Holland in 1965, and introduced to New Zealand in 1973.
It has a considerably better form than “Flevo” with a straight stem and finely-branched fairly narrow crown. The growth rate is similar to or slightly faster than “Flevo”. It is highly resistant to the two poplar leaf rusts in New Zealand, but slightly susceptible to the leaf spot disease Marssonina brunnea which, although not detrimental to vigorous growth, may cause early autumn defoliation following high summer rainfall. Its leafing period and salt wind and drought tolerance are almost identical to those of ‘Flevo’.
Populus trichocarpa. This is a balsam poplar, originally selected in Belgium, and released in New Zealand in 1979. Its growth rate is reasonably rapid, and it is highly resistant to both poplar rusts and to Marssonina. Although not suitable for very exposed situations, it is promising for horticultural shelter because it retains a very good low branch structure and autumn leaf fall is not until mid-June. However, it is very late coming into leaf in the spring.
Radiata pine is normally planted where other species cannot be successfully grown because of exposure to salt-laden winds, or where very tall evergreen boundary shelter is required. Trees are inexpensive, and where correct management is practiced this species can provide a very effective shelterbelt.
Trees must be side-trimmed from an early age to prevent their opening at the base. However, this will eventually occur, and underplanting with a shade tolerant lower growing species will be necessary.
Eucalyptus botryoides and E. saligna have been the main species planted, particularly for boundary shelter. This is mainly due to their fast growth rate and resistance to the leaf chewing insect Paropsis charybdis. In shelterbelts, they rapidly develop a spreading form and often suffer crown damage from strong winds.
Other Eucalyptus species which could be used as well as the above for boundary shelter are E. fraxinoides, and E. regnans. These species retain a more narrow crown form, particularly at younger ages, and are reasonably resistant to Paropsis. E. fraxinoides retains its lower branches longer than many other Eucalyptus species, and does not suffer from crown damage by wind.
Eucalypts are not suitable for internal shelter, and need underplanting when used for boundary shelter.
Sheoaks. Casuarina species.
Two species are widely planted: Casuarina cunninghamiana (river sheoak), and C. glauca (swamp sheoak). Of the evergreen species, the sheoaks are the most permeable, although regular trimming is necessary to prevent them from becoming too open. C. glauca will withstand heavier soils than C. cunninghamiana and is more resistant to infection by Phytopthora root rots.
C. glauca may be slightly more resistant to salt-laden winds though neither have high levels of tolerance.
C. cunninghamiana has considerably better form than C. glauca and is to be preferred where soil conditions are not limiting. Both species are fairly frost sensitive, and in frost prone areas should not be planted out until September. Young trees are particularly palatable to hares and rabbits.
Japanese cedar (Cryptomeria japonica).
This species is widely planted in northern districts of the North Island, where it grows moderately rapidly. It is often planted in conjunction with willows or poplars, because it is particularly shade tolerant. It can be used as the slower growing species in both boundary and internal shelterbelts. It does not perform well in the South Island.
Leyland cypress (Cupressocyparis leylandii). Several clones of Leyland cypress are available, the most commonly planted being “Leighton Green”. Other clones include “Naylors Blue”, “Green Spire”, “Haggerston Grey”, and “Castlewellan Gold”. All these clones can be affected by cypress canker, but they are more resistant than other Cupressus species.
They are relatively fast growing for an evergreen species, and eventually grow into large trees. Their major use is for boundary shelter in districts exposed to salt-laden winds. Regular side trimming is required (they are prone to heavy-side branching) but this may cause the shelterbelt to become too dense for optimum sheltering effect.
The clone with the narrowest form is “Naylors Blue” which, although somewhat slower growing than the other clones, may in fact be the most suitable for horticultural shelter on fertile sites. All clones can tolerate a wide range of soil conditions, and are quite frost tolerant.
Western Red Cedar (Thuja plicata). This evergreen species is slow growing, but useful for planting in combination with a faster growing poplar or willow species. Its major advantage is its resistance to cypress canker. It does not withstand very exposed conditions, although it is quite frost tolerant.
Alders (Alnus species). Alders are gaining in popularity for use as internal shelterbelts, for several reasons. Because they fix atmospheric nitrogen they do not compete to any extent with the crop for this nutrient and in fact can contribute additional nitrogen to the ecosystem, mainly through leaf litter. Although they are reasonably fast growing, they do not reach a very large size, and can be easily controlled by annual or biennial trimming.
They retain their lower branches very well when trimmed, and some species hold their leaves well into the autumn. This is an important factor for the protection of autumn or winter harvested crops. When trimmed regularly, their permeability is about the optimum 50%. Four species are commonly available in New Zealand: Alnus cordata, A. glutinosa, A. incana and A. rubra.
A. cordata (Italian alder) retains its leaves longer into the autumn than the other species, but can be susceptible to damage from severe frosts. It is probably the most promising species for New Zealand conditions. It is not suitable for very acid soils.
A. incana is the hardiest of the European alder species, and retains its vigour on dry soils. An unattractive feature is the production of root suckers.
A. glutinosa (common or black alder) is more tolerant of wet or clay soils. In Europe, some trees have a tendency to flower precociously at about age four years, and subsequently lose vigour. However, this characteristic has been noticed only rarely in New Zealand.
A. rubra (red alder) grows well in New Zealand, but its usefulness in shelterbelts has not been determined as yet. It has a tendency to become quite spreading at a relatively early age, but this can no doubt be controlled by trimming.
Species tolerant to salt-laden winds
Damage to shelterbelts from constant westerly winds over much of New Zealand during last summer highlighted the need for species more tolerant to salt-laden winds. Species which can be considered for areas where damage occurred are listed in Table 2. Several of these species have been commented on above.
|TALL BOUNDARY SHELTER|
|a) Most exposed sites|
|Araucaria heterophylla (Norfolk Island Pine)
Cupressus macrocarpa (macrocarpa)
|b) Slightly less exposed sites|
|Cupressocyparis leylandii (Leyland cypress)
|MEDIUM HEIGHT SHELTER|
|Banksia integrifolia (coast Banksia)
Casuarina glauca (swamp sheoak – moderate salt winds only)
|LOW SHELTER TO 5m TALL|
Tagasaste (tree lucerne)
Coprosma repens (taupata)
Dodonaea viscosa (purple akeake)
Lagunaria patersonii (Norfolk Island hibiscus)
Olearia traversii (Chatham Island akeake)
Phormium tenax (New Zealand flax)
Pittosporum crassifolium (karo)
Pomaderris apetala (tainui)
Teucrium fruticans (germander)
Norfolk Island pine (Araucaria heterophylla).
Norfolk Island pine will tolerate very exposed coastal conditions, and can be considered for first line defence where radiata pine or macrocarpa are burnt by salt-laden winds. Trees are initially slow growing, and require wind protection for the first two years. They can be trimmed successfully, but need to be underplanted with a lower growing species.
Coast Banksia (Banksia integrifolia).
This species is extremely tolerant to salt-laden winds, has a moderate growth rate and forms a narrow crown. It tolerates most soil conditions except waterlogging, but is not suitable for planting in areas with heavy frost. It should be planted in conjunction with another faster growing species.
Low growing species.
A reasonable number of lower growing species are quite tolerant of salt-laden winds, although most are widespreading. An exception is Olearia traversii (Chatham Island akeake) which has a very narrow crown, and grows well alongside taller growing species. Of the remainder of the species in this group listed in Table 2, Pittosporum crassifolium (karo) is the most useful, and can be planted very successfully alongside Pinus radiata to provide bottom shelter.
The characteristics of all the more commonly planted horticultural shelter species are summarised in Table 3. This Table enables direct comparisons to be made between species, and the most suitable species or species combination can be selected according to individual requirements.
Note – Table 3 above is an image file –
A larger clearer view is available by right-clicking on it (or equiv); choose open image (or equiv).
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