Labill.

blue gum, Tasmanian blue gum, common eucalyptus

none known

none

8

CalEPPC List A-1,CDFA nl

David Boyd

Blue gum (Eucalyptus globulus) is
a tall (150-180 foot), aromatic, straight-growing tree, with bark that sheds in
long strips, leaving contrasting smooth surface areas. Adult leaves are waxy
blue, sickle-shaped, and hang vertically. Juvenile leaves are oval, bluish
green, and have square stems. Fruits are blue-gray, woody, and ribbed. Trees
produce abundant fruit drop and leaf and bark litter. Blue gum is distinguished
by tall growth habit, smooth bark, long leaves, and large, solitary, waxy buds
and fruits (Chippendale 1988).

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Blue gum has been planted extensively
worldwide because of its rapid growth and adaptability to a wide variety of site
conditions. It does especially well in Mediterranean climate regions,
characterized by cool, wet winters and dry, warm summers, such as portions of
California, Chile, Portugal, Spain, and South Africa (Skolmen 1983). In
California it is most widely planted in central coast locations, but found below
1,000 feet elevation of the north, central, and south coasts, as well as inland
throughout the Central Valley. It is most frequently found growing in small
groves or windbreaks within grassland habitats where initial plantings took
place. Large specimen trees are found in urban and rural settings.

Blue gum grows well on a wide range of soils, but requires good
drainage, low salinity, and a soil depth of two feet (0.6 m) or more. In
California it grows best on deep alluvial soils because of the greater moisture
supply (Skolmen and Ledig 1990). Hawaiian soils supporting blue gum eucalyptus
are about three feet (0.9 m) deep. They are usually acidic, moderately well
drained, silty clay loams (Skolmen 1983). Blue gum does well with only
twenty-one inches (530 mm) of annual rainfall accompanied by a pronounced dry
season, primarily because frequent fogs compensate for lack of rain (Skolmen and
Ledig 1990).

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Native to Australia, where it occurs mainly
along the east coast of Tasmania, blue gum was first cultivated in California in
1853 as an ornamental. Widespread commercial planting occurred after 1870,
primarily for timber and fuel. A second planting boom took place in the early
1900s (Groenendaal 1983). By the 1930s planting in California had lost
popularity because of unsuitable characteristics of the wood for lumber
production and a decrease in demand for fuel wood. Blue gum aggressively invades
neighboring plant communities from original plantings if adequate moisture is
available for propagation by seed. Invasive in coastal locations, blue gum is
rarely invasive in the Central Valley or in dry southern California locations.
It is most invasive on sites subject to summer fog drip.

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Within groves, biological diversity is lost
due to displacement of native plant communities and corresponding wildlife
habitat. Abundance and diversity of understory vegetation is dependent on stand
density. Understory establishment is inhibited by the production of allelopathic
chemicals and by the physical barrier formed by high volumes of forest debris
consisting of bark strips, limbs, and branches. The fuel complex formed by this
debris is extremely flammable, and under severe weather conditions could produce
drifting burning material with the potential to ignite numerous spot fires.
Because stringy bark is carried away while burning, eucalyptus forests are
considered the worst in the world for spreading spot fires. The Oakland hills
firestorm was both intense and difficult to control because of the many stands
of eucalyptus. Individual trees growing near structures or in public use areas
are hazardous because of the potential for branch failure. Stature and growth
form are distinctive and unlike native tree species, which compromises the
visual quality of natural landscapes.

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Blue gum reproduces by seed and by resprouting. In California flowering occurs from November to April. Flowers are pollinated by insects and hummingbirds. Fruit ripens from October to March, about eleven months after flowering. Seed set begins at approximately four to five years of age. Good seed crops are produced in most locations at three- to five-year intervals. Seeds are small and abundant. Capsules open immediately on ripening, and seed is dispersed by wind within one to two months. Dispersal distance from one 131-foot (40 m) tall tree, with winds of six miles per hour (10 km/h), was sixty-six feet (20 m). Newly released seeds germinate within a few weeks under suitable conditions. California eucalypts have highly variable germination rates, ranging from 2 to 80 percent within a thirty-day germination period. Seedlings often survive in sufficient abundance to significantly invade neighboring plant communities. Establishment of eucalyptus saplings within groves is inhibited by forest litter and duff, but can be significant following disturbances such as fire or harvesting operations (Skolmen and Ledig 1990, Krugman 1974).

Blue gum sprouts readily from the main trunk, from stumps of all sizes and ages, from the lignotuber, and from roots. Large masses of foliage are produced by sprouting stumps after tree felling. Numerous clusters of shoots later thin to one stem per cluster. A number of small-diameter stems can continue to thrive on each stump, resulting in bush-like growth. Production of lignotubers, which may live for many years in soil, may account for sprouting that sometimes occurs away from the main stump of cut trees (Skolmen 1983).

(click on photos to view larger image)

Blue gum typically grows in dense
monospecific stands. Rapid growth is characteristic of this species. Most height
growth occurs within the first five to ten years, and 60 to 70 percent of total
height growth is achieved in about ten years. Growth is dependent on site
quality, but trees can reach heights of eighty feet (24 meters) in ten years
(Skolmen 1983).

Blue gum generally does not form a taproot. It produces roots
throughout the soil profile, rooting several feet deep in some soils. Blue gum
is shade intolerant, and failure to regenerate within forests in the absence of
fire is related to low light intensities. It is drought tolerant and somewhat
frost hardy. Frost resistance increases with maturity (Skolmen and Ledig
1990).

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Manual/mechanical methods: Removing trees is a difficult task
and can be expensive if individual trees are felled. It is also unlikely that
this cost can be offset because of the low value of the wood as fuel. An
effective method to control stump resprouting is absolutely necessary. Stump
grinding can eliminate sprouting, as well as remove all evidence of trees. Where
there are few stumps and the terrain is gentle, this may be a preferred method.
It is expensive to treat many stumps this way, even if a powerful and efficient
self-propelled grinder is used. Care must be taken to grind all underground
portions of stumps to a depth of approximately two feet. Provision must be made
to fill resulting craters with soil.

Manual removal of eucalyptus sprouts from stumps results in
eventual control as food resources are exhausted. This method is expensive and
impractical if a large number of stumps are to be treated. Manual removal should
be limited to situations where close attention can be given to a few stumps.

Prescribed burning: This method can reduce fuels in blue gum
stands, but the species is fire tolerant. Only seedlings can be killed by fire.
Fuel replenishment is rapid.

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Because blue gum is valued as an ornamental
tree in many settings, biological control cannot be considered as a control
option. However, pests have inadvertently been introduced, including the
long-horned borer, which may increasingly affect the health of these
trees.

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The most effective control of sprouting is achieved through
application of triclopyr or glyphosate directly to the outer portion of the
stump‰Ûªs cut surface at the time of tree felling. Triclopyr (as Garlon 4å¨ and
Garlon 3Aå¨) should be applied at the rate of 80 percent in an oil carrier.
Imazapyr (as Arsenal or Stalker) can be used as an alternate to Garlon.
Glyphosate (as Roundupå¨ or Rodeoå¨) should be applied at 100 percent. Stumps
should be cut as low to the ground as practical and brushed clean of sawdust to
maximize absorption of the herbicide. For best results, herbicides should be
applied to the freshly cut surface as soon after cutting as possible. Maximum
success is achieved if cutting occurs in fall (Carrithers, pers. comm.).
Complete control of sprouting on every stump will not always be achieved. Any
resprouts, when three to five feet tall, should be treated with a foliar
application of 2 percent of triclopyr or glyphosate.

Triclopyr (as Garlon 4å¨) offered the best results of the
herbicides currently available in California for a 1996 eucalyptus removal
project at Angel Island State Park in Marin County. A high concentration was
used (80 percent Garlon 4å¨, 20 percent oil carrier; an alternative is 100
percent Garlon 3A). Glyphosate (as Roundupå¨) was used in 1990 on a similar
eucalyptus removal project on Angel Island, but with less consistent results.
When sprouting occurred following the 1990 eucalyptus removal project on Angel
Island, excellent follow-up control was achieved by applying triclopyr as Garlon
4å¨ (80 percent Garlon 4å¨, 20 percent oil carrier) to overlapping frill cuts.
These cuts were made on portions of the vertical surfaces of stumps with live
cambium.

Application of these herbicides to the foliage or stems of
sprouts is less effective. Several years of foliar applications of triclopyr at
Annadel State Park, Sonoma County, following a major eucalyptus removal project
produced only incremental results. The visual impact of tall, herbicide-killed
sprouts must also be considered.

At The Nature Conservancy‰Ûªs Jepson Prairie preserve near Rio
Vista, Solano County, over 1,200 eucalyptus stumps were killed by repeated
foliar herbicide treatments over one summer period (Serpa, pers. comm.). The
stumps were not treated at the time of felling, and sprouts were allowed to grow
to a height of about ten feet. The sprouts were then cut and the resulting
resprouts were treated with glyphosate as Roundupå¨ (5 percent solution). By the
third herbicide application all of the stumps were dead. It is possible that the
dry climate of this site contributed to the success of this method.

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