Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Datasheet

Hakea drupacea
(sweet hakea)

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Datasheet

Hakea drupacea (sweet hakea)

Summary

  • Last modified
  • 27 September 2018
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Preferred Scientific Name
  • Hakea drupacea
  • Preferred Common Name
  • sweet hakea
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Plantae
  •     Phylum: Spermatophyta
  •       Subphylum: Angiospermae
  •         Class: Dicotyledonae
  • Summary of Invasiveness
  • The main factors contributing to the success of H. drupacea as an invader in South Africa are the high seed longevity in the canopy and efficient seed dispersal. The winged seeds of H. drupacea facilitate dispersal over several kilometres in some cas...
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Identity

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Preferred Scientific Name

  • Hakea drupacea (C.F. Gaertn.) Roem. & Schult.

Preferred Common Name

  • sweet hakea

Other Scientific Names

  • Conchium drupaceum C.F. Gaertn.
  • Hakea suaveolens R. Br.

Local Common Names

  • South Africa: hakea boom; soet hakea; soet speldebos

EPPO code

  • HKASU (Hakea suaveolens)

Summary of Invasiveness

Top of page The main factors contributing to the success of H. drupacea as an invader in South Africa are the high seed longevity in the canopy and efficient seed dispersal. The winged seeds of H. drupacea facilitate dispersal over several kilometres in some cases. In South Africa, dense H. drupacea infestations threaten the biodiversity of the Cape Floral Kingdom (fynbos).

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Plantae
  •         Phylum: Spermatophyta
  •             Subphylum: Angiospermae
  •                 Class: Dicotyledonae
  •                     Order: Proteales
  •                         Family: Proteaceae
  •                             Genus: Hakea
  •                                 Species: Hakea drupacea

Notes on Taxonomy and Nomenclature

Top of page The name Hakea suaveolens has been changed to H. drupacea (Barker, 1990), although it is still often found in the literature by this synonym.

Description

Top of page H. drupacea is a much-branched, rounded shrub or tree up to 6 m high. The adult leaves are dark green, hairless, up to 100 mm long. The leaves are usually divided into upright, 30-50 mm long sharp-pointed needles. H. drupacea produces juvenile leaves, which are broad, dentate, leathery and prickly. The flowers are white in colour with a pink tinge and occur in axillary racemes up to 20 mm long. The mature fruits are woody follicles (25 mm long and 20 mm wide) comprising two dehiscent valves, each valve containing one black seed. The seeds are 7 mm long and 3 mm broad. The mature fruit is a pale yellowish-brown colour with dark warts on the surface. The fruits become pale grey as they age.

Plant Type

Top of page Perennial
Seed propagated
Shrub
Tree
Woody

Distribution

Top of page H. drupacea is endemic to coastal regions of south-western Australia and some islands in the adjacent Recherché Archipelago. The natural vegetation where H. drupacea occurs is evergreen forest and xerophilous woodland with a Mediterranean-type climate. In South Africa it is restricted to the Cape Peninsula, Somerset West, Franschhoek, Bot River, Hawston and the Kleinrivier Mountains (Neser, 1978; Fugler, 1982).

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

Africa

South AfricaRestricted distributionIntroduced1850 Invasive Shaughnessy, 1980

Oceania

AustraliaPresentPresent based on regional distribution.
-New South WalesRestricted distribution Not invasive Royal Botanic Gardens Sydney, 2003
-VictoriaRestricted distribution Not invasive Royal Botanic Gardens Sydney, 2003
-Western AustraliaRestricted distributionNative1850 Not invasive Neser, 1968

History of Introduction and Spread

Top of page The first known occurrence of H. drupacea in South Africa was 1850. It was introduced for sand stabilization and for use as a hedge plant (Shaughnessy, 1986). Although H. drupacea was widely planted in the Cape Peninsula it has not become as invasive as H. sericea. This has been attributed to the long juvenile period (6 years) and lower seed production of H. drupacea (Richardson et al., 1987).

Risk of Introduction

Top of page Further spread is possible if accidentally introduced as an ornamental or a barrier plant. H. drupacea has been declared a noxious weed in South Africa under the 'Conservation of Agricultural Resources Act' (Act No. 43 of 1983).

Habitat

Top of page H. drupacea occurs on granitic outcrops along the south-western coast of Western Australia. The vegetation is evergreen forest and xerophilous woodland. In South Africa the plant grows in sandstone and granite soils. All the areas where H. drupacea occurs in South Africa experience a Mediterranean-type climate with an annual rainfall between 400 and 800 mm per annum. The rainfall range is wider in its native range. The plants are restricted to well-drained acidic soils.

Habitat List

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CategoryHabitatPresenceStatus
Littoral
Coastal areas Present, no further details Harmful (pest or invasive)
Terrestrial-natural/semi-natural
Natural grasslands Present, no further details Harmful (pest or invasive)

Hosts/Species Affected

Top of page H. drupacea is not a weed of crops but is an environmental weed sometimes invading natural grasslands used as pastures.

Biology and Ecology

Top of page Physiology and Phenology

H. drupacea flowers from May to August. The newly set fruits are green but when they mature they become woody with age, generally occurring in clusters of 4 to 7. H. drupacea has a long juvenile period and can take between four and six years to reach maturity.

Reproductive Biology

Propagation is by seeds only, which accumulate in the canopy during the lifetime of the plant. The seeds are protected in woody follicles and are only released following the death of a branch or plant, usually by fire. The seeds in the canopy remain viable for many years. Once released the seeds can germinate quickly under moist conditions. With frequent watering, seeds of H. drupacea start germinating after 30 days. Maximum germination of 90% is reached after 60-70 days (Richardson et al., 1987). The winged seeds of H. drupacea have the potential to disperse long distances.

Environmental Requirements

H. drupacea occurs on well drained soils derived from quartzite and sandstone. It can tolerate drought.

Latitude/Altitude Ranges

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Latitude North (°N)Latitude South (°S)Altitude Lower (m)Altitude Upper (m)
0 0 0 0

Air Temperature

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Parameter Lower limit Upper limit
Mean annual temperature (ºC) 13 22
Mean maximum temperature of hottest month (ºC) 16 40
Mean minimum temperature of coldest month (ºC) 2 16

Rainfall

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ParameterLower limitUpper limitDescription
Dry season duration4number of consecutive months with <40 mm rainfall
Mean annual rainfall2003000mm; lower/upper limits

Rainfall Regime

Top of page Summer
Uniform
Winter

Soil Tolerances

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Soil drainage

  • free

Soil reaction

  • acid
  • neutral

Soil texture

  • heavy
  • light
  • medium

Natural enemies

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Natural enemyTypeLife stagesSpecificityReferencesBiological control inBiological control on
Aphanosperma occidentalis Herbivore Fruits/pods

Notes on Natural Enemies

Top of page Neser (1968) recorded a number of insects attacking H. drupacea. The most promising insect was Aphanosperma occidentalis Britt. (Cerambycidae) that attacks the mature fruit in the native range in Australia.

Means of Movement and Dispersal

Top of page Natural Dispersal (Non-Biotic)

Propagation is by seeds only, which are produced in large numbers and stored in the canopy. The seeds are only liberated once the plant or a branch dies. The winged seeds of the closely related H. sericea are dispersed long distances by wind. These seeds then form the nucleus of new infestations.

Accidental Introduction

Only very occasionally are there instances of human dissemination via the collection of woody follicles for dried flower arrangements and their subsequent discarding on rubbish heaps.

Intentional Introduction

H. drupacea was introduced into South Africa as a hedge plant and has been planted for sand binding and firewood production. It may also have been introduced into other countries as an ornamental.

Plant Trade

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Plant parts liable to carry the pest in trade/transportPest stagesBorne internallyBorne externallyVisibility of pest or symptoms
Fruits (inc. pods)
Plant parts not known to carry the pest in trade/transport
Bulbs/Tubers/Corms/Rhizomes
Flowers/Inflorescences/Cones/Calyx
Leaves
Roots
Stems (above ground)/Shoots/Trunks/Branches

Impact Summary

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CategoryImpact
Animal/plant collections None
Animal/plant products None
Biodiversity (generally) Negative
Crop production None
Environment (generally) Negative
Fisheries / aquaculture None
Forestry production None
Human health None
Livestock production Negative
Native fauna Negative
Native flora Negative
Rare/protected species Negative
Tourism Negative
Trade/international relations None
Transport/travel None

Impact

Top of page H. drupacea poses a threat to the US$40 million South African wild flower export industry. The costs to control H. drupacea are high as the plant occurs in dense thickets and in inaccessible areas. The loss of water, biodiversity and amenity value of mountain areas are difficult to estimate but must be significant.

Environmental Impact

Top of page H. drupacea is a serious invader of the floristically rich and unique mountain fynbos in the Western Cape Province, South Africa. Dense stands of woody alien plants can alter the composition of natural plant and animal communities (Macdonald and Richardson, 1986). They also lead to increased fire intensities that may kill plant species that regenerate vegetatively and seeds on or in the soil. The most obvious impact of dense stands of H. drupacea on the native vegetation is the reduction in species richness and the altered appearance of the landscape.

Impact: Biodiversity

Top of page In South Africa, dense H. drupacea infestations threaten the biodiversity of the Cape Floral Kingdom (fynbos), which is one of the eight Floral Kingdoms of the World. Dense stands of H. drupacea (>2000 plants/ha) have brought about significant reductions in species richness of indigenous plants in the fynbos (Richardson et al., 1989).

Social Impact

Top of page H. drupacea is an unpalatable, prickly plant that forms dense impenetrable thickets restricting access to mountain areas.

Risk and Impact Factors

Top of page Invasiveness
  • Proved invasive outside its native range
  • Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
  • Highly mobile locally
  • Has high reproductive potential
  • Has propagules that can remain viable for more than one year
Impact outcomes
  • Damaged ecosystem services
  • Ecosystem change/ habitat alteration
  • Negatively impacts agriculture
  • Negatively impacts tourism
  • Reduced amenity values
  • Reduced native biodiversity
Impact mechanisms
  • Competition - monopolizing resources
  • Produces spines, thorns or burrs
Likelihood of entry/control
  • Difficult to identify/detect as a commodity contaminant
  • Difficult/costly to control

Uses

Top of page H. drupacea is grown as an ornamental and used as a barrier or hedge. It has limited use in the dried flower industry.

Prevention and Control

Top of page Mechanical Control

The most successful method to control H. drupacea is the fell and burn technique. Adult plants are cut down and stacked into piles and left for 12 to 18 months before they are burnt. Shortly after the plants are cut down the fruits, which have accumulated over the plant's lifetime, split open and fall to the ground. The released seeds germinate the following winter. It is important that the area is burnt before the seedlings reach reproductive maturity. H. drupacea produces its first fruits after four to six years. One or two follow-up operations are necessary after the burn to eradicate any regenerating or coppicing plants. Any regenerating seedlings can be hand-pulled using thick gloves. This is an essential facet of the operation as it ensures that no plants are left to produce viable seeds. It is not advisable to just cut and leave adult plants, as fire is needed to kill any seedlings that may germinate.

Chemical Control

Tebuthiuron and triclopyr are recommended for the control of H. drupacea. Tebuthiuron has been shown to be safe for use in environmentally sensitive areas.

Biological Control

Biological control has not been used for the control of H. drupacea but a number of natural enemies have been identified in Australia that may be potential candidates (Neser, 1968).

Integrated Control

H. drupacea is not as invasive as H. sericea and H. gibbosa and can be easily controlled mechanically. The 4-6 year juvenile period also allows more time before follow-up operations need to be carried out.

References

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Barker RM, 1990. New species, new combinations and other name changes in Hakea (Proteaceae). Journal of the Adelaide Botanical Gardens, 13:95-110.

Fugler SR, 1982. Infestations of three Australian Hakea species in South Africa and their control. South African Forestry Journal, No. 120:63-68

Macdonald IAW; Richardson DM, 1986. Alien species in terrestrial ecosystems of the fynbos biome. In: Macdonald IAW, Kruger FJ, Ferrar AA, eds. The ecology and management of biological invasions in southern Africa. Cape Town, South Africa: Oxford University Press, 77-91.

Neser S, 1968. Studies on some potentially useful insect enemies of needle-bushes (Hakea spp. - Proteaceae). PhD thesis. Canberra, Australia: Australian National University.

Neser S, 1978. Sweet hakea. In: Stirton CH, ed. Plant Invaders: Beautiful but Dangerous. Cape Town, South Africa: Department of Nature and Environmental Conservation of the Cape Provincial Administration, 76-79.

Richardson DM; Macdonald IAW; Forsyth GG, 1989. Reductions in plant species richness under stands of alien trees and shrubs in the fynbos biome. South African Forestry Journal, No. 149:1-8

Richardson DM; Wilgen BW van; Mitchell DT, 1987. Aspects of the reproductive ecology of four Australian Hakea species (Proteaceae) in South Africa. Oecologia, 71(3):345-354

Royal Botanic Gardens Sydney, 2003. Australia's Virtual Herbarium. Sydney, Australia: Royal Botanic Gardens. http://plantnet.rbgsyd.gov.au/cgi-bin/avh/avh.cgi.

Shaughnessy GL, 1986. A case study of some woody plant introductions to the Cape Town area. In: Macdonald IAW, Kruger FJ, Ferrar AA, eds. The ecology and management of biological invasions in southern Africa. Cape Town, South Africa: Oxford University Press, 37-43.

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