Heterotheca grandiflora
(telegraph weed)

Heterotheca grandiflora is an annual or biennial herb native to arid coastal regions of North America, from California to northern Mexico. It produces large numbers of wind-dispersed seeds and is naturalized on all of the main Hawaiian Islands, USA, as well as in coastal regions of both New South Wales and south-east Queensland, Australia. In Hawaii it invades dry, disturbed sites from near sea level to over 2200 m elevation, forming dense cover and competing with native plants species including the endangered Schiedea hawaiiensis. In Australia, it is considered a potential or emerging environmental threat that invades and dominates disturbed sites and is designated a Class 2 weed in Queensland (Queensland Government, 2013).

H. grandiflora is native to southwestern North America, in coastal regions to 900 m elevation, from California to northern Mexico; it is probably introduced to Arizona, Nevada, and Utah (Flora of North America Editorial Committee, 2015), though considered as native to all four US states by USDA-NRCS (2015). It is recorded from all of the main Hawaiian Islands except Niihau (Wagner et al., 1999; Oppenheimer, 2010). In Australia, It has become naturalized in the lower Hunter Valley of New South Wales (Auld and Medd, 1987; Harden, 1992) as well as at the northern end of Queensland’s Gold Coast (Csurhes, 2009). It is reported as present in Japan, with no details on status or distribution (Mito and Uesugi, 2004). 

Within its native range, H. grandiflora is promoted because it is beneficial to native pollinators (The Pollinator Partnership and NAPPC, 2008). Otherwise, there are no reported uses or commercially available sources of this plant and the probability of long-distance intentional or accidental introduction is low. 

H. grandiflora is a weed of pastures, rangelands, cultivated areas and wastelands.

In its native range of coastal North America. H. grandiflora occurs in disturbed ground, along roadsides, and in vacant lots and fields from 0-100(-900) m elevation (Flora of North America Editorial Committee, 2015). In Hawaii it occurs in dry, disturbed areas from 10 to 2270 m elevation (Wagner et al., 1999).

In South East Queensland, H. grandiflora also invades disturbed or open, sparsely vegetated coastal sand dunes. Csurhes (2009) describes the habitat as harsh, wind-blown by sand and salt spray and with seasonally dry and hot conditions.

Genetics

H. grandiflora has a chromosome count of 2n=18 (Flora of North America Editorial Committee, 2015).

Reproductive Biology

As an annual or biennial plant, H. grandiflora is able to flower after one to two years of growth (Wagner et al., 1999). After summer flowering in the first year, the plant dies back to the base, then re-sprouts and flowers again in the second year (Csurhes, 2009). In Queensland, Australia, peak seed production occurs in late summer and autumn (Csurhes, 2009). In California, USA, plants flower mostly during the summer, but can flower throughout the year depending on location, and seed germination occurs from autumn through to spring (DiTomaso and Healy, 2007).

Pollinator exclusion experiments demonstrate that plants are self-compatible, and capable of limited self-pollination, but seed set is improved in field settings (Flint, 1977). Flowers are visited and pollinated by a variety of generalist insects, including bees, beetles, flies and butterflies (Flint, 1977; The Pollinator Partnership and NAPPC, 2008).

As an annual to biennial plant, H. grandiflora persists for one to two years (Wagner et al., 1999). The disc achenes lack dormancy and will germinate whenever conditions are suitable. The harder coat of the ray achenes allows them to persist in the seed bank until the seed coat is worn down through abrasion, disturbance or other forms of scarification (DiTomaso and Healy, 2007).

Associations

In the Hawaiian Islands, H. grandiflora is now a common non-native component of subalpine shrubland at the base of Mau Kea, in the saddle between it and Mauna Loa on Hawaii Island. The community is dominated by the arborescent native shrub Chenopodium oahuense and scattered trees of Sophora chrysophylla and Myoporum sandwicense, with other weedy grasses and composites including Bromus rigidus, Nassella cernua, Tagetes minuta and Verbesina encelioides (Wagner et al., 1999).

H. grandiflora is also common in Sophora forest on Hawaii and East Maui, dominated by the native trees Sophorachrysophylla, and secondarily by Myoporum sandwicense, and with a number of weedy grasses and herbs including Bromus rigidus, Dactylis glomerata, Danthonia pilosa, Hypochoeris radicata and Verbascum thapsus (Wagner et al., 1999).

On the northeast slopes of East Maui and Mauna Loa, Hawaii, at 1680-1980 m elevation, H. grandiflora is a common component of subalpine mesic grasslands and shrublands, occurring with the native grass Deschampsia nubigena, the indigenous bracken fern Pteridium aquilinum and native shrubs consisting of Dubautia species and Leptecophylla tameiameiae. Other common weeds of this community type include the grasses Anthoxanthum odoratum, Dactylis glomerata, Festuca rubra, Holcus lanatus, and the herbs Cirsium vulgare, Hypochoerisradicata,Oenothera stricta and Verbascum thapsus (Wagner et al., 1999).

Environmental Requirements

The native range of H. grandiflora from California to northern Mexico has a temperate climate that is seasonally hot and dry. Within its naturalized range, including Hawaii, Australia, and probably Arizona, Utah and Nevada, it can tolerate climates that are tropical to sub-tropical and soils that are sandy, gravelly or otherwise well-drained (Csurhes, 2009; Flora of North America Editorial Committee, 2015). It also grows across a broad elevation range in the Hawaiian Islands, from 10 to 2270 m, demonstrating a measure of environmental versatility in the tropical to sub-tropical zone.

There are no natural enemies or biological control agents currently known for H. grandiflora. In the Hawaiian Islands, Secusio extensa (Lepidoptera: Arctiidae) was evaluated as a potential biological control agent for Madagascar fireweed (Senecio madagascariensis) from the Asteraceae family, and H. grandiflora was one of the species evaluated, and found to be an unsuitable host in no-choice feeding tests (Ramadan et al., 2011).

H. grandiflora is reported to have low palatability to goats (Green and Newell, 1982) and is one of the few plants remaining in sparsely-vegetated areas otherwise heavily browsed by feral goats in Hawaii (Oppenheimer, 2010). 

Environmental

• Wildlife habitat

H. grandiflora is included in the Weeds of Australia Lucid identification key (University of Queensland, 2013).

In the Hawaiian Islands, H. grandiflora may be mistaken for Verbascum thapsus (common mullein), a herbaceous plant in the Scrophulariaceae family V. thapsus is also invasive at higher elevations, and has whitish hairy leaves and tall inflorescences that superficially look like H.grandiflora. H. grandiflora can be distinguished from V. thapsus by its dandelion-like flowers and by smaller leaves with pointed tips that have a distinctive, sage-like odour when crushed (Wagner et al., 1999).

Erigeron canadensis (horseweed) and Laennecia coulteri (Coulter's woolwort) are both members of the Asteraceae family with stem growth and flowering habit that also appear similar to H. grandiflora. Both of these taxa have white to cream-coloured flower heads, versus the bright yellow of H. grandiflora. The flowering stems of E. canadensis have leaves that are linear to narrowly lanceolate and the foliage lacks glandular hairs. 

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Prevention

In Australia, H. grandiflora is considered a potential or emerging environmental threat that can invade and dominate disturbed sites. It is designated a Class 2 weed in Queensland, requiring landowners to make efforts to control the plant on their property and prohibiting the sale of the plants or seeds within the state (Queensland Government, 2013).

Containment

To prevent further spread, an infestation of H. grandiflora, distributed over 200-300 hectares at the northern end of the Gold Coast in Queensland, Australia, has been the target of control efforts, but is otherwise regarded as probably beyond eradication (Csurhes and Edwards, 1998; Csurhes, 2009).

Physical/Mechanical Control

H. grandiflora can be hand-pulled, burned or mowed to prevent flowering (Smiley, 1922; DiTomaso and Healy, 2007). Cultivation may also deplete the seedbank by stimulating germination of the hard-coated ray achenes (DiTomaso and Healy, 2007).

Chemical Control

Herbicides are reported to provide effective control of H. grandiflora (Csurhes, 2009). Specific herbicides evaluated include 2,4-D amine (Zandstra et al., 2004; Willsher et al., 2008).

USA: Haleakala National Park Resources Management - HALE RM, PO Box 369 Makawao HI 96768, http://www.nps.gov/hale/index.htm

Australia: Biosecurity Queensland Department of Agriculture, Fisheries and Forestry - DAFF, GPO Box 46, Brisbane QLD 4001, www.daf.qld.gov.au

31/05/15 Original text by: 

Charles Chimera, Hawaii-Pacific Weed Risk Assessment, PO Box 983, Makawao, Hawaii 96768, USA