Emerging Weeds - Current hill country weeds

Historical surveys
Survey of stakeholders

Current hill country weeds

Historical surveys

General observations on the importance of various weed species have been made over the years, although these have rarely been backed up by objective assessments except for a few weed species in the South Island.

Cockayne³⁰ conducted a survey of weeds of national importance in 1917. Californian thistle, twitch (or couch), fathen, and sorrel were then regarded as the most important weeds, followed by sweet brier, gorse, blackberry and broom.

Leonard³¹ (in 1962) reviewed the distribution, importance and control of scrub weeds in the South Island. Serious weed problems (in order of importance) were gorse, sweet brier, manuka and kanuka, broom, matagouri, blackberry and tutu. Potential problems were seen as heaths (Erica spp.), tauhinu, barberry, hawthorn, elder and flowering currants (Ribes spp.)

Several authors of papers in the Plant Protection Society Conference Proceedings reviewed the regional distribution and importance of weeds, especially in the South Island. These highlighted the problems caused by gorse, manuka and bracken. Sweet brier, matagouri, and tauhinu are all common in the South Island and although found in the North Island, have rarely been regarded as serious threats there.

Molloy³²outlined a scheme for recording the distribution of weeds, but this was never put into place except for a few noxious introductions.

Bascand & Jowett³³, ³⁴, in the late 1970s, conducted a detailed survey of the occurrence and importance of 13 scrub weeds below 1200 m a.s.l. in the South Island, and followed this up with a survey of landowners' opinions of serious weeds. The scrub weeds they surveyed and a summary of their results are presented in Table 3.

Table 3. Areas of the South Island covered by the scrub weeds surveyed by Bascand & Jowett

species	km²covered	% of farmable land covered
bracken	1881	2.85
matagouri	1756	2.66
gorse	1214	1.84
kanuka/manuka	812	1.23
broom	607	0.92
sweet brier	581	0.88
tutu	290	0.44
blackberry	271	0.41
tauhinu	152	0.23
tree tutu	33	0.05
elder	33
Himalayan honeysuckle	33
Hawthorn	26

They found that 1.84% of the farmable land in the South Island was covered in gorse - a total area of 121,400 ha. The greatest area of gorse was found in Canterbury, but the percentage cover was greater in Nelson and Westland.

Their conclusion was that scrub weeds occupied about the same status in farmers' opinions as they had 20 or 30 years before. Former concerns about barley grass expressed by Symons³⁵ and Dingwall³⁶ was apparently justified as that had become, by the 1970s, an important weed, as shown by Bascand and Jowett, and by Atkinson and Hartley³⁷. The hawkweeds (Hieracium spp), not mentioned by previous authors, had also by then become serious problems of depleted grasslands in inland Marlborough, Canterbury and Otago. Nodding thistle is another species surveyed by the authors that has certainly increased in incidence over the past 50 years.

Blashke et al.³⁸ analysed New Zealand's vegetation cover, based on vegetation information from the New Zealand Land Resource Inventory (NZLRI), a major survey undertaken by the Water and Soil Division, Ministry of Works and Development. The information they reported on was collected between 1973 and 1979 and included compilation of rock type, soil, slope, erosion and vegetation. The vegetation types included some scrub weeds - gorse, manuka, 'fern', tauhinu, broom, sweet brier, matagouri, heather and blackberry, associated with either scrub or pasture. The authors reported the data shown in Table 4.

Table 4 . Area of mapping units containing scrub weeds listed in Blashke et al. (1981).

	NI area (ha)	SI (ha)	NZ total (ha)	NZ total %
Total land area	11,413,400	15,021,900	26,435,300	100
Scrub dominated by manuka/kanuka or fern	106,300	48,800	155,100	0.6
Scrub containing gorse	18,500	34,500	53,000	0.2
Scrub containing Cassinia	4,000	3,100	7,100	0.0
Broom-dominated scrub	800	1,900	2,700	0.0
Scrub dominated by sweet brier or matagouri	-	800	-	0.0
Grassland & scrub dominated by manuka/kanuka or fern	1,467,200	690,800	2,158,000	8.2
Grassland & scrub dominated by sweet brier or matagouri	-	1,111,500	1,111,500	4.2
Grassland & scrub containing gorse	201,600	455,000	656,600	2.5
Grassland & scrub containing Cassinia	60,400	49,100	109,500	0.4
Grassland & broom-dominated scrub	4,600	31,000	35,600	0.1
Grassland & blackberry-dominated scrub	23,300	1,300	24,600	0.1

According to these figures, manuka/kanuka is the commonest scrubweed in both North and South Islands. Grassland and scrub dominated by sweet brier or matagouri is the next most important weedy association, but is only found in the South Island. Vegetation associations containing gorse come next, and Cassinia, broom and blackberry come a long way behind.

A detailed examination of the data referring to gorse in the NZLRI in 1981 by Popay, Hartley and Blashke (unpublished) showed that the total area of land infested with gorse (not the total area of gorse-covered land) was 304,540 ha, or 2.7% of the total land area of New Zealand.

None of these surveys or reviews allows any firm indication of whether areas covered by weeds have changed and, more importantly, none of them can tell us which weeds are increasing in importance. Yet this information is vital if potential problem weeds are to be detected early enough. Even the spread of weeds like nodding thistle within the last 50 or so years has only been reported in general and anecdotal terms (Popay and Medd³⁹). In contrast with this, NIWA maintains an aquatic plant database and has consistent data over the last 15 years on the spread of aquatic weeds.

Thus we have little documented evidence of the spread of weeds in New Zealand, and only occasional snapshots of the occurrence and distribution of a few species of weeds. This makes it difficult to know when future weeds are beginning their relentless spread.

Survey of stakeholders

Asking landowners and those concerned with weeds about the weeds under their care is fraught with difficulties. Some farmers, biosecurity officers and regional or district councillors can become fanatical about certain weeds. Many weeds, especially annuals or biennials like thistles wax and wane according to patterns of climate, increasing in area and numbers after a series of dry summers and declining after wetter seasons. The answer to the question 'Which weeds are increasing in importance?' will therefore depend on whether it is asked in an El Nino or a La Nina phase of climate cycles. Some weeds, and Californian thistle is a good example, have been regularly cited as 'increasing' for over 100 years.

Survey results

Fifty three survey forms (Appendix C) were distributed to Regional Councils, Provincial Presidents of Federated Farmers, agrichemical companies and appropriate scientists in the Department of Conservation and Agresearch. Some agencies forwarded copies of the form to field staff or people better qualified to complete it. Fifty three replies were received, as shown in Table 5.

Table 5. Number of replies received from each area

Region	Replies
Nth Auckland	4
Auckland	2
Waikato	4
Northern BOP/ Gisborne	5
Volcanic Plateau	3
Wanganui/Manawatu	4
Hawkes Bay	4
Wairarapa	2
Taranaki	3
Nelson/Blenheim	6
North Canterbury	3
Central Canterbury	4
South Canterbury	4
Otago/Southland	4
West Coast	1

Commonest scrub weeds

Some respondents questioned whether weeds like manuka and matagouri could be considered weeds: farming respondents were in no doubt about either. Respondents were asked to list, in order, the commonest scrub weeds in hill country pastures in their areas, and responses are summarised in Table 6. Gorse was regarded as the commonest scrub weed, being the most common for 32 out of 51 respondents. Other scrub weeds named, but not shown in the Table, were: buddleia, burdock, bush lawyer, coprosma, cotoneaster, crack willow, elder, inkweed, mexican devil/mistflower, muhlenbeckia, old man's beard, porcupine bush, prickly hakea, rata, ring fern, rowan, Russell lupin, tutsan, wilding pines. More detailed results are presented in Appendix D.

Table 6. Commonest scrub weed species reported in our survey.

Species	Ranked by # out of 53	Listed in top five by
gorse	53	48
blackberry	42	35 NI especially
broom	35	29 SI especially
manuka/kanuka	35	31
barberry	25	19
bracken	25	21
sweet brier	20	11 SI especially
tutu	19	10
matagouri	17	14 SI especially
hawthorn	17	6
pampas	13	7 Waikato northwards
privet	13	4 BoP especially
Himalayan honeysuckle	12	2
tauhinu	12	6
boxthorn	10	2
woolly nightshade	10	5 Waikato northwards

Most serious scrub weeds

Respondents were asked to name the three most serious scrub weeds determined by the time and money land-owners spend in worrying about and controlling them. Of the 53 responses:

gorse was placed in the top three by 49,
blackberry by 28 (especially in the North Island)
broom by 24 (especially in the South Island)
manuka/kanuka by16 (especially in the North Island)
barberry by 7
matagouri by 6 (especially in the South Island)

Other species named among the most serious were: boneseed, bracken, burdock, hawthorn, mingimingi, nassella, pampas, ring fern, Spanish heath, sweet brier, tutu, wilding pines, woolly nightshade. More detailed survey results are in Appendix D.

Emerging scrub weeds

Several respondents (7/53) considered native scrub weeds to be increasing in numbers and importance. This is probably the result of reduced topdressing, lower stock numbers, and less herbicide use. Native scrub weeds mentioned were coprosma, manuka, matagouri, muhlenbeckia, rata, tutu, wineberry.

Respondents also considered as 'emerging' scrub weed species already widespread and common in many places:

gorse (12/53 respondents)
broom (11/53), especially in the South Island
blackberry (6/53)
barberry (4/53)
hawthorn (3/53)
sweet brier (2/53)
inkweed (2/53), and elderberry (1/53).

Figure 3 shows both scrub and herbaceous species named by respondents as increasing, with responses grouped according to Regional Council areas. More details of survey results are shown in Appendix D.

Figure 3: Survey respondents’ emerging plant pests

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Of particular interest are the following species considered by some as emerging:

Boneseed (3/53) in NZ is an amenity rather than a pasture weed, although it is a serious noxious weed in parts of Australia. Differentiation into pasture, forestry and amenity weeds is not as important as it once was, because of land use changes and interchange of weeds between different land use categories.
Buddleia (Dunedin) is a serious forestry/amenity weed that may be increasing in bush margins.
Burdock, listed by some as scrub and some as non-scrub is a biennial species whose burs affect the value of wool. It is common in NZ and may be becoming more important as a result of seasonal changes.
Clerodendrum has the potential to be weedy, but the respondent did not clarify whether or not it has actually become established and is causing problems in the wild. Report should be followed up.
Cotoneaster (Nelson) could become a serious scrub weed and this report should be followed up.
Darwin's barberry is known to be spreading in scrub margin areas in southern NI
Heather is an amenity weed, but has the potential to spread into low-producing grassland. Report should be followed up.
Himalayan honeysuckle is not usually regarded as a pasture weed, but could be becoming more of a problem in bush remnants and on bush margins.
Lantana in Northland is already the subject of a plant pest strategy in Northland.
Pampas is not usually regarded as a pasture weed, but could be becoming more of a problem in bush remnants and on bush margins.
Rowan (Rakaia) is worth following up.
Spanish heath (3/50) may expand its range under lower fertility, lower stock numbers.
Tutsan is well known as a problem in Taumaranui - the report of it emerging in Northland deserves attention
Wilding pines in the South Island are of concern and respondents do not think the authorities are doing enough (in May 2001, Canterbury Regional Council announced a major drive against these species)
Woolly nightshade (5/50) has been extending its range southwards and becoming commoner. These reports seem to confirm that this trend is continuing.

Reports naming these species raise concerns that should be followed up:

Boneseed (listed as a plant pest by several Regional Councils)
Clerodendrum
Cotoneaster (listed as a plant pest by Southland Regional Council⁴⁰)
Lantana (listed as a plant pest by Northland Regional Council⁴¹)
Rowan
Wilding pines (listed as Biodiversity pests by Environment Canterbury⁴²)

Commonest herbaceous weeds

Non-scrub herbaceous weeds considered the most important by respondents are listed in Table 7.

Other species named as 'common' were: alligator weed, apple of Sodom, Australian sedge, barnyard grass, Bathurst bur, burdock, Chilean needle grass, daisy?, flatweeds, foxglove, hedge mustard, inkweed, Kikuyu, mayweeds, needle grass, nettle, oxeye daisy, oxtongue, parrot's feather, parsley dropwort, piripiri, Russell lupin, St John's wort, stinking mayweed, stonecrop, storksbill, summer grass, tarata thistle?, thorn apple, tussock, tutu, viper's bugloss, willow weed, yarrow

Table 7. Commonest herbaceous weed species reported in our survey

Species	Ranked by # out of 53	Listed in top five by
thistles	49	48
ragwort	38	36
barley grass	12	7 (especially SI)
hawkweeds	11	8 (SI)
pennyroyal	10	8 (NI)
horehound	9	7 (SI)
rushes	8	7 (especially NI)
buttercups	7	7
hemlock	6	4
docks	5	4
nassella tussock	5	4 (SI)
ratstail	5	2
ring fern	5	4 (NI)
woolly mullein	5	5

Where respondents listed thistles by species, Californian thistles were mentioned as common by 26 respondents, nodding by 18, Scotch by 17, winged by 12, variegated by 7, marsh by 2 and plumeless by 2. More detailed results are presented in Appendix D.

Most serious herbaceous weeds

Respondents were asked to name the three most serious non-scrub weeds determined by the time and money land-owners spend in worrying about and controlling them. Out of the 53 responses:

thistles were placed in the top three by 41 (named thistle species - nodding 11, Californian 7, variegated 5, winged 5, Scotch 3, plumeless 2),
ragwort by 30
barley grass by 7
nassella tussock by 7
parsley dropwort by 4 (Northland)
buttercups by 4
hawkweeds by 4 (SI)
horehound by 4 (SI)
ring fern by 3

Other species named as serious included: alligator weed, burdock, buttercups, Chilean needle grass, fireweeds, lupins, nettle, pennyroyal, ratstail, ring fern, rushes, summer grass, tutu, wild ginger. More detailed results are presented in Appendix D.

Emerging herbaceous weeds

Respondents considered as 'emerging' non-scrub or herbaceous weeds the following species:

thistles (10/53 respondents) (named species were Californian, marsh, nodding, Scotch, variegated)
hawkweeds (6/53)
ragwort (4/53)
pennyroyal (3/53)
nassella tussock (2/53)
parsley dropwort (2/53, Northland)
foxglove (2/53)
horehound (2/53)
nettles (2/53)

others species mentioned: Bathhurst bur, burdock, buttercups, Canadian fleabane, Cape daisy, chamomile, daisy?, docks, ground tutu, honesty, field horsetail, Maori onion, Mexican daisy, Mexican devil/mistflower, poppies, pyrethrum, ratstail, ring fern, rushes, St John's wort, stipa or needle grass, summer grasses, tall oat grass, tutu, veld grass.

Many of the species listed have been relatively common for many years and, especially thistles, come and go with the seasons. However, changing climatic and land use patterns may well mean that such species are becoming more prevalent.