Asparagus in tropical Australia:
The commercial and research experience

W.T. Bussell, Department of Landscape & Plant Science, UNITEC Institute of Technology, Private Bag 92025, Auckland, New Zealand.(e mail wbussell@unitec.ac.nz)
C. Robinson, Dept. of Agriculture Western Australia, P O Box 19, Kununurra
J.D. Bright, NSW Agriculture, P O Box 2491, Orange, and
J.K. Olsen, Policy Analysis and Industry Development, Department of Primary Industries, GPO Box 46, Brisbane QLD 4001.

Why grow asparagus in tropical Australia?

In tropical regions, crops are harvested from mid-June to early August when crops are dormant in temperate and sub-tropical Australia. Prices are high ($8 to $20 per kg) for fresh asparagus during this time period.

Emerging asparagus spears (Bob Davis, DPI Queensland)

Plantings in tropical Australia

Where have they been?

Commercial plantings, 14 in all, have been made in the Ord River Irrigation Area near Kununurra, WA, near Katherine, NT, near Mareeba, Qld and at Tumoulin, Qld. Trial plantings, 11 in all, have been grown either on research stations or grower properties at Kununurra, Katherine and Bowen, Qld.

What are the climatic conditions at these sites?

Temperatures at those locations are favourable for good spear growth in June and July. Maximum air temperatures are close to the optimum for spear growth of 28°C.

Minimum temperatures, mostly of at least 10°C, are higher than those required to induce dormancy or to prevent bud-break after dormancy at most sites. Dormancy induced by low temperature could occur at higher altitude (1000 m) in some winters, say at Tumoulin in Queensland.

Throughout tropical Australia, there are characteristic wet and dry seasons during the year. Whether these seasons fit in with being able to induce dormancy by drought prior to harvest or at other times of the year and lead to higher yields than have generally been obtained to date in tropical Australia will be considered later in this DPI Note.

The soils at both commercial and trials sites had a gradational texture with unimpeded drainage, well suited for asparagus. Water table depth was 10 m or more at most sites and water table did not override irrigation treatments at these sites. There have been occasional plantings on sites with a water table as shallow as 1 m and on these sites the water table could be expected to override the effect of irrigation.

How were the crops planted?

Crops were planted either by dormant crowns (approx. 15cm deep) or seedling transplants in rows 1.5m apart to achieve populations between 15 000 and 33 000 plants/ha.

Direct seeding is not recommended for any asparagus planting in tropical Australia because ferns in the first year were fewer and shorter and suffered more disease than from planting crowns or seedlings.

Weed control

It is essential to control weeds prior to planting, during crop establishment, and just prior to, during and immediately after harvest. Mechanical and chemical control methods are essential.

Post-harvest weed control has often lost its effectiveness four to five months into the long fern-growing period. When this problem occurs, fern may be slashed (ie cut to near ground level with a mulcher or flail mower) during the wet season to control weeds (see further information on slashing later in this note). Beds have sometimes also been cultivated after fern slashing but this practice is not recommended because it is likely to destroy spears that have either not emerged or recently emerged that would have formed part of the next fern growth.

Based on a recent pilot study at Kununurra, it is likely that mulching with hay after fern has been slashed and cleared in the wet season will assist in subsequent weed, pest and disease control in the crop.

Irrigation

Irrigation in the dry season in commercial plantings has been primarily with solid set sprinklers and the largest planting at Katherine was irrigated by a centre-pivot system.

Overhead irrigation is likely to increase the occurrence of fungal diseases on asparagus fern in a tropical environment, something already noted at Katherine.

New commercial plantings should use trickle irrigation to reduce the chance of infection by fungal diseases of the foliage. Irrigation scheduling, employed so far only in some trials, needs to become standard commercial practice.

Diseases and pests

The most serious foliar fungal disease, found so far at Katherine and Mareeba, is anthracnose (Colletotrichum gleosporioides). Fungal diseases have also been found to be caused by Cercospora asparagi and Stemphylium vesicarium at Mareeba and by the common temperate fungi Phytophthora sp. and Fusarium spp. at Kununurra.

Insect pests found at Kununurra have been the cluster caterpillar (Spodoptera litura), and grasshoppers (Austracris guthulosa and Ailopus tamulus). Corn earworm (Helicoverpa armigera), known well in southeast Queensland, is also at Mareeba.

Harvesting, spear quality and yield

Harvesting

At the beginning of the harvest season, fern is cut and mulched, beds are formed, and harvesting begins as spears emerge within a few days.

The season now usually begins in early June when crops elsewhere in the country are dormant and should ideally end about eight weeks later around mid-August, when harvesting starts in crops in sub-tropical Australia and there is a fall in the price of spears.

However, the length of the harvest period in most commercial plantings to date has been about 4 weeks. Thus, to obtain an 8-week supply of spears from a block of asparagus in tropical Australia, a block should be divided in half with each half being harvested for about 4 weeks.

A high proportion of good quality spears, with tight tips and diameter of at least 10mm, are obtained in the first 14 days, but later in a harvest season in tropical Australia the proportion of such spears generally decreases considerably.

Methods of improving spear quality in the last part of the harvest season, for example, cutting only thicker spears and leaving the thinner ones to grow to fern, require consideration and study.

Yield

Yields have been low (mostly about 1 t/ha) for the industry standard cultivar UC157 F1 and the newer Californian cultivars Ida Lea, Grandé and Apollo have not been consistently higher.

The main reason for low yield may be that asparagus is a vegetable adapted to a temperate climate with cold winters that induce dormancy of the crown. Other factors contributing to low yield or abandonment of the crop in tropical Australia include infection by anthracnose, either early or later in the life of the crop, poor spear quality through heat stress, and shortage of labour for peak harvests.

Taiwan experience

In Taiwan, where tropical crops are also not dormant prior to harvest, mother fern culture is practiced. This practice is suitable for the 5-6 month long harvest season in Taiwan but is unsuitable for the short harvest season in tropical Australia.

Asparagus mother fern production (Bob Davis, DPI Queensland)

The reasons include -

1. the higher cost of labour due to the more difficult harvesting conditions under mother fern
2. the long time to establish mother fern (probably about three weeks in the coolest temperatures of the year)
3. poor spear quality due to pale spear colour from the effects of shading by mother fern, and
4. additional costs, including the instalment of supporting posts and wires.

Pre-harvest, drought induced dormancy

The asparagus industry in tropical Peru can produce up to 22 t/ha in two harvests per year. This yield is achieved by inducing dormancy by droughting prior to short, four-week harvest seasons. Low annual rainfall (c 50mm) in the asparagus production areas of Peru facilitates the inducement of drought-induced dormancy before each harvest.

Inducing dormancy by droughting has rarely been attempted by growers in tropical Australia and has been investigated in only a few trials conducted at Kununurra, Katherine and Bowen. The results to date have been inconsistent.

Droughting may never be worthwhile in some current commercial sites such as those on clay soils at Kununurra which crust during droughting and on soils where the water table is high (2-3 metres below the surface). Crusting makes cultivation and bed formation difficult.

A lysimeter trial at Katherine has indicated that about 124mm total evapotranspiration (ET) is required to induce dormancy by droughting. This amount of ET occurs in about 20 days in May and the first half of June in tropical Australia. An inspection of long-term (1966-2000) rainfall records revealed a common pattern of no rain or light falls of less than ET for the day in 20-day periods from 1 May to 10 June. Dormancy should be induced in approximately 20 days at these times. It would have been induced in 31 of the past 35 years at Kununurra, in 29 years at Katherine and in 23 years at Mareeba.

Thus droughting prior to harvest to improve yield will not always be possible in tropical Australia, but conditions allowing droughting occur frequently enough for droughting prior to harvest to be seen as a very important tool to improve yields of asparagus in tropical Australia.

Managing wet season fern slashing

Asparagus beds after hoeing (Bob Davis, DPI Queensland)

Asparagus after slashing (Bob Davis, DPI Queensland)

Particular attention needs to be paid to the timing of wet season fern slashing. Slashing fern in the wet season is essential to:

When to slash

Slashing should ideally be done when:

• there has been an adequate (about 3-4 months) fern-growing period after the harvest season and
• there is enough time to have another adequate fern-growing period before droughting commences prior to the next harvest season.

The last half of November is probably the best time to plan for wet season slashing. Long-term (1966-2000) rainfall records showed that the first heavy rains of the wet season occurred in mid to late November in 25 of the past 35 years at Kununurra, in 29 years at Katherine and 21 years at Mareeba.

What happens after slashing and heavy rain

The first heavy rain of the wet season (more than 25mm in a day) may induce a new flush of spears. It is important that slashing is timed so that this new flush of spears is not cut when the wet season slashing is done because carbohydrate from the crown is removed without replacement, leading to a lower potential yield for the subsequent harvest season (June to August).

Slashing the older and decaying fern after the first heavy rain in November would ensure that an adequate period of fern growth (c 4 months) has occurred following the end of the previous harvest period in early August.

In years when no heavy rains fell before the end of November it would be possible to induce dormancy by droughting in about 14 days, (which is the time needed at this time of year for there to have been 124 mm of evapotranspiration required to induce dormancy). Inducing dormancy, slashing dead fern and then having growth quickly started again by irrigation or heavy rain may be more beneficial than just slashing necrotic fern of a non-dormant crop.

Further refinement of best practices related to fern slashing at the beginning of the wet season awaits more detailed field observations and experimentation.

Conclusion

The chances of boosting average yields and having a prosperous industry in tropical Australia are considered good.

Poor results to date are attributed to:

• unsuitable soil type
• poor disease control in the wet season
• insufficient carbohydrate reserves in the plants to support good yields of good quality spears at harvest time
• lack of experience in growing a perennial, temperate crop in a tropical climate with high humidity during the wet season.

Better disease control, better management of carbohydrate reserves in the crown by timely slashing of ferns and using drought-induced dormancy should boost average yields.

Further reading and information:

• Bonnardeaux J, Robinson C (1993) Export of asparagus from the Ord River. Journal of Agriculture, Western Australia 34(2), 57-59.
• Bonnardeaux J (1994) The prospect for an asparagus industry in northern Western Australia. Dept. of Agriculture, Western Australia. Miscellaneous Publication 5/94 22 pp.
• Bright JD (2000) Asparagus - Productivity response to droughting in a tropical environment. RIRDC Publication No 00/22 ix + 68p.
• Bussell WT, Robinson C, Bright JD, Olsen JK (2002) Asparagus in tropical Australia - the first fifteen years. Australian Journal of Agricultural Research 53, 729-736.
• Grundy T, Olsen JK (1999) Asparagus in Queensland: commercial production. DPI Note, Department of Primary Industries, Queensland, Australia; File No H99043 4pp.
• Olsen JK (1992) Tropical asparagus production opportunities. Proceedings of the Australian Asparagus Workshop; Wagga Wagga, NSW Agriculture, 10-11 August 1992, (no.1) p. 36.1-36.10; 5 fig.
• Olsen JK (1997) Tissue culture development of a tropical asparagus superclone. RIRDC report DAQ-138A iii + 19pp.
• DPI's Infopest CD-ROM: The Infopest CD-ROM contains current national information on registered agricultural chemicals and is available from: Infopest, DPI, GPO Box 46, Brisbane Qld 4001 or by email from infopest@dpi.qld.gov.au

To access DPI's information and services

• DPI Call Centre: Phone 13 25 23 (Queensland residents) between 8am and 6 pm weekdays; non-Queensland residents phone 07 3404 6999 (international: +61 7 3404 6999); email callweb@dpi.qld.gov.au
• DPI's web site: www.dpi.qld.gov.au

This DPI Note is published on DPI's website and DPI's PrimeNotes CD-ROM.

---

Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.

---