module 17D
Integrated weed management for horticulture and viticulture
Objectives
At the end of this module, you will:
- be familiar with the integrated management of weeds in
horticultural and viticultural production systems
- be able to be able describe the principles of integrated
weed management in horticulture
- explain the impact of different weed management methods
on horticultural production systems
Module Outline
- Introduction
- Cover crops
- Weed control technologies
- IWM technologies
- Summary
- References and further reading
Introduction
This module will focus on weed management in horticultural and viticultural enterprises. These enterprises can present specific difficulties for weed management. On the positive side these are usually high-value crops that are intensively managed and, therefore, provide significant opportunities for novel weed management strategies.
In this section it is not expected that you prescribe a weed management system for these crops, but will instead discuss a range of different techniques and their benefits and difficulties of use. Because of the great diversity of crops, crops will be generalised into three major types:
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Unfortunately, there is a limited amount of discussion on weed control in these crops under Australian conditions. For this reason some of the examples come from overseas.
Horticultural and viticultural crops are normally grown in rows, which means that two different management types can occur. Management within the row can be conducted separately to management between the rows. The latter is also normally used for traffic and presents special problems. Row cropping facilitates management, by allowing machinery to move amongst the crop and, in the case of grapes and other vine fruit, allows for trellising. The rows may be built into beds for vegetables or banks for vine crops. The area between the rows may be kept bare or a cover crop grown.
Cover crops
Cover crops are grown between the rows of some horticultural tree crops and vines, and may be annual or perennial, as is the case with a grass sward. There is no intent to cover in detail why cover crops are used or how they are managed. What is intended here is to consider how they can be used for weed control, and the problems, including weed control, that are associated with their use. The choice of whether a cover crop is grown and the type of cover crop grown will be dictated by a number of factors, including:
- type of enterprise
- soil type
- rainfall pattern
- erosion risk
A variety of different cover crops are grown depending on soil type and structure. Advantages of growing cover crops:
- reduce soil erosion
- add organic matter, either carbon or nitrogen, to the soil.
- improve trafficability for machinery
- suppression of weed growth.
Cover crops may be:
- legume pasture species
- grasses and cereals
- a mixture of both
- more rarely volunteer species
The cover crop may be permanent, including grassy sods, or may be killed and re-sown each year. Where a cover crop is grown, it also needs to be managed for weeds to obtain maximum value.
- in tree and vine crops it is also important that cover
crops are reduced in height, either by cultivation, mowing, or herbicide,
prior to bud burst in spring to reduce frost risk
- in dryland vine and tree crops, a permanent cover crop will compete for water with the crop, therefore growth of the cover crop is normally terminated in late spring or early summer
A cover crop if sufficiently competitive may also act as a form of weed control in its own right, crowding out less desirable species.
The following readings cover many aspects of the use of cover crops in horticultural tree crops. The first two readings are overviews of cover crop use in orchards and should be read to gain an overall impression of the advantages of cover crops. The next reading deals more with the disadvantages of cover crops, particularly the problems of excessive water use by the cover crop, which can reduce yields.
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Weed control technologies
There are four main weed control strategies in horticultural crops, these are:Tillage
Cultivation is still widely used in all three cropping systems. Once even more widely used, cultivation is now being slowly replaced by other methods of weed control. Cultivation is used in establishment of raised beds for vegetable crops. Tree and vine crops are also established with cultivation, but these crops are established less often.
Cultivation is also still a common practice between rows. This may be done to keep the ground bare, or to incorporate a winter cover crop. Despite the disadvantages of cultivation in terms of soil structure, cultivation is still favoured in some enterprises as all vegetation can be removed to reduce competition for water. The major equipment used for inter-row cultivation is the disk plough or the rotary hoe.
Cultivation within the row is also possible, for example using a silly-plough, or undervine knife in vineyards. These tools are used to keep the undervine area free of weeds, but need to be used carefully so as not to damage the roots of the crop. Cultivation between successive crops of vegetables is also still common. The disadvantages of cultivation in Australian soils have been described in Module 10.
These disadvantages also apply in horticultural and viticultural crops. In particular, cultivation between the rows can lead to soil breakdown, making it difficult to manoeuvre machinery during winter. Stirzaker (1991) in the following reading discusses the issues related to reducing tillage in vegetable growing in Australia. The reading compares the impacts of reduced tillage and growing mulches on soil structure, soil biological activity and crop production.
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Mowing/slashing
Mowing and slashing find useful employment in vine and tree crops. Mowing or slashing of the inter-row cover crop:
- reduces growth of the cover crop
- controls taller weeds
- provides mulch for weed control within the row (see section on mulches below)
Disadvantages of using mowers and slashers are that:
- mowing may need to be done often during spring
- the area immediately surrounding the tree and the undervine area needs to be kept entirely free of other vegetation which cannot be achieved only by mowing
Herbicides
Herbicides are widely employed in the vegetable industry for the control of weeds. Most commonly, herbicides are applied to the whole crop area, but they also may be used just within the rows. Details of specific herbicides used can be found in West (1998).
A range of selective herbicides are applied to the crop and weeds, but non-selective herbicides such as paraquat, diquat, and glyphosate are used to control weeds prior to sowing or transplanting the vegetable crop. Paraquat and diquat can also be used as a directed spray to control weeds between the rows.
Similarly, a variety of herbicides are used in tree and vine crops. These herbicides are used to control weed growth under the trees and vines and also to kill cover crops in late spring or early summer so as to reduce competition for water. Typically, residual soil-active herbicides such as simazine, diuron, napropamide, oxyfluorfen, norflurazon, trifluralin, and pendimethalin are applied under the trees and vines. For several of these herbicides, such as diuron and simazine, it is important that the tree or vine have a deep well-established root system.
These herbicides applied to young trees and vines can leach to the root zone and cause crop damage or loss of young trees or vines. In general, it is recommended that these herbicides not be applied near trees or vines that are less than three years of age. Glyphosate, and to a lesser extent, paraquat are used to control growth of the cover crop, if present, or vegetation growing between the rows.
The non-selective herbicides, glyphosate, paraquat, and glufosinate can also be used as directed sprays near trees and vines so long as the herbicide does not contact the foliage of the crop, or in the case of glyphosate, young or damaged stems. These herbicides are typically used to control weeds within rows of tree crops. For trailing crops, such as vines, glyphosate is best used early in the growing season before the canes near the ground. Paraquat or glufosinate are better used later in the growing season as these herbicides do not readily translocate within plants. Therefore, if some herbicide does contact the foliage, only those leaves that intercept the herbicide will be damaged.
Shielded or shrouded spray equipment should be used in tree and vine crops so that drift onto the crop is minimised. Controlled droplet applicators (CDA) (see Module 12) are also advantageous in many situations due to the narrow spectrum of droplets produced by this equipment. This narrow spectrum of droplets reduces the likelihood of drift.
Another advantage of this equipment is that water volumes can be reduced. The following reading from a trade magazine outlines the advantages of CDA equipment and its use in vine growing.
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Mulching
Mulches generally act by inhibiting light penetration to the soil. As the seeds of many weeds require light to germinate, they do not do so under the mulch. Mulches can also provide a physical and light barrier to emerging weed seedlings.
Several different types of mulches are available for weed control. The choice of mulch type is influenced by many factors. The mulches include:
plastic mulches are commonly used in annual vegetable crops, where they:
- inhibit weed growth
- limit moisture and nutrient loss.
One side effect of the use of plastic mulches, in particular black plastic, is an increase in soil temperature under the plastic. If required, this can be ameliorated to some extent by using white or reflective plastics. The following reading discusses the use of plastic mulches for herb growing compared to other weed control strategies.Reading
‘The effects of black plastic mulch and weed control strategies on herb yield’ by JA Ricotta and JB Masiunas, 1991, HortScience, vol. 26, pp. 539-541.
straw mulches are generally preferred in tree and vine crops due to:
- cost considerations
- they provide valuable organic matter to the soil when they do break down
Organic mulches allow water and air to pass through to the soil, but reduce weed germination. In addition, mulching can increase the activity of earthworms and other soil organisms involved in nutrient recycling.- cereal mulches, such as oat or wheat straw are preferred because they take longer to break down than lucerne hay.
It is important that organic mulches are applied thickly enough to prevent light reaching the soil surface. Some problems associated with straw mulches:
- the introduction of weed seeds with the straw
- the possibility of damaging insects or fungal diseases
building up in the mulch.
- need to consider the source of cereal straw to be used
as mulch because most cereal crops grown in Australia have been treated with
a herbicide to control broadleaf weeds and residues of the herbicide may remain
in the straw. These residues could potentially leach out and in some circumstances
cause damage to sensitive crops. A rule of thumb would be to reject any straw
from crops that had received an application of a broadleaf herbicide late
in the season. If in doubt, it would be prudent to obtain straw from another
source.
- need to control weeds that grow around the edges of mulched areas
A living mulch, grown originally as a cover crop and then cultivated, can also be valuable in some situations, particularly in vegetable growing when soil organic matter is low. Alternatively, a cover crop can be slashed and used as mulch under trees or vines. The uses of cover crops to provide mulch are discussed in the next two readings. The first reading discusses the use of an inter-row cover crop for subsequent undervine mulching.
You should particularly note the types of cover crops described in this reading, for example, fodder radish is a fast growing rosette plant and offers tremendous competition with winter growing weeds. The second reading examines slashing a cover crop to provide mulch between the rows. This reading looks at the development of new equipment that has allowed a cover crop to be mown and spread in long strands, as these have been found most effective at controlling weed growth between the rows.
IWM technologies
The type of weed management system used in any cropping system will depend on a host of other management factors. For example, herbicides have come into widespread use in vineyards because of concerns over excess tillage. Likewise, the introduction of cover crops has influenced the ways that weeds are managed.
Weeds can cause serious damage to vegetable, tree and vine crops, therefore, it is important that weed control planning is considered early in the set up of the enterprise. For long-lived tree and vine crops, it is prudent to attempt to reduce weed seed-banks prior to initially planting the crops. This is particularly important where the crop is being planted in new ground that may have been used as a pasture or for grain cropping in previous years. These areas can be expected to have large seed-banks of some weeds, particularly grass weeds, which can provide intense competition for newly planted trees or vines.
The recent occurrence of glyphosate-resistant annual ryegrass (Lolium rigidum) in an orchard following 15 years of use of glyphosate as the main weed control tactic, illustrates the herbicide resistance can occur in horticultural systems (see section Module 16 'Herbicide Resistance in Weeds'). Therefore, it is important that reliance on a single herbicide, or mode of action, does not occur.
A simple solution to the threat of herbicide resistance in horticultural systems is to rotate among modes of action. For the non-selective herbicides, there are three modes of action, represented by paraquat, glyphosate, and glufosinate (see section Module 14), allowing plenty of scope for rotation of herbicides. Where possible, other weed control tactics should be incorporated into the IWM strategy.
Management strategies in horticulture are more likely to target all weeds rather than any particular species, as the major emphasis is on reducing competition for water and nutrients. However, there can be specific concerns with hard to control weeds, or weeds that cause specific problems. One example is spiny weeds, such as Emex australis, in grape crops used for dried fruit. In this case a specific integrated weed management strategy may be implemented (see MacGregor 1990 and Pohlner 1996).
Summary
Weed management decisions in horticulture and viticulture are often driven by the need to conserve water. Therefore, practices that remove all other vegetation have been used in these industries in the past, centered on the use of cultivation and herbicides. The advantages of managing the area between rows with a cover crop or mulch to reduce problems associated with soil erosion and soil compaction are now obvious, despite concerns with water use. There are tremendous challenges ahead for weed management in the horticultural industry as reliance on cultivation and herbicides for weed control need to be reduced.
References and further reading
Harrington, KC 2000, ‘Tree crop and viticultural weed management systems’, in BM Sindel (ed) Australian Weed Management Systems, RG & FJ Richardson, Melbourne, pp. 373-392.
Henderson, CWL & Bishop, AC 2000, ‘Vegetable weed management systems’, in BM Sindel (ed) Australian Weed Management Systems, RG & FJ Richardson, Melbourne, pp. 355-372.
MacGregor, A 1990, ‘The potential for cultural control of Tribulus, Cenchrus and Emex in Sunraysia vineyards’, Plant Protection Quarterly, vol. 5, pp. 116-119.
McCarthy, MG, Dry, PR, Hayes, PF & Davidson, DM 1992, ‘Soil management and frost control’ in BG Coombe and PR Dry (eds) Viticulture: Volume 2 Practices, Winetitles, Adelaide, pp. 148-177.
O’Connor, G, Strawhorn, J & Orr, K 1993, ‘Soil Management for Orchards and Vineyards’ Department of Agriculture, Melbourne, Australia.
Pohlner, D 1996, ‘Emex australis and dried vine fruit production in Sunraysia’, Plant Protection Quarterly, vol. 11, pp. 150-153.
Tisdall, JM & Huett, DO 1987, ‘Tillage in Horticulture’ in PS Cornish and JE Pratley, (eds) Tillage: New Directions in Australian Agriculture, Inkata Press, Melbourne, pp. 72-93.
West, S 1998, ‘Weed control in vegetable crops’, in J Salvestrin (ed.) Australian Vegetable Growing Handbook, Scope Publishing, Pty Ltd, Frankston, pp. 266-297.
Young, KR & Hingston, L 1993, ‘Cereal cover crops for vegetable production in Tasmania’, Proceedings of the 10th Australian Weeds Conference, pp. 233- 237.
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