Cover crops for quality wine production – Blockout for Vineyards and Orchards
Inter row cover crops (sod culture) in vineyards incorporate many species of plants from annual cereal or pulse species to permanent grass or clover.
These cover crops are used for a number of reasons:
• Increased water infiltration by reducing run off
• Decreased soil compaction
• For increasing organic matter (organic carbon)
• Improved access, especially in winter
• Reduced herbicide applications
• Reduced temperatures in summer
• Reduced costs associated with cultivation
• Improved conditions for manual labor
• Dust is reduced
• Improved fertility management
• Weed control eg. Field Bind Weed Convolvulus arvensis.
While many cover crops will assist in achieving these outcomes, modern wine making demands more from cover crops. Two important management systems that have been introduced to assist in producing better quality wine are Regulated Deficit Irrigation (RDI) and Integrated Pest Management (IPM). These systems have highlighted deficiencies in many cover crop species.
RDI irrigation management is a system that relies on the soil moisture being controlled by the vineyard manager. Some cover crops interfere with correct soil moisture levels by either not drying out the soil sufficiently prior to instituting RDI or competing with scarce water resources.
IPM management relies on encouraging maximum numbers of predator insects and creating an environment with minimum sucking destructive insects. Some cover crop options are significant hosts of sucking insects and therefore interfere with IPM management
Regulated Deficit Irrigation (RDI)
‘It has been shown in grapevines and some deciduous fruit that controlled moisture stress, at the correct times during the growing season, can be used to control vegetative growth, without adversely affecting the process of photosynthesis. This process is known as Regulated Deficit Irrigation (RDI).
From the above it is clear that RDI is best imposed for maximum effect, with minimum losses as soon as possible after fruit set. To this end it is important that excessive soil moisture is mopped up, especially during the flowering period, right through the soil profile whilst monitoring very carefully to avoid any vine stress. This will enable the soil profile to be rapidly dried out after fruit set and then monitored levels of stress applied.
It is the ability of a cover crop to set up the soil moisture level for RDI that exposes the weakness in many cover crop species and blends. RDI is required in many regions with challenging rainfall distribution. In some regions RDI will start at a time when occasional rainfall reduces its effectiveness or prevents the manager from implementing it entirely. The Marlborough region on the north coast of the south island of New Zealand is an example of this type of environment. Other areas need an aggressive reduction of soil moisture after winter water logging of soils but then need an entirely summer dormant cover crop so as not to compete with scarce water resources over summer and through until harvest. The Barossa Valley is an example of this type of environment.
The following is an example of one area in central eastern Victoria between Yea and Alexandra and the time of crop development.
Life Cycle of Wine Grapes and Approximate Times
(Alexandra/Yea area in North Eastern Victoria)
- Veraison – 1st week of February
Veraison = Stage when fruit shows colour
- Fruit set – Last week of December (2 weeks after flowering)
Fruit set = Beginning of fruit appearance
RDI initiation stage
- Flowering – 1st week of December (2 weeks)
- Budburst – 1st half of September.
Budburst = Commencement of vegetative growth (canes)
- Harvest – 1st week of April
*Average dates subject to environmental changes.
In this region of Victoria winter waterlogging is common. Excess soil moisture needs to be removed during spring in time for fruit set. Many annual and most perennial cover crop species have a growth period that does not dry out the soil profile and or continue to grow and compete with the vines for moisture during the critical post fruit set RDI period.
A couple of new varieties of ryegrass have been bred in Australia and their growth habit contributes to RDI management and minimizes or eliminates competition for moisture. Roper Perennial Ryegrass was bred by Valley Seeds Pty. Ltd. in Victoria from native ecotype plant collections. Roper was bred to develop an extensive root system. In trials conducted by NSW Agriculture Roper developed 133% more roots than New Zealand varieties.
Most varieties of Ryegrass have the greatest period of growth during late spring and early summer. This time is likely to conflict with RDI management and is often too late to dry out the soil profile sufficiently. Roper matures much earlier than New Zealand bred varieties and completes all stages of its reproduction at about the same time as RDI is initiated. After this stage Roper returns to a moderate vegetative stage which it maintains until after grape harvest. Nui perennial ryegrass does not finish its reproductive stage until mid February. (See Table No.1) Vineyard managers in the Marlborough region of New Zealand’s south island report that Nui creates excessive de-vigoring of vines over summer.
Roper is currently being trialled in the Napa Valley in California USA. It is preferred for vineyards that have cellar door sales due to its low green summer growth, adding to the visual outlook around the winery. Roper is the main component in the Australian cover crop blend – Blockout No. 3.
A second new variety of grass was bred by the WAIT Institute in South Australia by Dr. Alan McKay. In collaboration with breeders at Valley Seeds Pty. Ltd. Dr McKay bred a new annual ryegrass called Guard. Guard matures about the same time as Roper but has the added advantage that it does not grow at all over summer. Plants of Guard finish their life cycle after reproduction and providing they are allowed to set seed will re-grow from that seed after the main autumn break.
This means that Guard provides the perfect solution for those vineyards that have limited water resources over summer and require no competing cover crop over that period. One such area where this type of cover crop is needed is the Barossa Valley in South Australia where the original plants of Guard were selected. Guard is available as the main component in the cover crop blend – Blockout No. 5.
Length of Reproductive Stage at Rutherglen Nth. Eastern Victoria.
Integrated Pest Management (IPM)
Much work has been carried out to assess the ability of legume species for use as a cover crop. While there may be some situations where they have a place many in the industry have questioned their value, particularly when used in an IPM regime. The following quotes were taken from papers written on the subject of IPM or legume cover crop options.
‘Capeweed and many broadleaf plants are preferred hosts for lightbrown apple moth. Botrytis rot can harbour in Bean cover crops’.
Clover and many other legume species are significant hosts of sucking insects. ‘If a cover crop is slashed or disturbed when pests are breeding they may well be driven up into the canopy,’ (Grape canopy) ‘thereby increasing the problem’. We have seen this with lightbrown apple moth in many crops, but other possibilities are thrips or Rutherglen bugs or similar sucking insects.
‘Medics and other broadleaf cover crop types are currently receiving a lot of adverse publicity in relation to pest and disease incidence in viticulture. One line of investigation into vine yellows is plant hoppers as a possible vector for this disease. Plant hopper populations can be high in legume cover crops and on some weed species. Lightbrown apple moth, soil nematodes and snails are also cited as potential drawbacks with legume cover crops.
Block Out for Vineyards and other mono grass cover crops are ideal for control of broadleaf weeds. Selective herbicides can be used to eliminate broadleaf weeds from a grass cover crop and therefore reduce undesirable insect pests.
Fertility levels are of particular importance in order to produce quality wine. Nitrogen is an element that can create too much vigor leading to lower quality fruit. Legume cover crops produce more Nitrogen than the 33.6 to 56 kgs per hectare required by vines. Most clover species such as Subclover, White clover and Strawberry clover produce from 207 – 337 Kgs of Nitrogen per hectare. Grass cover crops use nitrogen extensively. This can be a valuable tool if nitrogen levels need to be reduced as part of overall fertility management.