For most vineyards the aim is to achieve the best possible crop from healthy vines in the most environmentally sustainable way possible. Sharing space with other plants such as weeds – impacts the vine both positively and negatively as can weed control methods. Jo Cowderoy speaks to viticulturists and agronomists to find out their views on getting the balance right – and as usual no one size strategy fits all!
A weed is just a wild plant growing where it is not wanted – but as it can compete for nutrients and water it can have a significant effect on vine growth, fruit quality and yield. Weed control is one of the most challenging tasks for a vineyard manager. Historically there were herbicides, but these are less available and less acceptable, and the alternatives, mechanical or otherwise, are not always straightforward – as choosing a suitable method is dependent on many factors.
Many vineyards are looking towards an herbicide free future and considering the steps to achieve this. Duncan McNeill, Viticulturist, McNeill Vineyard Management, manages his own vineyard and several of his client’s vineyards successfully, without the use of herbicides. “There are a few ‘high level’ considerations,” advised Duncan. “Firstly, ensure that soil organic matter levels are good enough to hold reasonable soil moisture – as there will be more competition for moisture from other plants. Don’t make expensive machinery purchases without having researched and trialled their effectiveness on your land and with your tractor. Consider the per acre investment for a piece of equipment on your site – as maybe there are other, less appealing chemical free methods which are more cost effective. “Ensure your chosen piece of equipment is sufficient to cover your entire vineyard in a suitable time window as some equipment requires very slow driving speeds. Make a plan, and start non-herbicide weed control early in the spring. If undervine weeds become established, they are very hard to remove mechanically – and hand weeding can then be the only solution. I allow the undervine strip to green up over autumn and winter as this dormant season undervine green cover helps to avoid soil erosion and loss of soil structure,” Duncan added.
“Phasing out the habitual use of herbicide is inevitable for various reasons that we all know and we also have a duty to protect the soils that we grow our crops in. Whilst there is no ‘recipe’ to sustainable vineyard management, I like to simplify the theories and create practical, workable solutions,” explained Joel Jorgensen, Viticulture Director, Vinescapes. “Steering away from cultural practices that have become the norm over decades can be hard, but with a calculated approach it is possible to go herbicide free, with relative ease, and reap the benefits for years to come,” Joel added.
As vineyards in the UK are not generally short of water and are often planted on fertile soils does competition from weeds affect vine growth? This has been the subject of an ongoing research project at the NIAB-EMR plant research centre at East Malling, in Kent. The IWMPRAISE project involved trials over several years assessing the impact of weed control on the yield and quality of the crop in their experimental vineyard. The trials used two different types of mechanical weeders and an herbicide treatment in the undervine area. The results were then compared with an untreated control area. The study measured canopy growth and the concentration of different leaf pigments, to give an indication of the nutrient status of the vines. At an event hosted by NIAB-EMR last November, and reported in Vineyard, the project findings showed that in 2020, all weed treatments resulted in a yield 214% to 247% higher than the control, and in 2021 this was increased to 261% to 292% higher – indicating the negative effect that weed competition has on vine growth. The results did not vary significantly with the different methods of control – mechanical or herbicide.
Best practice methods
As there is no one strategy, best-practice weed management can be difficult to define, but it does need to consider an integrated approach with an understanding of the risks understanding the risks to the vines if the weeds are not controlled. Best practice management methods are influenced by the vineyard situation, the age of the vines, the soil and vigour as well as targeted herbicide or selecting non-herbicide options including cultivation, mowing, cover crops, mulches, or sheep.
Peter Hayes, Australia – based Viticulturist and Global Wine Industry Strategist, has advised many UK vineyards over the years. “For best practice weed management my personal preference is for a permanent sward mid-row, either a selected sown mix or a suitable volunteered, self-sown, and managed locally adapted population,” explained Peter.
“This facilitates year-round trafficability, minimises rutting and compaction – and for sloping or light-textured soils, inhibits erosion. However, such an approach is challenged when aggressive, competitive weeds such as bind weed are present in which case an extended campaign, specifically designed to eliminate or suppress the problem is required; this could entail targeted herbicide – although it’s best to gain control ahead of planting, with the use of suppressive sown sward or grazing etc – as cultivation is more likely to fragment and spread many of the problem weeds.
“For undervine, it depends in large part on the site characteristics including soil depth, texture, root volume and weed composition, all of which interact with and influence vine vigour. For higher vigour sites and where vine vigour suppression may be beneficial, an appropriately selected semi-competitive sward may be effective; this may then be grazed, treated with herbicide, mowed, flailed, topped, or strimmed as required, and if too competitive, cultivated.
“For weaker vigour vineyards, undervine vegetation may not be tolerated in which case the requirement for tillage should be recognised and utilised with well-timed interventions, appropriate to weed type, growth characteristics, stage of development, and soil conditions. Too many operators indulge in recreational tractor driving/diesel burning and are often ‘compelled’ – by logistical and scheduling issues – to cultivate when soils are too wet, resulting in soil structure degradation, compaction, and smearing; possibly not getting effective weed control.
“At the other extreme, when cultivated too dry, soil degradation is also an issue, but is perhaps a lesser concern in the UK. For young vines, the issues may be different, and certainly, in weaker areas, the benefits of a weed-free undervine strip, potentially supported by a compost-mulch blend warrants attention.
“As well as the effects of competition, undervine vegetation, especially as it grows taller increases humidity, restricts airflow, and can elevate disease risk, while impeding spray application. The consequences for physical entanglement and ease of harvest are also issues.
“The logistics and merits of any alternative need to be assessed against the downsides and costs with no singular approach being necessarily appropriate for all time or all situations across any vineyard,” said Peter.
According to Duncan McNeill: “Best practice is a weed presence that does not negatively impact on vine vegetative production, disease risk or fruit ripening. This might be zero weeds, some weeds or a green undervine cover.
“It enables soil structure to be maintained in the undervine strip, via some weed root growth, incorporating vegetative matter and not having prolonged periods of bare earth. It helps avoid soil degradation in the undervine strip, which can be caused by water or wind erosion.
In young vineyards (years one and two) weed presence should be kept to a minimum. This is to encourage more vine shoot growth, as increased leaf area equals increased root development – which equals better establishment. Greater leaf area also encourages more photosynthate to be excreted out from the roots, thus pumping ‘liquid carbon’ into the soil. This is storing atmospheric CO2 in the soil and improving the soil by increasing the levels of organic carbon,” added Duncan.
“I think leaving weeds unchecked can be OK, depending on the vine’s age and vigour,” commented Will Davenport, owner of organically managed Davenport Vineyards. “A deep rooting healthy vine should not suffer from too much water competition. With our high rainfall and generally fertile soils I think this is an area that could be researched properly and looked at as an option.”
“It depends on the weed and time of the year,” added Duncan. “Early spring weeds such as groundsel, dandelion and red dead nettle are not detrimental to spring vine growth, they attract insects and should be allowed to grow. However, other deep rooting weeds such as thistles or fat hen are certainly detrimental to vine growth – especially newly planted vines,” Duncan continued.
According to Chris Cooper, Agronomist with Hutchinsons: “For the herbicide glyphosate, there is realistically only Round Up Powermax – and this isn’t easily available for 2022. We have Fusilade Max, but this is only for grasses, and not very effective. Shark is available for sucker and weed control – but only does a limited amount of broad-leaved weeds and the Kerb season of use has passed. It’s early days for Finalsan, it’s expensive, has a limited period of use and I haven’t had much success to date,” explained Chris. “Kerb, Shark and glyphosate are in-registration so it’s up to the Chemicals Regulation Division (CRD) to decide if these products will continue as approved,” Chris added.
Mechanical vs chemical
Without the use of herbicides, mechanical weeding is an option, but does that bring with it increased costs, and carbon footprint? “It all depends,” commented Peter Hayes. “An absolute or definitive outcome cannot be defined for any singular situation. Synthetic inputs, in themselves have a carbon footprint that can be calculated, as can the carbon costs of operations in applying the herbicide or undertaking cultivation. Excessive passes, beyond those needed and which can be achieved through good observation, scheduling and optimal treatment efficiency should minimise time, diesel (or other energy), soil degradation, and the carbon and nitrogen losses associated with cultivation.
“It does seem to me that the swing-back to cultivation has conveniently ignored or forgotten the downside with carbon and nitrogen losses associated with cultivation; how then to minimise the losses while maximising the benefits?” Asked Peter.
“There are more machinery passes required if using mechanical methods over herbicide – regardless of a dry or a wet growing season, about an additional two to three passes per season. As a percentage of running costs this is negligible as our biggest cost is manual labour, rather than tractor operation,” commented Duncan.
“However, chemical herbicides are very harmful to the soil ecosystem. The harm caused to soil biology far outweighs some additional tractor passes involved in mechanical weed control. Glyphosate works by inhibiting the production of an enzyme called EPSPS in plants – and also in micro-organisms. This enzyme enables the production of three essential aromatic amino acids, without which the plant or organism is unable to synthesise proteins or secondary metabolites. Therefore, target weeds are affected, but also many of the essential micro-organisms that live in the rhizosphere – area around the roots – feeding off root exudates whilst supplying mineral nutrients in return. This creates a soil which is unable to nourish the vines growing in it without applying synthetic fertiliser.
“Unfortunately, synthetic fertilisers kill off even more soil microbes and leads to vines becoming reliant on applied fertilisers alone for their nutrition.
If wine is an expression of terroir, then to kill off the microbial population in the soil is surely the worst way to undermine the true expression of place and time which the wine is supposed to convey,” Duncan continued.
When asked if he recommends cover crops, Peter Hayes gives an emphatic, “yes! Many seed companies or local agronomists can suggest a locally adapted, potentially ‘indigenous’ mix,” he added. “For some sites, naturally developed swards as guided by the managers on-site practice, can also be effective.
“The usefulness of annual ‘green manure’ crops, which can possibly be under-sown with perennials, selected for a range of functions – such as bulk organic matter or nitrogen-fixing – may also be relevant in some situations and potentially break a cycle of developing perennial weeds. However, there’s considerable energy expended to prepare and seed and consider their potential to elevate frost risk and severity if not terminated early.
“Annual green manure crops are commonly reincorporated, with a related energy cost and loss of carbon to oxidation but may also be rolled down or mown and left to the elements and soil biology to incorporate and degrade. I suggest this topic is one that could readily see locally sponsored and implemented trials to achieve adapted answers for differing soils, vineyard vigour, local climate, and management objectives,” explained Peter.
At Davenport vineyards, Will Davenport relies on the natural vegetation for cover cropping. “If we sowed a low vigour cover under the vines it would almost certainly be out-competed by the more vigorous grasses in the seed bank – and after a few months we would be back to the plants that we already have in situ. Cover crops are more useful in the vineyard rows where they can be managed properly,” said Will.
“We don’t direct drill our cover crops yet, but this is in theory the best method,” explained Duncan. “ It avoids exposing bare soil and the right implement can achieve a very good seed strike rate.
“Cover cropping can also be done in a more simple way, without major topsoil cultivation. I have a front mounted seed spreader and a trailed set of Cambridge rollers behind to ensure soil/seed contact. This is a one pass method. If going into well-established grass sward even a pass with a simple chain harrow beforehand can be enough to disturb the surface sufficient for seeding,” Duncan added.
Woolly weed eaters
“Increasingly, this appears to be a realistic way to integrate animals into the vineyard system and to further the circular economy for soil carbon and other nutrients,” commented Peter Hayes. Getting the best effect probably requires attention to grazing management; stocking density, rotational cell grazing, timing etc may warrant specific evaluation. Winter grazing appears effective for many; attending to the bulk of organic matter and many escape weeds but may present issues on heavier soils with pugging and compaction risk under wet conditions,” added Peter.
“We graze one of our vineyards with sheep over winter,” explained Will Davenport. “This is a very useful tool in tidying up the vineyard. However, we can’t use sheep in all our vineyards because of lack of fencing and water or use by dog walkers,” he added
“Danbury Ridge are introducing sheep post-harvest, over the winter and up until spring,” explained Duncan. “It makes complete sense and is achievable as long as fencing is in place. Not only do the sheep graze the weeds but their manure nourishes the soil and introduces new microbes into the soil – helping achieve a diversity of soil microbiology. It is better to concentrate livestock in a small section which creates a high grazing density, otherwise if the livestock are allowed to roam the entire vineyard they will end up grazing only their preferred plants. This is the basis of a system called Planned Holistic Grazing developed in South Africa and Australia from the 1960s onwards. It is effective in ensuring that deep rooted succulent plants are not grazed to elimination, and that a diverse range of plant species are able to thrive rather than just those species which are not palatable to livestock,” added Duncan.
Going herbicide free
“In order to go herbicide free and manage weeds by alternative methods, first develop a good understanding of your site and situation,” advises Peter Hayes. “Identifying candidate persistent weeds and apply all legitimate and reasonable efforts to manage or eliminate these, ahead of any system change – especially for vineyards about to be developed,” Peter added.
“In an ideal world, one would eradicate weed control altogether and allow nature to take its course,” commented Joel Jorgensen. If left to its own device nature will maintain a balanced ecosystem allowing all plants to grow in harmony – but we want our vines to dominate the field. So, I find weed control in the early years of a sustainable vineyard essential to giving the vines a head start in getting established. A strip of thick compost mulch laid onto the bare soil soon after planting helps suppress weeds for a while in the first season and contributes organic matter – which is key to kickstarting the soil regeneration process.
“As soon as weeds start to take hold undervine, run through the vineyard with something like the Braun Rollhacke – it is cost effective and super quick with very shallow soil disruption. In the meantime, sow the alleyways with a diverse cover crop to suit your soil using a no-till drill. Its best to select plants with varying root types and depths to encourage microbial and earthworm activity in all soil horizons. Repeat passes with the Rollhacke are often necessary to keep a clean slate under-vine while they establish. Offence is the best defence – so don’t allow the weeds to take hold or you will be tempted to revert to herbicide use.
“Once the trunks have established and the vines have secured dominance, the competition from weeds will be less – but it’s still prudent to keep them under control whilst you regenerate the soil food web during the vine establishment years.
“Once your soils are healthy and alive again, and conditions are favourable, weeds should no longer be ‘weeds’. Instead, they perform nature’s intended purpose; they feed the soil food web, promote microbial activity, help regulate soil temperature and sequester carbon.
Monitoring soil health is key to understanding where you are in the journey of soil regeneration and once you are confident that you have a healthy community of beneficial organisms, it may be time to switch to under-vine mowing and mow-and-throw solutions to nurture a low growing cover crop under the vines and maintain airflow throughout the canopy. The cover crop and your vines will develop a symbiotic relationship if conditions are good and the healthy ecosystem restored,” advised Joel.
“Weed control is the most difficult aspect of organic vineyard management,” commented Will Davenport. “Obviously there are no herbicides, so we are reliant on mechanical weeding, ground cover, mowing or just tolerating the weeds. Every soil type and vineyard site requires a bespoke solution as there is no one-size-fits-all solution – and young vines can be especially tricky because anything mechanical risks damaging the plants.
“In one of our vineyards last year we managed with a single under-vine mow over the whole year. This was done by using sheep to graze and by persuading ourselves that grass cover under the vines is acceptable. The vines are 30 years old and the grasses are low vigour. I’m not sure we would get away with this approach every year in the long term. We mulch with compost on roughly one third of our vineyards each year, so these plots need very little weed management in that season. Labour costs are higher because under-vine weeding is a slow speed operation, and we can’t ignore the high capital cost of weeding machinery either.
“We have just purchased a new machine that can either hoe or roll hack at a faster speed than our previous machines. This will be a shallow cultivation. Combined with undervine mowing and compost mulch, plus a reasonable tolerance of weeds under the vines, I think we will be able to minimise soil damage and still keep the vines in a healthy state,” concluded Will.
There are emerging new technologies that are likely to be useful, and practical, for weed management. “Robotic mowers are already with us, so it’s likely just a matter of time before these become accessible and affordable,” explained Peter Hayes. “Others such as electric and microwave-based weeders are also in development, although applications for vineyards with practicality and efficacy are not assured; capital and energy costs are yet to be clear. Flame and steam weeders may have some local application, but costs, risks and logistical issues likely preclude their wide use. Likewise, some biologically derived actives are in development, but just how or when they will reach the market is unknown,” Peter added.
“I’m not convinced that equipment which takes the human eyes, brain and intuition out of the vineyard will result in better grapes or wine. My personal view is that as farmers we need to work with nature rather than trying to control nature – we will never win!” Exclaimed Duncan.