Herbicide options for nursery stock

David Talbot and John Atwood of ADAS consider future herbicide options for nursery stock growers.

Herbicide programmes need to be carefully planned to ensure target weeds are adequately controlled, while minimising phytotoxicity (crop damage), and should be included in the production schedule. Perennial weeds are easier to control on land before it is brought into production, rather than after the crop is planted.

If spraying herbicide applications are not possible due to weather conditions, short-term cultural controls such as strimming should be considered. This will prevent weeds seeding, particularly those that produce vast numbers of seeds that are readily dispersed by the wind, such as willowherb, groundsel and Canadian fleabane.

Herbicide application rates can vary depending on the crop or situation. For example, weed control can be achieved at lower rates than stated on the product label, particularly when tank-mixed with other active ingredients, such as Flexidor 125 and Devrinol. This can be useful when considering herbicide-sensitive subjects. However, where two or more active ingredients are tank-mixed, one of the ingredients may persist, giving weed control for longer than the other. This can lead to poor weed control, unless it is addressed with repeat applications.

Revocation of label approvals

Revocations of label approvals and final use dates further complicate herbicide programmes, although in many cases revocations only affect a particular herbicide product, leaving other similar products with the same active ingredient available for growers to use.

Difficulties usually arise when an active ingredient is revoked, rather than just one product, because users have to rely on different active ingredients for their weed control programmes. This can be a problem in ornamental crops, where there may be insufficient information about the safety of new active ingredients. Details regarding revocations can be found on the Pesticides Safety Directorate (PSD) website, www.pesticides.gov.uk, and are listed each month in the ADAS Hardy Ornamental Technical Notes.

Given the uncertainty over the availability of some active ingredients and herbicide products, research projects must screen potential new active ingredients to find replacements for those which are lost. The following HDC projects have been undertaken or are in progress:

HNS 132: Roses - Triazine-free herbicide programmes;

HNS 139: Control of problem weeds in hardy nursery stock;

HNS 155: Herbicide screening for tree seedbeds.

The final report is available for HNS 132 and annual reports are available for HNS 139 and HNS 155 to HDC members.

The following active ingredients have been revoked since January 2007, and storage and/or use of products containing these is now illegal in the UK: ammonium sulfamate, cyanazine, pentanochlor and simazine.

Active ingredients shown in the table have also been revoked, but are still in the use-up period. The latest final use date shown relates to active ingredient, as distinct from herbicide products. Always check the status of products on the PSD website.

Weed control programmes

Pre-planting treatments

Perennial weeds are best controlled in the season before a crop is planted. Most are controlled by the active substance glyphosate (eg Roundup). This should be given at least a week to kill weeds prior to planting, sometimes longer during cool conditions and the autumn and winter period.

Where species such as creeping yellow cress, horsetail and perennial nettle are established in fallow or non-crop situations, alternatives such as amitrole (Weedazole -TL) or a translocated hormone-based herbicide should be used. These herbicides should be applied six to 12 weeks before planting, allowing weeds time to die off prior to cultivations and avoid the risk of damage to the following crop. Products must be applied carefully as adjacent crops will be damaged if drift occurs.

Non selective, non-residual contact herbicides

Paraquat has been widely used and is a fast-acting, reliable, contact herbicide. It was originally included on Annex 1 under EC Directive 91/414, but the decision was annulled this month. Therefore, all approved products containing paraquat will expire on 11 July.

Diquat is a possible replacement for paraquat as it can be used during winter months, but is 10 per cent less effective on grass weeds. Also, label use is only 2 litres/ha compared with paraquat, which was 5.5 litres/ha, so is only effective at this low rate on seeding weeds. The use of an authorised non-ionic wetter is crucial for the control of certain weeds and tank mixing with residual products will further improve weed control.

Glyphosate is the only active ingredient that is really effective on perennial weeds, but is fairly slow acting, particularly at low temperatures. There is more risk of damage to an adjacent crop from drift. However, when it is too windy to spray and as an alternative to flailing/topping weeds to prevent weeds seeding, glyphosate can be applied to tall weeds via a weedswiper - a tractor- mounted bar with herbicide pad. This approach is preferable to topping as another pass with the tractor is not required a few weeks down the line. Glyphosate is translocated to the root, killing the whole plant in most cases; some weeds with extensive root systems, such as hedge bindweed, are unlikely to be eradicated.

Glufosinate ammonium (eg Challenge 60) may not eradicate perennial weeds in one application, but will suppress them for longer than diquat. Glufosinate ammonium is also more effective on willowherb and nettle than diquat. The present label restricts use to between March and September, when weeds are actively growing. It is most effective under warm, moist conditions. Light rainfall three to four hours following application should not affect activity, however, the active substance should not be applied to wet foliage or if rain is forecast within six hours.

The HDC has recently processed a Specific Off label Approval (SOLA) for Shark (carfentrazone-ethyl) for use as a horticultural herbicide for the post-emergence control of broad-leaved annual weeds, including willowherb, cleavers and cranesbill in ornamental plant production. Shark is less effective on groundsel, mayweed, chickweed and annual meadow grass, but it is possible to tank-mix it with diquat to improve the control.

The use of this product would not normally be allowed under the Long-term Arrangements for Extension of Use (from other edible crop approvals). However, this SOLA ensures continued approval for the ornamentals sector, albeit off-label and at the user's own risk. But there is no crop safety data available for the use of Shark with ornamental crops.

Shark can be used at any time of year, but is likely to be slightly less effective during winter when weed growth ceases and dormancy sets in. Other herbicides (eg glyphosate) are less effective under similar conditions. When temperatures are low and weeds are dormant, Shark must be used at the full label rate to obtain good weed control.

For weed control within the crop, overall applications may be applied, providing the spray does not come into contact with green or breaking buds, crop foliage, damaged or green bark.

A directed inter-row spray between plants may be applied via a inter-row shielded sprayer. When used on non-cropped land, avoid drift onto adjacent broad-leaved crops as the product is likely to be damaging, unless stock is fully dormant.

Although Shark is less effective in controlling grass weeds, some scorching will occur, so it would be unwise to assume that Shark can be used selectively to control weeds in ornamental grasses.

Field-grown nursery stock, residual herbicides

Stocks of diuron need to be used up before 13 December. This active ingredient should not be applied over foliage, on young stock or when the root system is active. It should only be used on stock that has been established for at least a year because of the risk of damage. Persistent residues can be a problem in following crops.

For many woody, field-grown nursery stock subjects, a weed-control programme based on oxadiazon (Ronstar Liquid) will be effective. However, Ronstar Liquid does not control chickweed, so a tank-mix with, for example, isoxaben (Flexidor 125) or propyzamide, such as Kerb Flo, may need to be used.

Although not approved for nursery stock, pendimethalin (Stomp) is frequently used as a tank-mix partner for Ronstar Liquid. This mix generally gives good residual weed control in field-grown nursery stock. At present, its use is permitted under the Long-term Arrangements for Extension of Use at the grower's own risk. However, with the phasing out of these arrangements, a SOLA will be needed, which is currently being considered by the HDC.

Ronstar Liquid has some contact action and will control most small annual weeds that have emerged when the herbicide is applied. However, care must be taken to avoid spraying over foliage or opening buds. Ronstar Liquid can be damaging in some crops or situations. For example, Berberis can be damaged and maiden Pyrus stocks are susceptible to damage if treated after heading back.

Container-grown nursery stock, residual herbicides

A greater range of residual herbicides can be used with trees and shrubs grown in containers compared with herbaceous perennials, alpines and heathers, as they are less sensitive to herbicides.

Axit GR has a final use date of 20 March 2009 as it contains triafluralin. It also contains isoxaben, which is the active ingredient in Flexidor 125. Flexidor 125 controls a similar weed spectrum to Axit GR, for example, hairy bittercress, chickweed and pearlwort, but does not control annual meadow grass so well. It has been trialled on herbaceous crops and alpines, and can be used on a limited range of such crops at the grower's own risk.

A programme using Ronstar 2G and Flexidor 125 provides good weed control as the two products complement each other, controlling different weed spectrums. However, the control of groundsel and willowherb can be difficult once the Ronstar 2G run out occurs 10-12 weeks after application, as Flexidor 125 does not give adequate control.

Fortunately, HDC project HNS 139 has identified some alternative herbicides that will control these weeds and which might be safe enough to use over a range of stock in the summer months.

For growers using bark mulches for weed control over the surface of the growing media, it is worth noting that Ronstar 2G granules are unlikely to give good weed control where such a mulch (or similar) is present.

Active ingredient Final use date of
active ingredient
Diuron 13/12/2008
Paraquat 11/07/2008
Triafluralin 20/03/09

EU HARMONISATION OF PESTICIDES

The EU harmonisation of pesticides means all active substances are being considered for inclusion in Annex I of Council Directive 91/414/EEC. This is a rolling process with active substances still being processed.

Some active substances, such as paraquat/diuron have been excluded from Annex I of Council Directive 91/414/EEC. Products containing these active substance have to used by a certain date - paraquat final use date is 11 July and Diuron 13 December. Such products will no longer be legally produced.

Revocation of an active substance can be due to safety issues, problems in the environment or the cost of producing data to meet requirements.

Article 8(2) of Council Directive 91/414/EEC provides a derogation allowing member states to continue granting authorisations for products containing existing active substances under national rules, prior to their inclusion in Annex 1, after which community rules apply.

In Great Britain, national rules include the Control of Pesticide Regulations (1986) (as amended) (COPR). Directive 91/414/EEC is implemented by the Plant Protection Product Regulations 2005 (as amended) (PPPR).

- Information contained in this article is intended to provide guidance, but cannot constitute a recommendation. All applications of crop-protection chemicals should be made in accordance with label recommendations, which should be consulted before application. Some of the herbicide products mentioned may not be supported by label recommendations for their use on ornamental crops, but are permissible under 'he Revised Long-term Arrangements for Extension of Use (2002).

A small area of the crop should be tested for adverse effects/crop tolerance prior to treating the whole crop.


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