Winter weed control – Teagasc

While plants are dormant there are some valuable weed control options available to the grower.
In all cases, check labels and PRCD website for up to date information and use when temperatures are above freezing.

Butisan S – Metazochlar
It is a residual herbicide absorbed by roots with limited foliar uptake. It is approved for use on outdoor woody ornamentals and limited to 1 treatment per year no more than 6 weeks before harvest. Good control of willow herb, groundsel, grasses, and liverwort.  It has good persistence of 12 weeks +. Best used in autumn or winter while plants are dormant as it can damage soft growth. Phytotoxic damage may be seen if used on sands, very light or poorly drained soils. To prevent damage make sure residue is washed off the foliage. Some woody plants are sensitive check label info etc for details.
Flexidor – Isoxaben
It is a residual herbicide absorbed by seedling hypocotyl ie as seeds germinate. It has very good persistence, lasting c. 18 weeks or more. Excellent bittercress control, but not as good on willow herb and groundsel. No control of grasses, moss or liverwort. Some sensitive species e.g.  buddleia and perennials.
Flexidor and Butisan are often tanks mixed to give excellent control however some species may be more sensitive – follow label guidelines
Kerb, Relva etc.– Propyzamide
Kerb is a residual herbicide that is absorbed by roots & translocated. It has no foliar effect. Good control of scutch, grasses and broad-leaved weeds during winter. The liquid application must wash off after application to avoid damage to crops.  Low residue c.2 months rapid breakdown by microbes in warm weather 3.75ltr/ha. C.€128/5L. Propyzamide is tolerated as an overspray by many woody crop species. It is virtually insoluble and requires plenty of rain to reach the roots/ rhizomes. It requires the weed to be slowly metabolising for the herbicide to be taken into the plant, and this process will stop in very cold conditions.
Devrinol – Napropamide
Devrinol is a residual herbicide that is absorbed through the roots of germinating weeds and is translocated upward through seedlings. Good control of groundsel, chickweed & willowherb however poor control of bittercress is reported. Will not control germinated weeds. Persistence is variable as it is broken down by sunlight so no two years will be the same. It binds tightly with clay particles and resists leaching. Do not use on sands or soil with 10%+ organic matter. Only 1 application is permitted per year. Avoid using it on Shallow rooting plants, alpines, conifers etc. plants in pots less than 1L, variegated or golden-leaved plants. Devrinol is reported to show little damage on herbaceous plants.
Summimax, (Digital & Guillotine) – Flumioxazin
Off-label use for outdoor ornamentals. It has contact action so is suitable for small recently germinated weed seedlings up to 3-4 true leaf. Also controls moss and liverwort. 1 treatment per year is permitted with 28 days harvest interval. Summimax is best suited to use on deciduous plant species. Some evergreen species show phytotoxicity e.g. Hebe, Escallonia, Vinca.
Grazon 90, Blaster pro, etc. – Clopyralid
Controls certain annual & perennial broadleaf weeds after germination, selective over many conifers. Vivendi is available for forest nurseries.
Afalon, Datura – Liunron 
Registration expires for Linuron in June 2018. Residual and contact action for the control of grasses and broadleaf weeds. More effective contact action if applied while weeds are active.

Further options but with more focused uses
Starane – Florasulam
Sharpen, Stomp Aqua anthem etc – Pendimethalin
Cutter, Callisto – Mesotrione