Plants accidentally sprayed with contact weedkillers usually have a scorched appearance or brown spots wherever the spray droplets landed on the leaves. Bulb foliage may emerge yellow if accidentally sprayed with a contact weedkiller before it had died back in the previous year. Affected bulbs may be weakened for several years.
Hormone or growth-regulating weedkillers (such as 2,4-D from lawn weedkillers or rough grassland weedkillers like Vitax SBK Brushwood Killer containing triclopyr) leave grasses unharmed but can cause damage to broad-leaved plants. The symptoms are very distinctive:
- Narrowed or cup-shaped leaves with parallel veins
- Twisted or distorted leaf stalks
- Adventitious roots on some plants, including root crops
- Swollen stems, galls or warts in brassicas and other plants
- Plum-shaped and distorted tomatoes with hollow centres
Roses, tomatoes, potatoes, brassicas and vines are particularly sensitive to this type of weedkiller.
Clopyralid and composting of mowings:
Weedkillers containing clopyralid (e.g. Vitax LawnClear 2, Vitax Green Up Lawn Liquid Feed & Weed and Scotts Weedol Lawn Weedkiller) have to be used with especial caution. This herbicide binds to leaves and stems which is useful in preventing accidental damage and pollution, but mowings retain active herbicide. Once bound to grass the herbicide takes time to break down. For this reason it is essential that the label recommendations are followed concerning disposal of mowings. Typically this will involve avoiding composting the first mowings after treatment or using them as a mulch. The ideal way to dispose of such mowings is to mow frequently or with a mulching mower so that clippings fall back into the sward. If this is not possible we suggest composting mowings separately and later applying composted material only to turf.
Manufacturers may also give a period of composting for subsequent mowings (but not the first mowing) after which the herbicide will have decayed – typically this will be around 9 months. Treated clippings should not be added to municipal green waste composting as there is the risk of spreading contaminated compost. Also some councils will not accept green material in ordinary domestic waste or for landfill at waste disposal centres. In such cases dealing with treated grass in the garden is the only option as well as good practice.
A number of cases in 2016 suggest some sources of growing media (e.g. growbags) may be contaminated with hormonal weedkiller, most likely from composted green waste containing clopyralid. See section below on how to run a germination test if you suspect a compost may be a source of weedkiller residues.
Glyphosate (e.g. Scotts Roundup, Bayer Garden Rootkill Weedkiller or Doff Glyphosate Weedkiller) damage shows as leaf yellowing and browning, and shoots collapse on soft-stemmed plants. Roses and raspberries are particularly sensitive to glyphosate from July onwards. This causes stunting and leaf malformation, which may not be evident until the following season. Roses produce clusters of short, often pale shoots appear that resemble mini witches’ brooms.
Residual weedkillers (e.g. Bayer Garden Path & Drive Weedkiller or Scotts Weedol Pathclear range) can be taken up by underlying tree and shrub roots. It may take some time before symptoms such as leaf yellowing or dieback appear in affected plants.