Weedkillers are designed to kill unwanted plants. Unfortunately, it is all too easy to damage or even kill desirable plants if care is not taken when applying weedkillers. Since there are no remedies, prevention is best.
Areas affected Beds, borders, fruit and vegetable garden
Main causes Deliberate or inadvertent application or release of weedkiller
Timing Any time of the year but usually effects are most noticeable in spring and summer
What is weedkiller damage?
Weedkiller damage can occur if the wrong type of weedkiller is used or the chemical is misapplied. Always read the label before applying and choose the best method of application for your situation.
Occasionally, vandalism, contaminated manure or contaminated green waste compost is to blame for weedkiller damage.
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 LawnClear 2 Feed & Weed and 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. SBM Job done General Purpose Weedkiller or Doff Advanced Concentrated 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 that resemble mini witches’ brooms.
Residual weedkillers (e.g. SBM Job done Tough Weedkiller (ready-to-use only), SBM Job done Path Weedkiller (ready-to-use only) or 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.
Weedkiller damage can occur in various ways:
- From drift when spray is applied during windy or warm, sunny conditions
- Using a poorly rinsed sprayer or contaminated watering can
- Leaching of residual weedkillers into adjacent beds
- Absorption of residual weedkillers by underlying plant roots. This can happen when the application rate has been exceeded, or on light, sandy soils where the weedkiller has penetrated deeper
- Mulching with recently treated mowings (or compost that contains them)
- Plant vandalism
- Composted green waste or manure contaminated with persistent hormonal weedkillers
Some plants are more sensitive to weedkiller damage than others. Tomato and potato plants are very susceptible to hormone weedkiller damage. Roses and raspberries are particularly sensitive to glyphosate from July onwards.
Prevention and remedies
The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.
Prevent damage by following these steps;
- Only use weedkillers in the ways described on the label
- Thoroughly wash out sprayers or watering cans after use and dispose of the rinsings on a treated area or uncultivated patch of ground
- Use separate sprayers for herbicides, insecticides and fungicides
- Do not use the first lawn clippings after application of a lawn weedkiller on the compost heap. Instead, mow regularly so the clippings are short. Allow them to drop back onto the lawn where they will quickly disperse. The next three mowings should be used as a mulch only after composting for at least nine months. If you use a lawn care company to look after your lawn, be sure to ask if weedkillers have been applied and if you need to take the precautions with subsequent treatment of clippings as described above
- Do not put grass clippings treated with a persistent lawn weedkiller into your council's green waste collection as this risks residues contaminating green waste compost sold back to gardeners as soil improver or in potting compost mixes
- Do not use residual path and patio weedkillers on paths or drives close to beds or lawns or tree root runs as this can leach
- If treating lawns or soil, avoid walking over the treated area which may spread the weedkiller into other areas of the garden
- When using weedkillers around bulbs that have died back, ensure that it does not enter the open neck of the bulb. Even if treated when dormant, some bulbs can still go on to exhibit weedkiller damage
- Take care when using weedkillers around plants or fruit that produce suckers, especially raspberries
- When spraying, protect nearby garden plants by covering with a plastic sheet or screen with a board. These can be removed when the spray is dry on the foliage
- Only spray in calm weather to reduce drift. Use targeted formulations such as a gel (Roundup Gel)
- To reduce the amount of drift when treating a lawn it is better to use a watering can with a dribble bar or a fine rose rather than a sprayer
If damage occurrs;
- Cut any abnormal shoots or those that are brown, hard back. It can be some time before the affected plants start to produce normal shoots
- To promote recovery, feed and mulch damaged plants
- Perennials treated with contact weedkiller will recover. Those treated with glyphosate may also survive but root damage is possible. If replacement is necessary, only replant after sufficient time has elapsed for the residues to break down (check weedkiller label for details)
- Beans, carrots, lettuce, peas and tomatoes affected by hormonal weedkiller can still be eaten. Brassicas (except sprouts) do not usually recover and should be destroyed
- If vandalism is suspected, photographs of the damage can be useful. These may be needed for evidence if the matter is taken further. Weedkiller residues are difficult to test for in soil or plant material (note: the RHS does not offer this service) but a report from a consultant is worth considering
Inclusion of a weedkiller product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener.
Weedkillers for gardeners (Adobe Acrobat pdf document outlining weedkillers available to gardeners)
If you suspect a problem with a particular batch of peat-free or peat-reduced compost or manure it is possible to do a simple test for weedkiller residues. This involves germinating plants that are sensitive to herbicide such as tomatoes or beans.
How to do the test
STEP 1: Fill two clean pots with suspect growing media (fresh, not used) and label.
STEP 2: Fill another two clean pots with another brand of growing media or John Innes potting compost as a control (i.e. a standard to compare to) and label.
STEP 3: Into each pot sow four broad bean seeds or insert four tomato cuttings (use sideshoots from healthy tomato plants). Water well with clean water.
STEP 4: Place the pots in a greenhouse or warm windowsill in winter, or for beans outdoors in summer. Keep the compost damp but ensure the drainage water from pots containing the suspect growing media cannot contaminate the control pots.
If after three weeks the seeds fail to emerge, the seedlings or cuttings are distorted, show less growth than in the controls or exhibit fern-like growth in the suspect growing media, while control plants are normal, there are strong grounds to believe weedkiller residues are present.
What to do
If the test is positive, gardeners should contact the manufacturer of the compost or growbag. Suspect compost can be used as a topdressing for lawns.
Plants showing symptoms early in the growing season may grow out of the problem as chemical residues break down. Harvestable produce from plants exhibiting symptoms is considered safe to eat. Vegetables such as courgettes, brassicas and salads are unlikely to show any distorted growth.
If you suspect that your garden or allotment has been affected by clopyralid from bought compost, you can report the damage on the Manure Matters page.
SylvaGrow is an RHS-approved peat-free compost that does not contain green waste so is worth considering as an alternative.
The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.