Documents: Special Interest: Gardening In England:

RHS Update on Weedkiller Residue

For Those Living in England
by Nicole Russell
September 7, 2008

The Royal Horticultural Society (RHS) advisory service continues to receive many calls from gardeners worried that their crops are being damaged by weedkiller residues from contaminated manures or soil improvement compost containing the chemical aminopyralid.

As the UK’s leading gardening charity, the RHS has been issuing the latest information to update gardeners on the problem. Dow AgroScience, who manufactures products containing aminopyralid, have asked for their approvals by the Pesticide Safety Directory to be modified while the situation is under investigation. All products containing aminopyralid have been suspended for the approval of sale and supply with immediate effect.

Storage is unaffected and it remains legal and safe for these products to be stored by anyone. For more details: or If gardeners are concerned about the health of their crops they should follow the latest RHS advisory service guidelines listed below.

Am I affected?

Weedkiller damage symptoms are very distinctive. Look for cupped leaves and fern-like growth on sensitive crops such as tomatoes, potatoes and beans. There are no remedies once damage has occurred, though some crops may show no symptoms and others may show sign of recovery. If in doubt, RHS members can contact the RHS Advisory Service and other gardeners can use the bulletin board on our website:

What to do

To speed up the rate of breakdown of residues on contaminated land, rotovate or dig over the soil several times, preferably between summer and autumn when the soil is at its warmest. This ensures the manure is fully incorporated into the soil and increases microbial activity.

Residue levels in the soil peak at three weeks after digging before breaking down relatively rapidly. The ground is safe to plant with edible crops next spring. Fruit trees and bushes damaged by contaminated manures are likely to survive and grow well next season. Feed the plants in the spring to encourage good cropping next year. Flowers such as phlox and delphinium that show symptoms should be cut back at the end of summer and also given a well-balanced feed next spring.

It is not advisable to compost ruined crops. If they cannot be incorporated into the soil, bag them up and put out with household refuse (NOT green waste collection). Seek advice from your council if they won’t accept green waste in domestic refuse.

Mulch or no mulch?

Mulching is a good thing and gardeners should not be reluctant to continue this practise. Adding any organic matter to the soil will increase its moisture-retaining properties, improve soil structure and help gardeners to combat potentially damaging forces of a changing climate such as drought and flooding.

When buying manure in the future

At the request of the manufacturers, products containing aminopyralid have been withdrawn from supply, sale and use while it (aminopyralid) is under investigation by the PSD. It is not illegal to still store such products. Even following withdrawal there is still likely to be contaminated manure on farms and stables that may be offered to gardeners. Therefore, gardeners should be cautious about accepting manure from sources that cannot give assurances that the manure has not come from animals fed on grass or forage treated with persistent hormonal weedkillers, especially aminopyralid products such as Forefront.

Why has this happened?

In the past, occasional instances of weedkiller damage from contaminated manure have been reported to the RHS advisory service. Most instances have no easily identified cause. However, this year there has been an upsurge in reports, and, in many cases relayed to the RHS and the Pesticide Safety Directorate, this has been traced to aminopyralid, a weedkiller used to control pasture weeds and only introduced in 2006. This herbicide binds to plant material fed to horses and cattle such as hay or silage. This can pass through animals without breaking down. Manure from animals fed on treated grass contains chemical residues sufficient to damage susceptible crops. Bedding materials such as straw are less likely to be affected as aminopyralid does not currently have approval for use on cereals crops from which straw is made.

Unfortunately, it is often impossible to trace the source because important information on weedkiller treatments is not always passed down the supply line – from farmers to hay or silage merchants, to owners of cattle or horses, to compost-makers, and finally to the consumer. An increased awareness of this issue at all stages of the chain should help reduce this problem from happening in the future.

The RHS is committed to help gardeners with the best advice on good garden practice. For more details visit

About the RHS

The RHS believes that gardening improves the quality of life and that everyone should have access to great garden experiences. As a charity we help to bring gardening into people’s lives and support gardeners of all levels and abilities; whether they are expert horticulturists or children who are planting seeds for the very first time. RHS membership is for anyone with an interest in gardening. Support the RHS and secure a healthy future for gardening. For more information call: 0845 130 4646, or visit RHS Registered Charity No. 222879/SC038262

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