It all started with a discussion on an allotments forum about rhubarb leaf tea, alleged to be efficacious against red spider mite. The moderators removed the post on the grounds that such home-brewed remedies are illegal to make, store, or use.
What nonsense, I thought. An Englishman’s home may no longer be his castle, but surely it can’t be a crime to spray a little rhubarb leaf tea on one’s own plants! Eccentric, certainly. Ineffective, probably. But not against the law.
I was wrong.
I wrote to the Chemicals Regulation Directorate, and was promptly answered by Ian Rowland, whose comprehensive reply I reproduce below:
Dear Mr Norman,
Thank you for your enquiry. Strictly speaking, the use of home-made insecticides, herbicides and other pesticides – the term “pesticides” is used in legislation as an umbrella term to cover any substance, preparation or organism prepared or used for destroying any pest (where pest means any organism harmful to plants or to wood or other plant products, or any undesired plant; or any harmful creature) – would be covered by the Control of Pesticides Regulations 1986 which apply not only to recognised pesticides but also to substances, preparations or organisms prepared or used as if they were pesticides. These are UK regulations that require that all pesticides must have been granted approval by the relevant Government ministers before they can be legally advertised, sold, supplied, stored or used so the use of home-made pesticides would represent the use of unapproved pesticides and this would be an offence under the Regulations.
We are aware that some gardeners and allotment users do make their own pesticides from household chemicals or other substances to control particular pests; for example, using rhubarb leaves as in your example, or using coffee or salt to repel or kill slugs. However, we cannot condone or encourage the use of such pesticides because such use is likely to result in the user committing an offence under the regulations even though such offences are unlikely to be prosecuted; it is usually difficult to obtain appropriate evidence and it is not generally in the public interest to do so. This does not mean that appropriate enforcement action, including prosecution, would not take place – while some of the home-made pesticides that gardeners have produced over the years are unlikely to have health or safety implications, others most definitely could – and where evidence was obtained that the use of such substances had led to serious environmental or health consequences or where “home-made” pesticides were being sold or supplied to third parties, this would potentially result in prosecution for a criminal offence.
Similarly, we would not encourage the publication of information relating to the ingredients of home-made pesticides and/or methods of manufacture or use, although we have no actual powers to prohibit this. However, the publication of such information could be interpreted as encouraging the use of unapproved pesticides, and this could, potentially, result in the person publishing the information also committing an offence. We would therefore hope that anyone considering publishing information in this area will avoid references to specific home-made pesticides or their ingredients and methods of manufacture and use.
By comparison, the use of home-made fertilisers such as compost or comfrey tea are not directly covered by any legislation, although we would probably not recommend that anyone try and manufacture their own “commercial” NPK fertilisers at home.
I hope that this answers your questions but if you would like clarification or further information on any point, please let me know.
Yours sincerely,
Ian Rowland
Communications Branch
Chemicals Regulation Directorate
Health and Safety Executive
So when we use Fairy Liquid to kill blackfly, it’s a crime. I’ve creosoted the henhouse against red mite: several serious offences there. And goodness knows what witches’ brews are in use across the whole allotment site. I’m getting a whiff of the atmosphere of Al Capone’s Chicago.
I should be outraged about the loss of liberty. For goodness sake, a chap can’t even put coffee grounds in his own flower bed without having his collar felt. But a sense of how risible these regulations are keeps breaking through.
I’ll sign off with one glorious example. There’s had to be a special derogation to the regulations, covering “foodstuffs used as an attractant or repellant”. Apparently zealous officials were following up reports of the use of unapproved jam in wasp traps, and unapproved peanut butter in mousetraps. Beer in slug traps is still banned, of course, since it’s the biocide as well as the attractant.
Dr Strabismus (whom God preserve) of Utrecht is clearly alive and well and working for the EU.
John Norman's Blog 
Whilst on the surface it does look ridiculous to have this legislation it seems fair given some of the very nasty homemade concoctions many selfish people continue to use, made, not from organic sources, but from household chemicals in vast quantities (surfactants, bleaches, caustic soda, Differentiating between “good” and “bad” requires defining the line between the two. Difficult and subjective. Better, as suggested in Mr Rowland’s letter, to ban them all and turn a blind-eye to the less serious breaches – “it is usually difficult to obtain appropriate evidence and it is not generally in the public interest to do so”. To define a difference would require more intensive, and more expensive, enforcement.
People are too ready to knock H&S legislation without examining its detail or necessity, especially when,,in a knee-jerk reaction, it can be used as a stick with which to beat the EU. The alternative would be to remove the law altogether and allow anything to be poured onto the soil and into the water supply.
Incidentally. rhubarb leaf tea spayed on brassicas is, anecdotally, very effective against white-fly. Rhubarb leaves are brilliant on the compost heap, Rhubarb tea and biscuits could be fatal.