Sunday 8 December 2013

Can you use ‘Roundup’ in Winter in the UK?

Yes, you can use glyphosate in Winter….

Photographed today the strip of rhizomatous grass was sprayed 1st November

- even though it sometimes says on the pack that you can’t - and its uses are more limited than in Summer.

Readers of my regular glyphosate posts know that I use this herbicide all the year round. Apologies to them, they might want to skip this one! Many gardeners ask this question and usually get a confusing answer - so here is mine!

It is important to understand that glyphosate kills the roots via the leaves. Active green leaves absorbs the chemical and translocates it to the roots. Glyphosate is renowned for killing perennial weed. Many perennial weeds such as convolvulus and ground elder go completely dormant and lose their leaves in Winter and at that time glyphosate is utterly useless

Some perennial weed such as rhizomatous grasses like couch often hold onto green leaves well into December. No doubt in warmer climates they hold them even longer. Such growth is receptive to glyphosate which is effective - but slow.

Controlling weeds that are still green
The use of glyphosate I find so important in Winter is against weeds that come from seed. Often referred to by the uninformed as ‘annual weeds’ they are not exclusively so; they might be annuals, biennials or perennials, they are merely seedling weeds - or if your maintenance is poor they might be rather larger! 
Weeds are very opportunistic and some will germinate at any time of the year and when conditions are suitable they grow! Weeds like cleavers germinate in November and December. That wretched ubiquitous hairy bitter cress will germinate at any time. Don’t ask me about controlling it in March when its seeds are dispersing all over the garden! Spray in Winter before any problems arise.

The fact that many garden plants have died down means that glyphosate is particularly useful as it will not harm these dormant plants. Take the opportunity to spray weeds growing amongst perennials such as hostas when your garden perennials are leafless.

Glyphosate in Winter is very slow
Even in Summer it takes in the region of two to three weeks for glyphosate to kill. In January and February glyphosate is effective but its speed is positively glacial. Six weeks if you are lucky, eight or more if you are not. If you are the sort of gardener who wishes to immediately transform a weedy patch to a pristine weed-free garden, glyphosate is not for you!
If you are the sort of gardener who regards weed control as frequent on-going garden management and the only person who even notices any weed is yourself then a quick winter spray-round is ideal. Once sprayed, a weed is ‘as good as dead’ it stops growing and you can file it away as ‘job done’. It will not be there in March to explode into vigorous growth! Weed control becomes easy and pleasurable if you keep on top! Spraying around in Winter is very speedy and easy when most of your garden plants are dormant. My own garden is an acre and is planted intensively and intricately with many thousands of plants. I can completely knapsack-spray the whole garden in less than two hours.

Glyphosate of course will not kill any further succession of newly germinated weed. Expect to spray again in another six weeks time. 

Efficient absorption of glyphosate
The literature says that glyphosate should remain on the weeds at least six hours before it is washed away by rain. In Summer I find this advice very conservative against small ‘weeds from seed’ and if conditions are warm and humid, a lethal dose will be absorbed in an hour or so. These conditions do not occur in Winter. Plan your Winter spraying when rain is not expected.
Dews in Winter can be very heavy. If your spray dislodges water that falls to the ground, according to the literature, this will waste some of your spray. It might be best to wait until the heaviest of dew has gone.

Does air temperature effect efficiency of uptake and subsequent effectiveness?
The technical literature suggests that to achieve optimum effect, growers should not spray when the temperature is below 5ºC and not expected to rise to 10ºC the same day. The significant factor would seem to be absorption rather than effectiveness when absorbed. Although I do not personally concern myself about how warm it is when I spray - other than my own comfort - it might be wise to heed this advice.

Sulphate of ammonia to enhance speed of absorption
A closely guarded secret is that sulphate of ammonia enhances a plants ability to absorb glyphosate and makes it ‘go further’. My own glyphosate comes so cheap that I never bother to add this common fertilizer to my spray. I have known it to be recommended at perhaps 15 gram per litre of diluted spray to increase glyphosate’s speed of absorption. I have not done this for many years and it might not even be legal! (I have recently seen ammonium enhancers recommended for a specific commercial glyphosate brand)

I see that 360gm/litre commercial glyphosate product now retails on Amazon at about £60 per 5 litre (A gallon in old money). I buy elsewhere in 20 litre containers - it was Rodeo this time, previously Hoedown, but they are all the same glyphosate (although not the same spreader). With VAT last time it cost me £100 for 20litre, so compared to the garden centre it is very cheap. I do not  in Winter consciously depart from my usual dilution rate of 1in 50 for ‘general’ weeds. Read my posts on buying and dilution rates.

I  carefully sprayed the grass growing at the base of this bog iris 12th November

Photographed today 7th December the grass is not quite yet dead and the near dormant iris is unharmed.

Weed nicely yellowing in my garden today


  1. Hi Roger,
    I stumbled across your website whilst searching for round up use in the UK. You may want to watch this video. It is vitally important.


  2. is this funny? (from

    A weed scientist goes into a shop. He asks: "Hey, you got any of that inhibitor of 3-phosphoshikimate-carboxyvinyl transferase? Shopkeeper: "You mean Roundup?" Scientist: "Yeah, that's it. I can never remember that dang name."

    1. Glyphosate CHEMICAL name: Isopropylamine salt of N-(phosphonomethyl) glycine

      I am surprised that any son of mine would get the chemical name wrong!

      Great joke Ben, glad it got past the spam filter!

      Good to see that you are a Guardian reader now.

    2. Joanna Haigh, professor of atmospheric physics, Imperial College, London got the chemical name wrong! I'm just the messenger.

    3. They always shoot the messenger.
      Having rechecked I did not read it carefully enough- your quote does not say its the name of the chemical, just what it does to plants like arabadopsis. Apologies to the prof!

    4. Lighten up big Roger. It was a joke for crying out loud.


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