Monday 7 January 2019

Crash course on roundup

Twenty important things to know about glyphosate

Although glyphosate is the basis of my weed control I also use other methods
I use the name 'roundup' generically to catch more attention. Of course I mean glyphosate. You don’t want to pay premium prices for trade names when all glyphosate is the same, products differing only in the ‘soap’ used as a wetter and spreader. 

The patented name is used for both amateur and professional products. In dilution they are VERY different
1. Glyphosate comes in a range of prices and concentrations. It is very expensive to those who insist they are amateurs but very cheap to those who buy professional products. Whichever product you use when diluted to the necessary concentration it is equally effective

2. In my opinion glyphosate is the safest pesticide ever invented. If you believe propaganda put out by campaigners and listen to easily swayed politicians and silver tongued lawyers, then you and I do not agree

I sprayed this intact couch grass for an allotment holder with glyphosate. None of it will regrow and although as yet rather inconvenient the plot can be planted straight away!
3. For practical purposes glyphosate completely degrades on contact with the soil. One consequence is that within reason you can plant or sow new plants very soon after application. Also, if you apply it to weeds close to your plants (again with a minor qualification) the roots of your plants will not absorb residue.

4. Glyphosate is NOT of itself selective. Potentially it can kill all plants (There are exceptions such as liverwort and moss). It is used selectively by such means as directed application and timing

5. There are some products such as those used for weed control on roads and pathways that are cocktails with other chemicals. You are generally advised not to make up your own mixtures - but I often quote exceptions

6. Glyphosate is not very rain fast and it is often stated it needs six hours in rain free conditions. This might be true for optimal uptake but for most garden conditions the advice is inappropriate. A short shower of very light rain in this uncommon circumstance can even enhance its effectiveness. In most cases a rain free hour is sufficient for most common ‘weedlings’ Both Peter Williams and myself will set out with our knapsack unless rain immediately threatens or we know a depression is coming over!
If you do have a project against severe established perennial weed you will need the full six hours or better a couple of days.

Just a silly illustration of how accurate you can be with a knapsack sprayer at almost zero pressure. Such accuracy is almost never needed!
7. Uptake of glyphosate is most efficient when applied as a fine spray short of ‘run off’. A coarser spray will be more wasteful but in respect of drifting more accurate. To apply with a watering can is very wasteful indeed and I do not recommend it.

8. It is best to spray on a completely still morning and for delicate spraying this is essential. I personally find in most cases this is over prescriptive and with my own often ‘coarser’ application I am prepared to spray in a light breeze.

The most accurate and speedy spraying is done with a professional knapsack sprayer (which is much cheaper than your mower)
9. Probably the most important thing about spraying with glyphosate is that it needs to be applied to a large and proportional leaf area. Sprayed onto an intact young weed however small it will have efficient ‘take up’ but sprayed onto an emerging large perennial as it peeps out of the ground  is completely useless.

10. To emphasise the above point, if you have already chopped up a perennial’s roots by cultivation you are  wasting your time

Sprayed  three weeks earlier, nicely dying in November
11. It actually takes quite along time for glyphosate to kill weeds. Perhaps two to three weeks in Summer but as long as eight in Winter. Don’t worry once sprayed the weeds are as good as gone. Weeds in flower might continue to set seed for a short while.

12. Strong long-established perennial weeds with a large root system will inevitably regrow after a single spray. Leave such weeds intact with zero cultivation and later spray regrowth. (It’s ok to just cut them back for tidiness a week or so after spraying).
It might need two or three resprays to completely eliminate such weeds - a season or so -  and for really vigorous or part-resistant weeds such as horseradish, equisetum or ground elder it needs repeated respraying over a couple of years - as much as a total of (say) five applications.

At this stage or sooner these herbaceous perennials are vulnerable to damage. Be very careful and direct  your spray head down and at low pressure close to the ground as you spray round the clumps
13. I find herbaceous weeds are killed most efficiently when growth is soft, extensive and vigorous rather than old and tough. Conversely old growth on garden plants is less likely to be damaged by misdirected spray

90% of this bindweed was completely killed by a single spray in early July
* It's not in my garden!
14. Large plants are less likely to be damaged by a little herbicide drift than small ones - they have more resources to out grow misdirected weedkiller and normally you will never know.

Couch sprayed in November, dead in December. Note the iris in this boggy area was already brown when I sprayed
15. Glyphosate is effective all year round provided the weeds are intact, green and preferably growing. Weeds such as convolvulus (bindweed) that have died down or gone brown brown are completely unaffected. Similarly dormant garden plants will be undamaged and early winter is an excellent time to spray such as still green couch grass within plant clumps that have died down in your herbaceous border!

You will need a strong mix for this one - and best to apply a first spray before it flowers
16. The optimum dilution rate for glyphosate varies with the weeds and conditions. My routine strength is 1 in 50 commercial 360gm concentrate to water. Against ‘difficult’ weeds my spray might exceptionally be as strong as 1 in 30. Contrary to general advice, within reason the stronger the mixture the more effective glyphosate will be. 
On the other hand it horrifies and amazes me how much glyphosate some so called gardeners get through.

I demonstrate selective spraying
17. You will need to read some of my other posts to learn all the tricks and wheezes to achieve selectivity by means of direction

When annual limnanthes dies down in July you can spray weeds before self seeded seed germinates in August
18. As to selectivity by timing don’t forget spraying weeds in otherwise bare soil in winter, spraying bulbs when they have died down and clearing weeds between one crop and another.

19. Glyphosate is particularly useful when the soil is too wet to dig or hoe as it might be in winter.

20. If horror of horror you carelessly spray a precious plant you might save it by immediately drenching the spray away, even lifting and washing a very small plant! In desperation if far from a tap I sometimes dust with dry soil to absorb the spray. For plants with a strong root system that readily regrows just cut off the leaves. 
In defence of glyphosate you will damage or kill far more plants with routine digging and hoeing.

In this case too much of the plant becomes the weed
No links today. You can just browse my previous articles by going to glyphosate in the links column or insert topics or named weeds in the search box


  1. I am rubbish at maths so just to clarify 1 in 50, does that mean 20 ml in a litre. I thought in one of your previous blogs I'd read 10-15 ml per litre which is what I've been using and it seems to work.

    1. Yes Jo your maths is correct! -so for example 15ml per litre is 1 in 75
      I have not been entirely consistent in my advice and drifted to the slightly stronger dose - perhaps for not very good reasons or perhaps I find more difficult weeds in client's garden
      Carry on with the weaker rate if that is working for you.


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