That’s what the article says, cut down to 8” then twist a screw driver down the hole.
The problem with horsetail (sometimes ref'd as marestail) is there is not the diameter in the stem to do this. Bruise it and paint with Round-up stump killer in a solution of washing-up liquid which holds it in contact with the plant for longer. I had some limited success with this.
Me too but in a bigger patch or when it's infesting a bed of other plants I found hoeing or pulling a lot less bovver and equally effective and quicker.
Vendée - 20kms from Atlantic coast.
"The price good men (and women) pay for indifference to public affairs is to be ruled by evil men (and women)."
After reading your comments perhaps cutting the top two thirds off isn't going to be at all effective, which is what we have been doing so far. Perhaps pulling the whole darn plant out by hand would be a more effective option. Thank you @Dave Humby my OH insists the weed is called Horsetail but I have always known it to be called Marestail - it's still a nuisance whatever it is called.
Thank you for the helpful link @Dovefromabove, I have just been out to pull a few more of the monsters up and bagged too. I hope we can keep them under control so they don't have a chance to produce spores next Spring.
Roundup is pretty awesome on it but you have to keep it up for years to starve it out. Alternatively I know someone who grassed their whole garden, kept it mown short for years and got rid of it that way. Any that came up on paths, drives etc got sprayed or had a fresh kettle poured over it. There is no easy quick fix.
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https://www.theguardian.com/environment/2019/apr/03/country-diary-field-horsetail-shoots-rise-and-prepare-to-swish
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.