What I've done that works mostly. Cut them down and dig up the roots you can access and find. (Depending on climate) Water and fertilize. When the new shoots are about 6 inches tall, paint them generously with Roundup (not diluted, full strength). This should kill most of the main roots. The next set of sprouting should be much smaller and weaker stems, these can just be spritzed with regular diluted Roundup (paint any big juicy stems, of course).
If this is in close proximity to plants you want to keep, keep it off any leaves and away from stems and major roots (although it is absorbed poorly by (not leaves), full concentration should not be put in contact with non-target plants. Roundup does degrade quickly in the soil.
I did this to a friends yard that was 8 feet deep in blackberries, and it was down to little 3 inch sprigs in a year.
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What I've done that works mostly. Cut them down and dig up the roots you can access and find. (Depending on climate) Water and fertilize. When the new shoots are about 6 inches tall, paint them generously with Roundup (not diluted, full strength). This should kill most of the main roots. The next set of sprouting should be much smaller and weaker stems, these can just be spritzed with regular diluted Roundup (paint any big juicy stems, of course).
If this is in close proximity to plants you want to keep, keep it off any leaves and away from stems and major roots (although it is absorbed poorly by (not leaves), full concentration should not be put in contact with non-target plants. Roundup does degrade quickly in the soil.
I did this to a friends yard that was 8 feet deep in blackberries, and it was down to little 3 inch sprigs in a year.