To overhaul a border, I would lift any plants you want to keep and pot them up. Then you can fork out all the pernicious weeds such as couch grass and bindweed. When all the perennial weed roots have been cleared, I would then add fertiliser and compost, and then replant the border. Then use mulch over all the spaces.
Its a fair size rough guess 3 meters wide and around 11 meters long and has a mix of shrubs, trees and some dog roses in there! It runs along a fence! Im looking forward to the challange as im aiming to make it into a wildlife friendly border! We live near a wood and a brook, so have a lot of frogs, toads, newts, slowworms amongst others!
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