Right start afresh - Dig up the plants you want to keep & put them in containers & prune back & water well - dig & fork your plot over a spit deep {spade} get some top soil & landscape the area into a crown in the middle - remove any perennial weds as you go on - get a roll heavy duty membrane off e bay & metal pegs do a double cover the water seeps through it so don't worry when clear order the rounds pebbles from a builders merchants & cover no more than 1/2 an inch -
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Plant wise you need to aim to have the taller plants in the middle evergreens if possible & cascade down look at the height & width of the plants youe plant - aubretia & Arabis at the edge - Scabious, Lupins, Hydrangea the small hybrid types - Carnations choose double flowering ones - Hosta's etc - think before you plant - if you purchase them buy plug plants & grow them on yourself = cheaper for you - this will be maintenance free use bone meal when planting with a good compost -
Water every plant in well & if dry repeat until established - to plant move the gravel back cut a X with scissors & dig your hole with a trowel & place the Ground cover back & gravel around each plant it the best way -
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My garden is now full hardly any gravel to be seen - Yes we make changes - if any plants don't perform etc - aim for colour all year around too - read up on what you like & what you buy - ornamental grass - etc etc - you choose what you want etc plan it pout before planting etc