Can you recommend border plants that will grow upwards rather than outwards, and will flower for as long as possible throughout the summer months?
Few plants will only grow upwards; most increase in spread as well as in height as they mature. Plants left to their own devices often outgrow the beds they were intended to fit and spread outwards to find light. The trick is to choose plants that thrive on being pruned, so they can be made to fit your space without damaging the plants. Also look for plants that are upright in habit and slower growing. You could try growing climbers trained up a vertical support. Clematis ‘Étoile Violette' (pictured), available from Taylors Clematis, produces deep purple flowers profusely from July to September. Train it up an obelisk or through existing trees and shrubs. Simply prune it back at the beginning of spring to a pair of buds about 20cm above ground level.