Straggly Olive Tree . To encourage strong growth, it’s a good idea to feed each spring with a general fertiliser. Prune once a year in the late spring or early summer. Big straggly olive tree — BBC Gardeners' World Magazine from forum.gardenersworld.com Also, you can prune it anytime from spring onwards. It takes, on average, 10kg of. The wild olive is a small, straggly tree or shrub to 8.