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Thuja occidentalis
What plant to use to screen a view that grows fast, tall and very narrow is one of the most commonly asked questions. If the space is VERY narrow, the best solution is to put up a trellis and cover it with a well behaved evergreen vine like Trachelospermum jasminoides, Solanum jasminoides or Hardenbergia violacea. Another option is to espalier plants that have a habit of growing flat to begin with.Grewia, Xylosma, Podocarpus gracilior, and Cocculus being good choices.The other possibility is bamboo, but be SURE that you plant a clumping bamboo, rather than a running species, or you and your neighbor will live to regret it. Good clumping bamboos for screening are varieties of Bambusa multiplex. They have foliage nearly to the ground, are very dense clumps, and are from 6' to 15' tall. They can even be sheared.

If there is a bit more space, a fastigiate shrub or small tree may be the answer. These are plants that have mutated from the normal growth habit, and grow much narrower than usual. Italian Cypress is the most commonly seen fastigiate plant, but there are many others that are potentially useful. Possibilities are Juniperus scopulorum 'Skyrocket', Deodara 'Gold Cone,' Juniperus communis 'Gold Cone', Thuja occidentalis 'Emerald', Buxus sempervirens 'Graham Blandy', Ilex crenata 'Sky Pencil', Ilex vomitoria 'Will Fleming', and Euonymus japonicus 'Chollipo' and 'Beni Komachi'.

There ARE a few plants that naturally grow tall and narrow, Nandina, Mahonia japonica and Podocarpus macrophyllus are the easiest to find.

If you are trying to extend the height of a fence, you could plant a large shrub or small tree alongside the fence, then prune up the lower branches to provide room to walk (and even plant) underneath. Photinias, Pittosporums , Oleanders, Arbutus unedo and Prunus caroliniana 'Brite n tite' and 'compacta' are good candidates for this treatment.

The last (and most labor intensive) option is to plant a row of evergreen shrubs and keep it sheared to the width desired. Keep in mind frequency of shearing will depend on the growth rate of the plant. I have an evergreen privet hedge (one of the fastest growers) that needs to be sheared twice a year to keep it under control, and could use a third if I was a perfectionist and less lazy. I highly recommend purchasing and using electric shears to make the job more bearable. However, if you go this route, DO NOT plant a broadleaf plant like English laurel (electric shears result in mangled and shredded leaves that look awful). Highly recommended species for a sheared hedge are Pittosporum tenuifolium, Myrtus communis and Eugenia.

You also need to think of what is on either side of the planting area. If it's an asphalt driveway or your neighbor's foundation, you don't want the heartbreak of having to remove your hedge (tree) 10-15 years down the road when it's looking perfect but starting to cause underground structural damage.