Victoria Summerley: City Life

'I've acquired a garden with a mammoth occupant, planted by a previous owner when it was a cute, teensy little thing'

With urban gardens, as with so much else in life, size matters. And when every inch counts, you don't want to waste it on something that's outgrown its welcome. Unfortunately for us city dwellers, estimating the ultimate size of a plant is a very inexact science. The eventual height depends on so many factors – situation, soil, level of care. And even while you might be able to visualise what a height of 1.5m might be, how many of us remember how wide the spread is going to be? How many of us actually look at the eventual size on the label in the first place?

So many times, I've acquired a garden with a mammoth occupant, bought by some long-gone householder when it was a teensy little thing they thought would look cute in the corner of their 10ft plot. By the time the property has fallen into my hands, what was once a ditsy little bay tree (and it's so often a bay tree), intended perhaps to form the centrepiece of a dinky little herb bed, is a neglected 20ft whopper that has long ago shaded out the lavender and rosemary growing around it.

Other miscreants in town gardens tend to be Viburnum tinus; Forsythia suspensa (the one that flops everywhere); Eucalyptus gunnii, or cider gum; Aucuba japonica (spotted laurel) and weeping willows. Then there's the infamous Leylandii – although at least the people buying these tend to know they're buying a big plant. It's just the neighbours who get the nasty surprise.

All of these are perfectly good plants in their own right, especially Viburnum tinus, which is one of the few things that flowers in mid-December. But so often, it's clear that whoever planted them had absolutely no idea how big they'd get. And unfortunately, no one really warned them.

Let's face it, how many times have you seen a baby bay tree labelled: "If unchecked, this will eat your garden". How many times have you seen Eucalyptus gunnii labelled: "Warning: 100ft monster in the making". (The BBC gardening website says it reaches "1,000cm". Judging by the specimen in a neighbouring garden, I think someone's missed off a zero.)

Conifers are another minefield. The growers are very fond of giving you a "height after 10 years" figure, which neglects to mention that the height in 20 years' time will probably be double that. If you're buying any sort of conifer, try to find the annual growth rate. Anything that grows 3cm a year is a safe bet for a small urban garden or even a container, whereas anything that boasts 40cm per annum is best avoided unless you're planning to leave an arboretum to the nation or you're handy with a chainsaw.

Crocus.co.uk, the online nursery, has a useful feature which shows you not only the ultimate height of a shrub or tree in metres or centimetres, but also a scale comparison with a human figure (for some reason this figure looks a bit like a Charlie's Angel). Viburnum tinus looms three or four feet above the figure, while on the scale drawing for E gunnii, the Charlie's Angel is a mere dot beneath a majestic giant.

Unfortunately, it's only a two-dimensional guide, so if you're not very good at imagining the spread, go out in the garden with a tape measure and see how much of your lawn or sunbathing area will be swallowed up by something with a girth of 6ft.

What can you do if you've inherited a monster? You can hire a tree surgeon to get rid of it altogether (conservation area regulations and local authority arboricultural officers permitting). Look up the Arboricultural Association website to find a reputable contractor in your area, or the local council may have a list of recommended operators.

You can also prune it back. Better still, you can prune it into a neater shape, or, indeed, into a shape, something vaguely rectangular or globular – we're not talking teddy bears or peacocks – which works well with plants such as privet, bay or even laurel.

You can also try raising the crown. I always think this is a slightly misleading term, because it sounds as if you're making the tree taller. Basically, it means removing all the lower branches from a big blobby shrub to turn it into something shaped more like a conventional tree. I've done this with both Viburnum tinus and bay, and it's not particularly difficult. It just requires courage and a pair of sharp loppers.

Have a good rummage around in the plant to see what sort of framework of branches is underneath all the foliage. There's no point trying to turn an overgrown bay back into a big lollipop if it's become multi-stemmed, and anything too twiggy isn't going to work either.

If you're wondering where to make the first cut, look for branches that face inward or are crossing or rubbing against other branches, as they should really come out anyway. Then take a deep breath, start at the bottom and carefully prune off the branches until you get the effect you want.

It may astonish you to see how much space, and light, is salvaged. And then you can go and buy more plants to plant underneath. But please check the labels first.

v.summerley@independent.co.uk