The most desired shrub is the mythical one that grows rapidly to the desired height and then stops. Of course it doesn't exist. Shrubs grow more or less continuously throughout their lives, some quickly and some slowly. An extra 6"/year on a 20' bush may not be noticeable but it does add up. In some conditions, winter dieback or a short growing season can effectively stop shrubs at a smaller than normal size, but the only reliable way to keep shrub at a specific size is through pruning. Once a shrub stops growing, it is halfway to dead, susceptible to disease and won't produce a strong display of flowers or fruit.
How Big do they Grow?
Shrubs come in a wide array of shapes, from pencil thin spires to flat ground huggers. Some are very amenable to training or pruning into different shapes. Shrubs grow as large as conditions allow and for as long as they are healthy. It has become a common practice for nurseries to label shrubs with the expected size after 5 years. At best this is an indication of fast and slow growers, at worst it is extremely deceptive. I have seen labels stating that a gum tree will be 6' tall after 5 years (small print says with annual pruning). In practice such a plant is likely to be 40' tall in 10 years. A holly 6' tall after 5 years could grow into a 50' tree. Of course there are shrubs that will never get more than two or three feet tall but they are usually slow growers or wide-spreaders.
Try to find out the mature size of a typical specimen and how long it takes to get there. Pay attention to the spread as well as the height. A 6' pencil could be lost against your house, but a 6'x10' shrub is an imposing beast. Research the growth habit and recommended pruning. Some shrubs can easily be sheared to size, others will resent pruning.
How Fast do they Grow?
How fast do you want? There are vigorous shrubs that will grow 2', 3', or more per year. Some scramblers and climbers will grow much more. Others only grow a few inches. The important thing to remember is that if it grows fast it is going to be big. The magic shrub that grows 3' in each of the first two years and then stops does not exist - it will grow another 3' in each year after that.
Shrubs at the cold limits of their range grow more slowly and might even shrink in a cold year. Shrubs in general grow more slowly in colder climates. Very hardy shrubs grow more slowly even in warm climates. Shrubs grow more quickly with the correct moisture levels and good fertile soil.
Image by: Ian Nartowicz
Entered by iann
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