Tree shrub shapes

When you choose a tree or shrub for its natural shape...

Consider four things:

First:

Shape and height reveals width

If a source tells you shape (also called "habit") of a plant species or variety and its height, then you can figure the width. Round plants are as tall as wide, and that ratio holds for three foot mini shrubs like dwarf boxwood as well as for 8- or 10-foot viburnums.

  • Spreading or weeping: Width is at least twice the height. May be many times wider than tall.
  • Round: Width = height
  • Columnar or pyramidal: Width/Widest part is usually half the height
  • Narrowly columnar, narrow pyramid or "fastigiate": Width is usually just 1/4 to 1/3 the height
All plants from the smallest to largest have a natural shape or "habit". Viburnum sargentii is round: 8-10' tall and wide.

All plants from the smallest to largest have a natural shape or "habit". Viburnum sargentii is round: 8-10' tall and wide.

Evergreen boxwood 'Winter Gem' is round, too: The plants in this arc would be 3-4' tall and wide if given their head.

Evergreen boxwood 'Winter Gem' is round, too: The plants in this arc would be 3-4' tall and wide if given their head.

Just as round as the boxwood or Sargent viburnum but elevated by its trunks, a healthy kousa dogwood (Cornus kousa) can be expected to reach 20' x 20' or larger.

Just as round as the boxwood or Sargent viburnum but elevated by its trunks, a healthy kousa dogwood (Cornus kousa) can be expected to reach 20' x 20' or larger.

A single 40 year old spreading dwarf Chinese juniper, never pruned. It remains true to its spreading habit,  5' tall and 40' wide.

A single 40 year old spreading dwarf Chinese juniper, never pruned. It remains true to its spreading habit, 5' tall and 40' wide.

In catalogs and plant encyclopedias you'll also see descriptions such as "upright spreading" which means there is a trunk or narrow base beneath the spreading top.

"Upright vase" is another commonly used term for a plant's habit. It's an inverted pyramid, and as for pyramidal plants, figure the width at the top of an upright vase as about 1/2 its height.

The boxwood called 'Vardar Valley' is not round like so many boxwood varieties. It's about the same height as the round boxwood 'Winter Gem' but its spreading habit makes it much wider.

The boxwood called 'Vardar Valley' is not round like so many boxwood varieties. It's about the same height as the round boxwood 'Winter Gem' but its spreading habit makes it much wider.

The second thing to consider about plant shape:

Do not assume related plants are the same in shape (or in size).

There are pyramidal spruces, globe spruces, weeping spruces and dwarf spruces. So it goes almost across the board among related plants.

As an example, you already saw a round viburnum, V. sargentii. Here is an upright character from the same clan, the Japanese arrowwood (V. erosum) -- taller than wide.

Notes for plant nuts: You're right if your antennae are tingling. Japanese arrowwood (Vibrurnum erosum) is not common in the trade, so it may not be among the couple dozen Viburnums you've already collected. It's an evergreen, hardy to zone 6. Its average performance on this continent may still be to be seen, too. It's more columnar that many viburnums, certainly, but it's said to grow only about 4-5' tall yet this one at Arnold Arboretum in Massachusetts is easily 8'. Like many Viburnums, it can't set such a beautiful crop of berries without another of its species nearby, a different clone, close enough so bees can cross-pollinate them.

Notes for plant nuts: You're right if your antennae are tingling. Japanese arrowwood (Vibrurnum erosum) is not common in the trade, so it may not be among the couple dozen Viburnums you've already collected. It's an evergreen, hardy to zone 6. Its average performance on this continent may still be to be seen, too. It's more columnar that many viburnums, certainly, but it's said to grow only about 4-5' tall yet this one at Arnold Arboretum in Massachusetts is easily 8'. Like many Viburnums, it can't set such a beautiful crop of berries without another of its species nearby, a different clone, close enough so bees can cross-pollinate them.

The third thing to consider about plant shape:

Know what shape you're selecting for and design options increase. On a wet site, for instance, the moisture loving standard Fothergilla can be a size- and shape matched stand-in for this similarly shaped but moisture sensitive Sargent viburnum (V. sargentii).

Standard Fothergilla, F. major. Same shape at twice the size of its more common cousin, F. gardenii but equally happy in damp sites.

Standard Fothergilla, F. major. Same shape at twice the size of its more common cousin, F. gardenii but equally happy in damp sites.

The fourth thing to consider about plant shape:

The easiest pruning you can do is to simply enhance a plant's natural shape. If it wants to be round, let it be round. Let pyramidal plants have points, and so forth. Even if you are keeping a plant smaller than it would be on its own, it will require less cutting, less often if you accept it for its genetically driven habit.

Barberry, for instance, is very round. These dwarf barberries are not pruned except perhaps to cut them to the ground every 4 or 5 years to keep the wood young and leaves most colorful. Every time we see a squared-off barberry hedge we think, "Ye gods, that gardener's working too hard - so much more cutting and on such a prickly plant!"

Barberry, for instance, is very round. These dwarf barberries are not pruned except perhaps to cut them to the ground every 4 or 5 years to keep the wood young and leaves most colorful. Every time we see a squared-off barberry hedge we think, "Ye gods, that gardener's working too hard - so much more cutting and on such a prickly plant!"

Naturally round 'Winter Gem' boxwood before...

Naturally round 'Winter Gem' boxwood before...

...and after its every-two-year cut. We can get away with pruning it at such long interval because we prune to keep it small yet retain its natural shape.

...and after its every-two-year cut. We can get away with pruning it at such long interval because we prune to keep it small yet retain its natural shape.

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