As Gertrude Stein might have said, a shrub is a shrub is a shrub-unless you've decided to transform a shrub into a small tree. Most of us consider that a tree is a tree by virtue of its central woody trunk; similarly, we regard a multi-stemmed woody plant as a shrub. Yet, we've all seen stands of multi-stemmed white birches as well as large, single-stemmed hibiscus. So where does a shrub leave off and a tree begin?
Virtually any shrub that doesn't sucker excessively from its base is a good choice for pruning into a tree (see “Ideal Candidates”); one that does sucker or flowers on new wood (roses, for example) is not suitable. A shrub trained into a tree is less aggressive than a dwarf tree (a term often used to just mean “slower growing”), and its dimensions are ideal for smaller spaces. As well, its augmented height is especially useful as a vertical accent in a perennial border of any size.
Training a shrub into a tree does require some patience. While I prefer to buy small, one-year-old specimens, anything up to a two-gallon container will do. A more mature plant is better trained into two or three trunks to form a multi-stemmed tree; just treat each stem you select as though you were training a single-stemmed specimen.
1 At the nursery, select a healthy, vigorous plant, preferably with an obvious upright central stem, which will become the leader, or main trunk, of your tree. Then plant the shrub as you would any other.
2 Next, remove all branches on the bottom third of the shrub. Leave the top two-thirds intact for the rest of the year: the extra foliage is essential for providing food and energy as the plant becomes established.
3 Insert a stake as close to the central leader as possible and attach using soft string, fabric or a commercial tree tie.
4 Repeat step No. 2 (limbing-up the shrub to one-third of its total height) each spring until the desired height is reached, usually between 75 and 150 centimetres. This process can take anywhere from three to five years if you begin with a one-year-old specimen.
5 Once the desired height of the trunk has been reached, it's time to develop the tree's canopy. The following spring, select three or five of the largest branches at the top of the plant. They should be evenly spaced to ensure an open framework, enabling light and air to penetrate. Clip these stems back by seven to 12 centimetres to encourage lateral growth. Remove all other branches in the canopy and along the trunk, and discard the support stake.
6 To maintain your tree-form shrub, prune off any branches that sprout along the trunk, and thin out the canopy as necessary.