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Small Trees Become Large Shrubs
When do Small Trees Become Large Shrubs?
By midsummer, small trees and large shrubs grow upward, out, and bend over from the weight of leaves and fruits. Unfortunately, heavy, drooping branches invade sidewalks, patios, block views, and brush up against buildings and cars. Trimming back these stray branches in summer won’t harm them, although pruning woody plants is normally done in winter.
Certain large species do this more than others, especially in the lush environment of a mulched and watered garden. These multistemmed species (and their average unpruned mature heights) include buttonbush (Cephalanthus occidentalis 10–15), elderberry (Sambucus canadensis 8–10), indigo bush (Amorpha fruticosa 8–10), ninebark (Physocarpus opulifolius 8–10), Hercules’ club (Aralia spinosa 20–25), bladdernut (Staphylia trifolia 8–10), hazelnut (Corylus americana 8–10), smooth sumac (Rhus glabra 15–20) and winged sumac (Rhus copallina 10–12), arrowwood viburnum (Viburnum dentatum 6–8), smooth alder (Alnus serrulata 10–15), rough-leaved dogwood (Cornus drummondii 15–20), and gray dogwood (Cornus racemosa 10–15). (If annually pruned, shrub heights will be about half these heights.)
These are the species that have the potential to grow into small multi-stemmed trees if you let them go. The more shade they get, the taller and lankier they grow. This can be ideal if you want to see through them and enjoy the trunks and bark, because the mass of foliage is above the eye in this situation. You can further enhance this see-through effect by trimming away the lower leaves and short side branchlets.
But if your long-term goal is to keep these types of plants lower and shrubby, or create an eye-level screen, these are great native plant choices. To take advantage of their ability to screen, prune them every year or two. This should happen after plants are established (2 to 3 years), and involves removing 1/4 to 1/3 of the stems (the oldest) to the ground, every winter or two. This is a process called rejuvenation. It forces multi-stemmed woody plants to continuously gen-
SCOTT WOODBURY Horticulturist
Horticulturist Scott Woodbury is the Curator of the Whitmore Wildflower Garden at Shaw Nature Reserve in Gray Summit, Missouri, where he has worked with native plant propagation, design, and education for 30 years. He also is an advisor to the Missouri Prairie Foundation’s Grow Native! program. Find suppliers of native plants, seeds, and native plant services at www.grownative.org, Resource Guide.
Buttonbush shrubs
Photos by Scott Woodbury.
Buttonbush tree Hazelnut
erate new stems and densely grow leaves and branchlets in the lower parts of the stems, blocking all views.
The decision to encourage small trees vs lower-growing shrubs is your choice. Nowhere is it written that gray dogwood in a garden is a shrub or a tree, because it can be either. There is no right or wrong way to go. Plants need to work for your situation. That said, keep in mind that maintaining multi-stemmed shrubs takes some effort, and it generates debris. Also know that plants flower, fruit, and grow more vigorously when they are located in more sun. Maintaining trees and shrubs in the shade is less intensive, because they produce fewer stems and branches.
The nature of multi-stemmed shrubs is that they grow wider over time, some more quickly than others. For example, the distance between hazelnut, ninebark, buttonbush, bladdernut, alder, dogwood, and arrowwood stems is 4 to 6 inches (or less). The stems are congested and difficult to prune, and therefore they are considered densely clumping shrubs, though their diameter continually increases over time. Because of this, they make excellent screens.
By contrast, when pruned into shrubs, Hercules club has branches 2 to 4 feet apart (or more). Indigo bush (pictured page 18) stems are about 1 foot apart and somewhat open and spreading in nature in part shade. Because of this, their screening ability is diminished. They are more see-through in nature and may require a groundcover companion plant like golden groundsel (Packera aurea) to prevent weeds. Elderberry and sumac also have stems 1 to 2 feet apart, and they tend to grow densely in full sun and more open in part shade.
Multi-stemmed natives woody plants have many applications in the garden. Getting to know each plant is key to finding success with small specimen trees or low shrubby screens. Resources that will help you learn about these plants can be found in the book Tried and True Missouri Native Plants for Your Yard. Also check out the Grow Native! and Shaw Nature Reserve websites for gardening tips and plant information. And lastly, don’t be afraid to try a new native woody plant. You may find a new use for a native plant in your own backyard. Happy gardening!
