Urban Gardener: Arbor Day honors trees with plant giveaways, events

The first Saturday in November is Arbor Day in Hawaii, and this Friday and Saturday, communities across the islands will gather to plant trees and learn and celebrate their many benefits. Trees require care and thoughtful consideration. If planted improperly or out of place, a tree may cost more than it’s worth. With proper management, trees will provide a return on investment.

When deciding where to plant a tree, make sure to give enough space to infrastructure such as buildings, roofs, power lines, underground utilities, pavement and walls. Master gardeners often get questions about which trees have the most aggressive roots, as homeowners are often concerned about causing damage to their walls, sidewalks, and foundations. Not just trees, but many plants such as shrubs, clumping palms, bamboo or bananas can be strong enough to damage infrastructure. However, problems generally arise from giving the plant inadequate space. With proper site and species selection, property owners should not be afraid to plant a tree for shade, beauty, fruit or other benefits.

Tree roots need space to spread and grow. The larger the tree, the more access to surface area and soil volume the roots need. A common misconception is that tree roots are a mirror image of the branches, or that they end at the “dripline,” or edge of the tree’s canopy. Tree roots are opportunistic, and can travel very far in search of water and nutrients. The majority of roots will be in the top several inches of soil to collect water and nutrients that come from the surface. However, these feeder roots typically don’t cause concern.

A tree’s structural support system includes the main structural roots and root collar. The root collar is the area where roots join the main stem or trunk, with the trunk flaring out to the major roots. These tend to cause the most damage when planted out of place. Visualize the size of a mature tree and how much space this root flare will need. When planting palms or bananas, consider how large the clump or mat will be when the plants have grown to the size you want. Typically, if you give a tree enough space for its canopy, that will be enough space for its root collar and major roots. If you want to plant a tree with its canopy spreading beyond a wall or pavement, give the tree at least 5 feet from the object for small trees, or 10 feet for larger trees. For very large trees such as monkeypod or banyan, at least 20 feet is ideal.

Learn more about trees and how to care for them this Arbor Day. There will be plant sales and free plant giveaways at locations across the islands, with local experts on hand to answer your questions and share advice and information.

Visit arbordayhawaii.org to find an event near you.

Kalani Matsumura is a junior extension agent with the University of Hawaii’s College of Tropical Agriculture and Human Resources and coordinates the UH Master Gardener Program on Oahu.