Plants Under Trees: Protect the Root Flare

Under-tree planting is tricky; installing new plants underneath an existing shade tree can not only damage the tree's fine absorbing roots, but there’s competition for water, oxygen, and nutrients, and a high risk of burying the trunk’s root flare if topsoil is added for the newly installed plants. If you want to plant under a tree, building a living understory that improves soil health, and never smothers the bark or roots, is your best bet.

Read on to learn about the importance of planting in the right spot, spacing rules, regional plant ideas, and root-safe planting methods.

Root Flare 101: The Line You Never Cross

Tree root flare refers to the area of the trunk’s base that widens and meets roots below the soil. Root flares help with structural stability. If visible, it indicates that the tree was planted at the correct depth, facilitates gas exchange, and helps channel nutrients and water into the tree's transport system. Burying it with soil, mulch, or dense ground cover or turf can trap moisture, invite decay, and encourage root girdling.

The quickest way to determine if you’re smothering your tree’s root flare is to check if you can see the buttress flare all the way around your tree. A certified arborist can inspect your trees to determine if your tree root flare is too deep underground and perform a root collar excavation if necessary.

“Breathing Companions”: The Selection Criteria

While under-tree plantings can risk damage to your tree’s root flare and fine absorbing roots, certain plant species won’t cause severe damage to a tree's root systems. Make sure to pick plants with these characteristics:

• Plants that are in clump form instead of spreading and forming a mat allow for better airflow and maintenance access.
• Shallow, non-woody root systems weave around feeder roots without strangling them.
• Airy foliage architecture, characterized by open canopies, allows leaf litter and water to reach the soil.
• Modestly high plants, less than 12-18 inches, allow for better airflow and access for inspection.
• Non-climbing, non-suckering, non-running plants. Avoid ivy, aggressive stolons, or rhizomes. These types of plants tend to crowd the root collar/flare area of a tree.

When planting under a tree, it’s best to avoid landscape fabrics and dense carpets, like English ivy, that retain moisture and turf. If planting turf, ensure that there is a two-to-three-foot buffer away from the trunk. When mulching, make sure that excessive mulch is not applied. Typically, 2 to 4 inches is ideal, and never volcano mulch, a process that involves mounding excessive amounts of mulch over and up against the trunk and root flare areas.

What Can I Plant Around the Base of A Tree?

The best plants to grow under trees are those that avoid disturbing the tree’s roots and root flare and can withstand a shady environment. Here are some low-maintenance examples of compatible under-tree plantings:

• Rhododendrons and Azaleas (Rhododendron spp.) – These evergreen or deciduous shrubs have shallow, fibrous root systems that don’t compete aggressively with tree roots. They thrive in partial to full shade and acidic soils and are often planted under oaks or pines.
• Hydrangeas (Hydrangea spp., especially H. macrophylla or H. serrata) – Hydrangeas have relatively shallow roots and excel in shaded areas, making them ideal under trees with dense canopies. They add lush foliage and blooms without aggressive root growth.
• Podocarpus (macrophyllus ‘Maki’) – This shrub has non-invasive roots, primarily for dwarf varieties such as ‘Pringles Dwarf.’ This variety is evergreen and compact, with shallow root systems that don’t invade tree root zones. They thrive in partial to full shade and are adaptable to a wide range of soil types. They can be pruned into formal shapes or left in their natural state.
• Yew (Taxus spp.) – Yews are shade-loving evergreens with shallow, non-aggressive roots, making them excellent companions for trees. They’re also drought-tolerant once established. Varieties like 'Densiformis' or 'Hicksii' are well-suited for shaded understories. They prefer well-drained soil but tolerate a range of conditions.
• Mahonia (Mahonia spp., e.g., Oregon Grape Holly) – This evergreen shrub has shallow roots and thrives in shade, making it ideal for under-tree planting. Its holly-like leaves and yellow flowers add texture and color. Mahonia aquifolium, or Soft Caress, is a compact option. They tolerate dry, shaded areas.
• Camellia (Camellia spp.) – Camellias are shade-tolerant evergreens with shallow roots, perfect for planting under trees like maples or dogwoods. Their winter or spring blooms add seasonal interest. Select compact varieties, such as 'Winter’s Star' or 'Sasanqua' types, for smaller spaces. They prefer acidic, well-drained soil.

Common Pitfalls (and How to Fix Them)

Now that we’ve discussed what to plant under trees, it’s also important to know what to avoid. Here are some common landscape choices that can be detrimental to your tree’s root flare and what to do with them.

• Landscape Fabric and Rock Beds – These starve and overheat trees. Remove the fabric and replace it with organic mulch and clumping plant companions.
• Volcano Mulching – Avoid volcano mulching, as it can cause tree root flare to suffocate and become more susceptible to disease and insect infestations. When mulching, make sure you pull mulch back to expose your tree’s root flare.
• Ground Covers – Plants that create dense mats which can creep into a tree’s root flare area and reduce air circulation to root flares. 
• Level Beds – Adding soil to create a level bed will suffocate a tree’s roots and root flare. Instead, lightly terrace plants away from the flare if needed. 

When to Call a Davey Arborist

Davey’s certified arborists have the knowledge and expertise to help make sure your trees and their companion plantings thrive. Call your local Davey arborist if you can’t see the root flare, your tree is showing signs of stress like early color and dieback, your tree’s major roots are exposed, or you need help designing a region-appropriate, root-safe understory.

 LEARN MORE ABOUT PLANTING TREES AND SHRUBS TOGETHER