Planting under an established tree transforms a bare, shady patch into a textured, visually appealing garden feature. The environment beneath a canopy presents unique challenges, including dense shade, intense competition for water and nutrients from the tree’s expansive root system, and often dry soil conditions. Successful underplanting requires prioritizing the tree’s health while selecting plants adapted to this competitive, low-light environment. Preparing the area without causing root damage is the foundation for establishing a thriving understory garden.
Safely Preparing the Planting Area
The primary goal when preparing the planting area is to minimize disturbance to the tree’s roots, which are essential for stability and nutrient absorption. Use a hand trowel or a garden fork instead of a traditional shovel to gently clear the area of weeds and debris. If you encounter a root two inches or more in diameter, stop digging, refill the hole, and move the planting site slightly to avoid severing a major root structure.
Planting holes should be kept small, only slightly larger than the root ball of the new plant. Using small transplants or spring seedlings is recommended. Never attempt to create a raised bed around the trunk by adding a large volume of soil or compost, as this can suffocate the tree’s roots and prevent gas exchange. The tree’s root flare, where the trunk widens at the base, must remain exposed to prevent bark rot and girdling roots. When amending the soil, spread no more than one or two inches of compost or leaf mold over the planting area to improve drainage and fertility.
Recommended Plant Groups for Tree Bases
Choosing plants with shallow, non-woody root systems is the most effective strategy for ensuring they can coexist with the tree’s established roots. These shallow-rooted plants are less likely to compete aggressively with the tree’s fine feeder roots for moisture and nutrients. Dry shade perennials, which are naturally adapted to low light and low moisture, are a strong choice for this harsh environment.
Shade-Tolerant Groundcovers
Shade-tolerant groundcovers offer a practical solution for covering the soil without requiring deep planting. Examples include Epimedium (Barrenwort), which is drought-tolerant and spreads slowly, and Vinca minor (Periwinkle), known for forming a dense, evergreen mat. Check the regional invasiveness of vigorous spreaders like Vinca before planting, as they can become problematic in some areas.
Shallow-Rooted Perennials
Shallow-rooted perennials provide texture and color without invading the tree’s root zone. Many varieties thrive in dry shade conditions.
- Hosta varieties
- Helleborus orientalis (Lenten Rose)
- Various ferns, such as the Christmas fern (Polystichum acrostichoides)
- Heuchera (Coral Bells)
- Pulmonaria (Lungwort)
These options offer attractive foliage and are successful competitors in this setting.
Spring-Flowering Bulbs
Spring-flowering bulbs are well-suited because they complete their life cycle before the tree canopy fully leafs out, maximizing sun exposure early in the year. Species like Daffodils and Snowdrops can tolerate the root competition and dry summer conditions that follow their spring bloom. Planting these bulbs between the larger tree roots allows them to establish and return year after year.
Managing Difficult Tree Species and Ongoing Maintenance
Some tree species present unique difficulties that go beyond simple competition for resources. The Black Walnut (Juglans nigra) is notorious for allelopathy, where its roots and fallen debris release the compound juglone, which is toxic to many common plants, including tomatoes, peppers, and azaleas. Planting under a Black Walnut should be limited to species known to be tolerant of juglone, or containers should be used instead.
Extremely shallow-rooted and aggressive competitors, such as certain Maples and Birches, can create an impenetrable mat of roots near the surface, making in-ground planting nearly impossible. In these cases, the best approach is often to plant in containers placed on a layer of mulch, or to simply accept the area as a mulched zone.
The long-term success of any underplanting relies on proper ongoing care, particularly concerning water and nutrients. New plants require consistent deep watering for their first year or two to establish a root system capable of competing with the tree. Instead of light, frequent sprinkling, a thorough soaking once a week encourages the new plants to develop deeper, more resilient roots. Applying a light, slow-release organic fertilizer annually can help replenish nutrients depleted by the tree. Maintain a thin layer of organic mulch, two to three inches deep, across the planting area, ensuring it is pulled back several inches from the tree trunk to keep the root flare exposed.