Under the canopy: planting beneath trees

Trees offer many benefits to gardens and gardeners. They provide shade, help moderate your microclimate, create organic material, and promote biodiversity.

However, large, mature trees often have spreading canopies that can leave some areas of the garden in deep shade. Their large root systems can also limit water and nutrient availability for nearby plants, leaving the gardener to wonder what they can plant there.

In this article, we’ll talk about planting under mature trees, with a focus on tips to help your plants grow.

Scoping the Site

Before choosing plants, take some time to understand the conditions beneath your tree. Light, the tree itself, and the soil all play a part in what will grow well there.

Light

Not all shade is the same, and knowing which type you have makes choosing plants much easier.

• Full or deep shade: Little to no direct sun, often under dense, low canopies or evergreens.
• Partial shade: A few hours of direct sun each day, usually in the morning or late afternoon.
• Dappled shade: Light filtering through the canopy, with patches of sun moving across the ground through the day.

To find out what you have, check the area a few times on a sunny day, and note roughly how many hours of direct sun it gets.

The Tree

The type of tree you have shapes the conditions beneath it. Consider how large the tree is, how dense its canopy is, and how it behaves throughout the year.

Deciduous trees drop their leaves in fall, which lets in more light through winter and early spring. This creates a window for spring-flowering bulbs, like snowdrops, crocus, and daffodils, which can grow and bloom before the canopy fills in.

Evergreen trees, by contrast, cast shade all year round. Conifers in particular can be challenging, as their dense canopies block much of the light, and fallen needles form a thick mat on the ground that can smother small plants.

Some trees are harder to plant under than others. Maples, for example, tend to have dense canopies and many shallow roots that compete heavily for water and nutrients. Trees with lighter, more open canopies, like honey locust and birch, create a more gentle, dappled shade that suits a wider range of plants.

The Soil

While most trees create dry shade beneath them, wet shade with boggier ground is not uncommon, especially in low-lying areas or under trees that naturally prefer moist soils. To check yours, dig down a few inches a couple of days after rain and feel the soil. If it’s dry and dusty, you have dry shade; if it’s still wet and sticky, you have wet shade.

A soil test is also helpful. It will tell you your soil’s pH and nutrient levels, which can be depleted under large trees as their roots take up much of what’s available. Knowing what’s there will help you choose the right plants and amendments, and avoid adding fertilizer you don’t need.

Plant Considerations

It’s generally best to choose plants that are a little more shade-tolerant than your site suggests. Canopies grow slowly, but shade will deepen over time, so planning ahead can save you a replant in a few years.

There is a wealth of plants suited to all types of shady conditions, from damp to hot and dry. Because there is such a variety of growing conditions, it’s difficult to recommend a single list of plants. In the Desert Southwest, shade-tolerant plants like Mexican honeysuckle, desert ruellia, and some agaves, such as Agave bracteosa, will do well. In fact, many desert cacti, including young saguaros, naturally start life in the shelter of “nurse trees” like palo verde and mesquite. In the Midwest or Northern Europe, hardy cyclamen might be a great choice.

Visiting a local plant nursery, or checking out a book from your local library on the plants native to your area, will give you a wonderful array of plants to choose from. You can also look through the Planter plant list and filter by light requirements to see what could work for you.

Putting In Plants

It can be challenging to plant underneath trees. Here are some important considerations:

• Use small plants (plugs or 4-inch pots) and tuck them into pockets between roots rather than digging large holes.
• Avoid cutting roots thicker than about an inch, since larger roots help anchor the tree and carry water and nutrients.
• Plant in fall, winter if that's an option for you, or early spring, when there’s more moisture and less competition from the tree.
• Mulch the whole area with an inch or two of compost or leaf mulch, keeping it away from the trunk, to conserve water and feed the soil. Leaving fallen leaves in place works well too.
• Water new plants regularly for the first 1–2 years as they establish their root systems.

If Conditions Aren’t Right

Sometimes it’s impractical to grow plants directly under a tree. If the problem is a dense root mat, overly dry soil, or a lack of nutrients, containers and elevated raised beds can be a practical solution. They let you control water, nutrients, and soil composition. Smaller containers can also be moved to a sheltered spot or indoors during winter, so you can grow plants that are more weather-sensitive.

It’s generally best to raise containers off the ground, and the same goes for raised beds. Elevated beds on legs or wheels work well, but avoid building beds at ground level and filling them with soil over the tree’s roots. Tree roots need air and water from the soil, and covering them with extra soil or heavy containers can reduce both and compact the ground.

Tree roots are also known to spread aggressively up into ground-level beds and break through wooden containers. Standing containers on bricks gives a few inches of clearance, allows good air circulation, and discourages tree roots from breaking in.

Wrapping Up

Shady spots under trees are not an immediate write-off. While growing conditions are more challenging, with a bit of investigative work, you should be able to find plenty of plants suited to these niche conditions. Shaded areas are a great opportunity to show off a host of plants that can’t be grown in many other places in the garden, and that will complement your trees beautifully.