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Gardening: Gardens under trees

People are interested in planting under existing trees for a variety of reasons. Have you struggled to keep turf grass alive under a tree canopy? Do you have a small yard and the area beneath the tree is the only place to have a flowerbed? Have you admired blooms planted by others in your neighborhood and would like to follow suit?

“Underplanting” is term used when gardening in the space between the lower branches of trees and the ground. Consider the process of plants forming tiers beneath trees is completely natural in a woodland setting. Usually in shades of brown and greens, the natural understory adds an intimate feel. But many of us have struggled to figure out what to plant under mature trees and shrubs in the landscape.

What types of shade-tolerant plants make a good understory and how to use them in combination? A good understory can include everything from moss and groundcovers to perennials, shrubs, even other trees. Woodland plants may not be as colorful and showy as those grown in sun-loving gardens, but they do offer form, foliage texture and color. There are challenging conditions like shade, dry soil, and tree roots that make underplanting more difficult, especially trying to make the understory look natural.

Some gardeners plant in drifts while others prefer a more haphazard look. If you like to see plants intermingle you could use plants that re-seed and allow them to spread. This would blur the lines and your planting would look more like nature. You could take care to use both large groups of plants (drifts) while allowing some intermingling and have a great underplanting. Whatever approach you take, try not to do the tree circle as nature does not plant a perfect circle around the base of a tree. Spread plants out in a gentle curving shape as you would do with other garden beds.

Use a trowel to loosen the soil rather than a shovel so you avoid cutting into roots that are more than 2 inches in diameter as these are the primary feeder roots. If you run into a root, just move over a little and try digging again. Note: Never till under a tree. If the soil seems poor where you are working, try adding a little compost to each planting hole, but don’t raise the soil level over the tree’s roots. The majority of roots that trees depend on for water, nutrients, and air are in the top few inches of soil. Burying these roots makes it difficult for trees to get what they need to survive.

It is best to work with bare-root plants, seedlings and small divisions of plants when gardening under trees as you will create smaller digging holes. The plants’ roots will find their way around the tree roots.

Keep in mind that the dry shade found under mature deciduous trees or evergreens does make tough growing conditions for understory plants. But many plants can do well once they are established if the shade is not too deep. You may discover that your woodland plants do better in a dappled shade with bright morning light that you can achieve by removing lower tree limbs.

A few groundcovers that thrive in dry shade are Ajuga (bugleweed), Lamium (dead nettle), Pachysandra (spurge), Asarum (wild ginger), and Galium (sweet woodruff).

Perennial choices include geranium, Brunnera (Siberian bugloss), bergenia, goatsbeard, Digitalis (foxglove), Solomon’s seal, Alchemilla (lady’s mantel), and ferns. Hosta plants and can add lush green, gold, and white foliage as well as varied heights. All hosta can tolerate some shade, but they do need dappled or filtered light. Hosta may also need additional water to thrive.

If you are planting under deciduous trees, spring ephemerals would add to your underplanting (they naturally fade before the tree would leaf-out in spring). Snowdrops, trout lilies, trillium, Virginia bluebells, anemone, and May Apple are a few plants in this category. Bulb choices for dry shade would be daffodils and squills.

If you have a tall tree canopy, consider understory shrubs to create a tiered effect. Pogada dogwood, red-twig dogwood, Tiger Eyes sumac, Viburnum, and hydrangea would fit this category. Try not to plant large container sized shrubs under mature trees because digging a large planting hole would be difficult and you could do serious damage to the tree roots. The recommendation is to plant nude sticks – but you will be surprised by how fast they grow.

Because you are working with small, very young plants in your underplanting, you may have to wait for several years to see the results you hoped for. Be patient!

Notes:

You can actually plant right up to the trunk of a tree.

Resist the urge to plant too close together, you can always add more plants in the future. After a few years you can divide some of the plants you like best and add to your understory.

As you plan your underplanting design, try to keep texture in mind. Example: light, feathery ferns to contrast with hosta. There are even shade tolerant sedges and grasses for contrast.

Your plants are going to need water until they are established (more than the 1 inch of water per week recommended for lawns). Once established, the 1 inch rule should be OK unless your tree is large and dense and the rain cannot filter through.

Black Walnut trees (and the related Butternut tree) can be problematic in coexisting with other plants. The roots of these trees produce a toxin known as juglone, which can stunt, deform or even kill other plants. Underplanting would be difficult, not impossible, but care in choosing compatable plants would be necessary.

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