Growing anything under mature trees is difficult at best, but you can establish a garden under a tree

Plants of any sort under a mature tree will have to compete with an enormous consumer of nutrients, moisture, sunlight and root space. That is a lot to ask of any little plant, but consider the understory plants in forests. Clearly there are plants that can either adapt to if not thrive in the environment created by large trees.

The most important consideration in planning an under tree garden is the needs of your tree. Many years have been invested in the growth of the tree, and yes, you could not only damage it but potentially set off its’ slow death. Root damage is the most common. Be sure you carefully investigate the cultural requirements of your tree and sensitivity of its’ roots. For examples some Oaks can be very sensitive to root disturbance but some are quite tolerant. In any case, you should try to work with existing conditions as much as possible and avoid both disturbing the soil and roots as well as avoid substantially altering or adding to the soil conditions.

Alterations to conditions that can adversely affect the health and life of your tree:

• Adding deep soil around the tree can virtually suffocate your tree. Oxygen will have difficulty reaching the roots as will moisture.

• Chemically clearing or aggressively digging out grass and weeds before planting can seriously damage roots and even kill the tree. Chemical herbicides in the soil are taken in by roots and dispersed through the tree.

• Never plant within 12 to 18 inches of the trunk. Damage to the trunk and large anchor roots can be devastating to the tree.

• Never install a raised bed around a tree. Installing any retention wall or structure will damage roots. Deep soil over the roots will limit oxygen and moisture availability. And the tree’s first instinct to survive will be to as quickly as possible send new roots through the whole raised bed anyway.

There are things you can do that will minimally impact the growing conditions of your tree and still help with the establishment of a garden.

• Eliminate grass and weeds within the drip line by smothering with a thick layer of wet newspapers and cover with an inch or two of mulch or compost. Dig only minimally when the soil is moist to tease out weed roots if necessary.

• Add only an inch or two of soil or compost at most within the tree drip line. Roots may prevent you from working it into the soil much if at all. But soon the earthworms will get to work moving matter and nutrients deeper into the soil.

• Choose the smallest size bedding plants available to minimize the hole size, and therefore root disturbance, necessary for planting.

• Design cannot dictate where the plants go, existing tree roots must be avoided. Accept that placing your plants around and in between the larger tree roots will create a natural and very pleasing effect. The new plant will also have a little space of its own to spread roots.

• Pruning off low hanging branches will help give your plants much appreciated sunshine. Even shade plants will be healthier with more sun.

After planting your under tree garden thoroughly water the area to settle the soil and to hydrate the new plants. A couple of inches of mulch can be applied after planting, but never apply mulch within a foot of the tree trunk. Excessive moisture invites insects and disease to invade your tree. Continue to supply up to one inch of water a week as the new plants establish. After a few weeks start reducing the soakings. Some may need spot watering if the tree is not sharing well.

Until the new plants are well established, avoid fertilizing. Feeding will encourage top growth and it is essential in an under the tree environment that the roots spread and grow strong first. It is best to wait until the second growing season to feed the plants. Apply a slow release granular fertilizer and water well. Refreshing the compost top dressing or organic mulch can also add nutrients to the soil for your plants. When organic material is adding minimally each year promotes good drainage, aeration and good microbial activity without affecting your tree roots. Gradually shallow roots will continue to move as close to the surface as they need to.

Although some shrubs may be able to survive under a tree, generally they will not thrive. Consider instead using shrubs at the outer edges of the root zone rather than directly under the tree. There will be less competition for sunlight, moisture and nutrients. Placing shrubs at the rear of the planting area will also give the new garden a nice backdrop and lend structure to the planting.

Selecting Plants for an Under Tree Garden

We all know that to grow well under a large tree plants need to be shade tolerant and adaptable to dry conditions. It will also help tremendously to choose plants that are native to your specific region. Look for natives that naturally grow in a woodland setting. There are many ground covers that can thrive in shade among tree roots, but often they are invasive spreaders. Some are quite difficult to confine and prevent spreading rampantly through your landscape. And if you choose some annuals for your under tree garden, keep in mind that every year you will be disturbing the tree roots. Perhaps you could just sprinkle in some shade loving impatiens where gaps in the roots allow for a bit of digging.

I have had great success with a several plants under very large shade trees. My favorites include hosta for a variety of bright foliage colors, Pulmonaria with delicate blooms and speckled foliage,and cheery and colorful impatiens annuals of course. Max Frei Cranesbill are not bothered at all by tree roots and dry conditions. Bright purple pink flowers are eye catching on rich green foliage. They will also self seed just a bit. But I also love Wood Poppy with bright cheerful yellow blooms in early summer and golden hues in autumn. It does self seed if you want to fill an area easily, but you can also surface cultivate out excess. Hakone Grass and Heuchera often do well at the outer areas of a woodland setting, they like a bit of moisture and sun. Early blooming Rhododendron gets the sun it needs to bloom before the trees leaf out. Astilbe thrives with minimal sun but don’t let it dry out. Sun King Aralia is a bright limey large shrubby perennial that competes well with trees and does fine with about 4 hours of sun, but does enjoy more if it can get it. And of course Irish Moss is a perfect woodland ground cover.

Best plants for Under Mature Trees

Be careful to choose plants that are closest to the specific environment you intend to plant them. The outer edges of a woodland garden may receive more sun as well as more water from lawn sprinklers but areas further back may need to be more drought tolerant. You will also need to identify areas of deeper soil for placing plants with larger root systems or those that demand a richer amended soil. But there are many to consider and don’t be afraid to experiment a bit and move things around if necessary.

• Astilbe

• Azalea

• Bergenia

• Bleeding Heart Lamprocapnos spectabilis

• Bugleweed Ajuga reptans

• Coral Bells Heuchera

• Columbine Aquilegia

• Creeping Phlox Phlox stolonifera

• Creeping Jenny/Moneywort Lysimachia nummularia

• Foamflower Tiarella

• Hakone Grass hakonechloa macra

• Hen and Chicks

• Hosta

• Hydrangea

• Irish Moss Sagina subulata

• Japanese Painted Fern Athyrium niponicum

• Japanese Spurge Pachysandra

• Lady’s Mantle Alcehmilla

• Lily of the Valley (invasive spreader)

• Lungwort Pulmonaria

• Mondo Grass Ophiopogon japonicus

• Monkey Grass Liriope

• Mountain Laurel

• Ninebark

• Periwinkle Vinca minor

• Rhododendron

• Sedge Grass Carex

• Sedum

• Spotted Dead-nettle Lamium maculatum

• Sweet Woodruff Galium odoratum

If you want to add bright summer color with annuals every year consider impatiens, coleus, begonias, pansies, and violets

And if soil depth is available, bulbs that bloom in spring will get all the sun they need to produce big gorgeous blooms before the trees leaf out later. If extensive roots prevent getting deep enough into the soil try rhizome spring blooms such as iris and daylily that prefer their bulb like roots close to the surface.

Now get creative with an interesting edging for your planting area. You will again want to be careful about disturbing roots by digging deeply if at all. Consider some interesting brick, stone or flagstone for a finished look.