Garden Planning: Gardens under Trees

Garden Planning: Gardens under Trees

Perennials

Perennial gardens are a natural for areas around and under trees, serving to tie the various landscape elements together -- and making mowing easier. Also, lawn grasses often struggle in the shade cast by trees, so it might seem like a great place for some flowers. Some of the most common landscaping questions involve getting plants -- grass or flowers -- to grow under dense trees. Here are some things to consider before digging in.

Shade. It’s a fact: Most mature trees cast a lot of shade, and this dense shade limits the types of plants that will thrive there. You may be able to open up the canopy of the tree with some judicious pruning, allowing filtered light to reach the ground. Also, removing some of the lower limbs will let in more light. (It’s best to have a professional arborist to tackle major tree pruning.)

Competition. Tree roots will compete for water and nutrients with plants nearby, so these plants may need supplemental water and fertilizer.

Anywhere grass doesn’t thrive may be a tough spot to establish a perennial bed.

Protecting the tree. If you’ve ever tried to dig a hole underneath a tree, you’ve probably encountered the tree’s roots. Some trees, such as poplars, maples, and beeches, tend to have very shallow roots, some of which are barely covered with soil. Other trees have somewhat deeper roots, but all trees have the bulk of their feeder roots in the top 18" of soil. (Anchoring roots can go much deeper.) If you damage too many of the feeder roots, you will injure and possibly kill the tree.

Though it seems logical to bring in topsoil to create a nice deep planting area, don’t do it! Never spread more than an inch or two of topsoil (or dense mulch) under a tree, and never create a raised bed around the trunk. A tree’s feeder roots are located near the soil surface because they need oxygen, as well as the water and nutrients that filter down. Bury them deeper, and they can suffocate. Also avoid piling soil up against a tree trunk, because this invites disease organisms to attack the buried portion of the trunk.

What’s a gardener to do? Don’t expect to grow your most colorful perennial garden under a mature tree. It’s not worth the frustration, expense, and risk. Instead, consider introducing shade-loving ground covers and foliage plants with colorful or otherwise interesting foliage, including perhaps some ferns and hostas. You can then incorporate this island into a larger perennial bed, planting the larger, deeper rooted perennials beyond the perimeter of the canopy. Take your cues from nature. If you’ve ever walked in a mature woodland, you’ve probably noticed that little grows under the densest trees. Then, as you move toward the edge of the trees’ canopy, you’ll notice more and more plant growth as you get further from the tree and into higher and higher sunlight levels.

Smaller, younger trees are more resilient (and cast less shade), so you may be able to establish some perennials near young trees. Don’t plant too close, or you’ll create competition for water and nutrients for the tree, slowing its growth.

Sample Garden Plan for a Shady Garden

  • includes area under tree
  • color scheme: silver/white/green, plus pink/red flowers on lamium and astilbe
  • USDA Zones 4-8

 

Plant Common Name Height Flower Color Season of Bloom Sun/ Shade USDA Zone
Athyrium niponicum pictum Japanese painted fern 8-12" foliage: silvery green with wine red midribs -- full or part shade 3-8
Astilbe astilbe 22"' red early to midsummer part shade 4-8
Hosta hosta, plantain lily 18" deep blue-green foliage, white flowers early summer part shade/ shade 3-9
Lamium lamium 6-8" ground cover silvery white foliage, pink flowers  spring to fall part shade/ shade 3-9
*Tucking in some spring-flowering bulbs, such as daffodils, tulips, and crocus, will provide color from early to late spring.

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