Explore Plants

Share

Ask Mr. Smarty Plants

Ask Mr. Smarty Plants is a free service provided by the staff and volunteers at the Lady Bird Johnson Wildflower Center.

Search Smarty Plants
    
 

Can't find the answer in our existing FAQs, submit a question to Mr. Smarty Plants.
Need help with plant identification, visit the plant identification page.

rate this answer
Not Yet Rated

Wednesday - June 12, 2013

From: Lakeville, MN
Region: Midwest
Topic: Drought Tolerant, Shade Tolerant, Herbs/Forbs, Shrubs, Trees
Title: Perennial Suggestions for Under Ash Trees in Minnesota
Answered by: Anne Van Nest

QUESTION:

With our house we have inherited 2 ash trees in our front yard with a large amount landscaping underneath. I'm replacing the landscaping but there are tree roots at the surface preventing me from being able to dig and plant new perennials. Our ground is very compacted and clay-like. Is it safe to cut the roots out or will this damage the trees? Is it a safe option to mound above the roots with black dirt and plant new landscaping? The trees are approximately 11 years old and I believe they were planted too close together - about 10-15 feet apart. Is it best to remove the trees and start over?

ANSWER:

Your issues with growing plants under trees and dealing with the tree roots are a big challenge for many gardeners. Colorado State University Extension have put a very good article online about tree roots. It is by J.M. Sillick and W.R. Jacobi and is entitled “Healthy Roots and Healthy Trees.” Some of the key points are:
• Most tree roots are located in the top 6 to 24 inches of the soil and occupy an area two to four times the diameter of the tree crown. Roots grow where water, minerals and oxygen are found in the soil. Because the greatest supplies of these materials are usually located in the surface layer of soil, the largest concentration of feeder roots exists in this zone.
• Large roots and small feeder roots occupy a large area under ground. Typically, the root system of a tree extends outward past the dripline, two to four times the diameter of the average tree's crown.
• Roots obtain water, oxygen and minerals from soil. They do not grow toward anything or in any particular direction.
• Soil compaction, change in soil depth and improper watering can injure roots, increasing stress and susceptibility to disease and insects.
The Extension people say that changes in soil depth around trees can cause injury to root systems. The addition of only 4 to 6 inches of soil over a root zone drastically reduces the amount of oxygen and water available to the roots.  So cutting the roots out or adding additional soil on top of the roots is not a recommended solution.

Even though your trees are 10-15 feet apart, once they grow together they will focus their growth to the outward facing areas where the branches have the most sun exposure. More than likely they will fare just fine. If you are concerned, an arborist can give you a more detailed recommendation.

The best suggestion is to plant a low, native groundcover that tolerates dry shade and compacted soil under the trees or replace the old landscape plants with a mulch layer and create some new planting beds away from the ash trees where the roots will not interfere and there is more sunlight.  You should put your new planting beds two to four times the diameter of the mature ash crown.
The Ladybird Johnson Wildflower Center has a list of recommended native plants for your state in our plant database page. There are 173 plants to consider in the Minnesota Recommended list.


If you do decide to plant under the ash trees, Theresa Rooney of the University of Minnesota Master Gardeners has put together an information sheet with suggestions for planting in dry soil for shade or under trees. She gardens under a 100-year old elm tree and says that locations in dry shade are some of the most difficult to grow plants because of the competition for water, light and nutrients.

To compile a list of native plants for planting under the ash trees, go to our Native Plant Database. Under Combination Search, select the following categories: State – Minnesota, Habit – shrubs, and then search again for herbs (for herbaceous), Duration – perennial, Light Requirement – shade, and Soil Moisture - dry. You can further narrow down the list of potential plants by indicating whether you prefer a deciduous or evergreen plants, specific blooming times or blooming colors.

Some dry shade shrubs that grow in Minnesota that are in our native plant database include:
Amelanchier alnifolia (Saskatoon serviceberry)
Arctostaphylos uva-ursi (kinnikinnick)
Ceanothus americanus (New Jersey tea)
Diervilla lonicera (northern bush honeysuckle)
Elaeagnus commutata (silverberry)
Rhus aromatica (fragrant sumac)
Shepherdia canadensis (russet buffaloberry)
Symphoricarpos orbiculatus (coralberry)
Viburnum rafinesquianum (downy arrowwood)
And some native perennials for dry shade in Minnesota include:
Astragalus crassicarpus (groundplum milkvetch)
Geum canadense (white avens)
Lobelia spicata (palespike lobelia)
Maianthemum stellatum (starry false lily of the valley)
Mitchella repens (partridgeberry)
Polygonatum biflorum (smooth Solomon’s seal)
Pteridium aquilinum (western bracken fern)

 

From the Image Gallery


Saskatoon serviceberry
Amelanchier alnifolia

New jersey tea
Ceanothus americanus

Northern bush honeysuckle
Diervilla lonicera

Fragrant sumac
Rhus aromatica

White avens
Geum canadense

Starry false lily of the valley
Maianthemum stellatum

Partridgeberry
Mitchella repens

Smooth solomon's seal
Polygonatum biflorum

Western bracken fern
Pteridium aquilinum

More Trees Questions

Screening Suggestions in Brooklyn, NY
March 08, 2013 - My neighbor directly in back of me has shrubs that are growing all over my fence. Also his 9-foot-tall shed facing me is rusted. What can I do to improve my view so that I can enjoy my backyard more?
view the full question and answer

Determination of native plants of North Carolina
April 24, 2006 - I am compiling lists of native plants to use in NC, and found that Dirr (Manual of Woody Landscape Plants) lists introduction dates (xxxx) for MANY of the trees you list as natives, e.g., Acer rubrum ...
view the full question and answer

Trees for privacy shield in Dublin, Virginia
April 16, 2009 - What are the best trees to plant for privacy in a small yard?
view the full question and answer

Need trees & shrubs for a 2.5x45 ft. planter box in Chatsworth. CA.
August 07, 2012 - We recently built a pool in our backyard and need to redo all the landscaping. We have a planter that is 45 feet long and about 2.5 feet wide. We'd like to put some trees in this planter that are n...
view the full question and answer

Evergreen for privacy screen in Northern California
October 19, 2012 - Hello, My neighbor just logged their property and we need a very quick growing evergreen shrub/tree (for privacy of ugly cabin) that grows to at least 10' -15' tall. We live in northern Cal. about ...
view the full question and answer

Smarty Plants's Facebook profile Support the Wildflower Center by Donating Online or Becoming a Member today.

Mr. Smarty Plants wants you to be his Facebook friend. Click the Facebook icon to add yourself to Mr. Smarty Plants list of friends.