Raising soil level and how tree roots react


The Japanese Upright Yews and Pyramidal Cedars I planted years ago are now an 18’ tall ‘forest’ that reaches more than 15’ out from the fence.
This created a ‘forest floor’ where no plants grow even with mottled light through out the day.
I see no visible roots on the ground, just soil, and was told yews are thirsty and their fine, non-visible roots suck up the available water for other plants.
In order to reclaim this area to add a path and woodland plants….I was told to add 6” of compost onto the forest floor making sure to not pile it near the base of the trunks.
I was also told that surface roots access oxygen and adding soil onto of them will suffocate the tree… I’m not sure what they mean by surface roots… in my case there are none of the 2”-3” thick roots you see above the surface… just the fine web of roots you find just below the existing soil surface when you dig.
About 6 weeks ago I went ahead and added compost to a depth of about 3” everywhere except near the trunk bases and created a path under the trees with two slight ‘hills’ made of compost/clay/garden soil that are probably around 6”-7” deep.
Could raising the soil level as described suffocate fine roots and kill the trees?
I was told to look at the yews to see if the colour has begun to yellow from stress… but they are still a rich dark green.
I don’t want to lose they yew/cedar ‘forest’ and am prepared to remove all of the soil if it will harm them.
Your advice would be greatly appreciated.
Thanks. – A


To reach your goal of having both a nice pathway through the mini-forest and adding woodland plants in the area, adding soil/compost in the area of interest (but not mounding it near the tree trunks) makes sense but could be a source of damage to your trees.

Shallow roots: Tree roots can be sensitive to damage. The fine web of shallow roots you describe should be disturbed as little as possible.  Yews have deep roots as well as fine roots in the upper areas of the soil.  Cedar root systems vary from one variety to another, many have shallow, spreading fibrous root systems.   So it is likely that both the yews and cedars have shallow roots that could be susceptible to damage.  If you are digging near or around these roots when planting, be careful and use a trowel, not a large shovel.  If you dig into a root system, move to another area.

Covering roots or trunk with soil: You mention that you have added 3 inches (7.6 cm) of compost, which looks from the photo to be along a path that does not touch the tree roots.   The path looks quite narrow, with the trees appearing to be quite close together.  It is possible that the tree roots extend “across” the path.  This would mean that the soil you added could cover the tree roots and prevent them from receiving needed water and nutrients, leading tree damage.  It is not possible to predict if this would kill the trees – I suggest that you consult an arborist for advice.   Monitoring the colour of the yews/cedars for stress is a good idea, but once damage has been done, it may be difficult to repair.

You mention that you’ve added a couple of compost /soil “hills” around 6-7 inches (15-18 cm) deep – it’s not clear why you added these or if these hills are in close proximity to the tree trunks. (these do not seem to be visible in your photo)

As you indicate, it is important to ensure that the base of the tree, just above the roots where the trunk starts to widen, is left exposed and not covered with soil.   This area is called the “root flare”.  If roots are under too much soil, they do not have access to oxygen and warmth and roots could girdle.   If you find that the root flare has been buried by the extra soil you have added, you can remove the excess soil.

Underplanting: When selecting plants for underplanting, choose those that will survive the shade caused by the tree canopies, and also consider that these plants and the shallow tree roots will compete for water and nutrients, particularly during dry periods.   If the tree canopy provides excessive shade, you may want to consult an arborist about pruning/thinning branches of the trees, to let in more light.

As an alternative to underplanting, adding strategically placed and unobtrusive containers can provide lots of interest and colour to the garden.


  • University of Maryland Extension. Grade Change problems with trees.
  • Oettinger D. Planting under trees. The American Gardener 2015 Nov-Dec:30-34.    This author recommends shallow-rooted perennials, bulbs and groundcovers for sharing space with established tree root systems. Note: This article mentions Lily of the valley (Convallaria majalis) as a groundcover for dry shade. It has been listed as invasive by the Ontario Invasive Plant Council and should not be planted. See Grow me Instead, Southern Ontario.
  • The November/23 issue of Cross Pollination by the Halton Master Gardeners has an excellent article about Gardening in dry shade that may interest you.

Landscape Ontario provides a list of certified arborists.

All the best with your forest!

October 30 2023