In the last year we purchased a house that has eastern white cedars bordering our small backyard (18’ x 15’ of green space). It was mainly bare with a small patch of grass, many weeds and a few invasive plants which we quickly got rid of. We would like to create a native garden in this space but we do not want to disturb the roots of the cedars as it’s my understanding that the roots are quite shallow though I haven’t been able to find information on how shallow they are? I was wondering if you might be able to help with this and if planting around them would disturb the roots or if it would be best to plant natives in containers as not to disturb the root system of the cedars? The cedars are approximately 10 meters tall, if that helps?
Many thanks, in advance, for your help!
Thank you for contacting the Toronto Master Gardeners – and thank you for wanting to re-design your new backyard with native plant species!
Aside from your question about root depth, you might also find it helpful to have a look at this section on the Toronto Master Gardeners website about native species.
Planting perennials and shrubs directly in the ground is usually preferable to planting in containers and this can be done successfully by taking a few precautions. (As a sidebar, planting in containers creates issues of needing to overwinter the plants or replacing them each spring. Containers also compact the soil underneath them creating a less hospitable soil environment for the other plants in your garden.)
The root systems of eastern white cedars (Thuja occidentalis), as described in this article, are shallow, wide-spreading and opportunistic. This means that in well-drained soils the roots are deep, in wet soils they are shallow, and in rockier places they are far-reaching allowing the roots to take advantage of cracks in rocks.
Generally when planting under trees, this resource details a few helpful guidelines. A good rule of thumb is for every 2.5 cm of tree trunk diameter avoid cutting roots 30 – 45 cm from the tree. For example if the average diameter of your cedar trees is 10 cm, consider planting no closer than 120-180 cm from the tree. Instead of taking the chance of severing some of the roots, some gardeners choose instead to add 5-10 cm of planting medium on top of the existing soil before planting. A mixture of compost and mulch, such as shredded bark, can be used for this purpose.
Choose perennials and shrubs with shallow root systems, rhizomes or small bulbs to minimize damage to the roots of the cedars and be sure to buy them as seedlings or young plants so that they require only small holes to be made in the soil. As outlined in this article, dig carefully, using a small trowel, to avoid cutting any roots. Sticking to perennials, rather than including annuals in your choices, will also minimize damage as you only need to plant perennials once. Checking the moisture and sunlight requirements of your native species, and where they will be planted, will also help to ensure their health and the health of your hedge.
I hope these are helpful tips for you to incorporate into your spring planting. Good luck with this next stage of your project! You are now at the exciting step of turning your garden into what you truly want it to look like.