20170917A.Alex Indigo, Flickr
When planting a woody plant, just backfill with the original soil. Photo: Alex Indigo, Flickr

Forget the old technique for planting shrubs and trees, the one I learned when I was a kid. We were told at the time to amend the soil removed from the planting hole of a tree, an evergreen or a shrub by mixing in a good dose of compost, organic matter or fertilizer. Some gardeners even went so far as to replace all the soil removed with quality top soil.

20170917C distinctivetreecare.com
Tree and shrub roots tend to remain in the area with the best soil, eventually girdling the plant. Photo: distinctivetreecare.com

However, studies led notably by the American tree guru, Dr. Alex Shigo, have since shown that when the soil in the planting hole is better than the soil all around, the roots tend to stay in that area, circling within that patch of richer, more friable soil. In fact, with time, they can literally girdle the tree, strangling it.

Instead, backfill with the original soil, adding no soil amendments (fertilizer, compost, manure, etc.). Since the soil is now of the same quality everywhere, the roots will do what Mother Nature wanted them to do: extend out in all directions.

You can, however, apply mycorrhizal fungi to the plant’s rootball, especially in soils that have been perturbed. These beneficial fungi improve root growth and, contrary to fertilizers that, when added to the planting hole, encourage the roots to stay nearby, actually tend to spur them to spread out widely.

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To improve the soil around a tree or shrub, apply compost or fertilizer throughout the entire root zones… and it extends beyond the drip line. That will encourage the roots to extend outward.

If you do want to improve the texture or quality of the soil, do so after planting, fertilizing or applying a layer of compost over the entire area, as far as the roots will eventually reach, that is out to and beyond the tree’s eventual drip line. That way, you’ll encourage the roots to spread in all directions in search of the added nutrients … and you’ll also improve the growth of all the plants in that zone.20170917A.Alex Indigo, Flickr

Larry Hodgson is one of Canada’s best-known garden communicators. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He has written for many garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 60 other titles in English and French. He is a past president of the Garden Writers Association (now Garden Communicators International) and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. He resides in Quebec City, Quebec, Canada.

1 comment on “Plant Trees without Amending the Soil

  1. Pingback: Fall is the Perfect Time to Plant Shrubs – Laidback Gardener

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