People have been gardening and cultivating the earth beneath our feet for millenia upon millenia; just when this hip, non-hipster gardener has seen and heard everything, I get from a friend an article posted in The Guardian discussing the idea of planting trees in square holes to prevent circling roots. Yes, you read that correctly: square holes. The rationale behind this supposed age-old practice, arises from the fact that root tips hitting an impenetrable surface such as the side of a flower pot, deflect to the side and begin an internal one way path around the circle. Circling roots, when planted in the landscape, continue their circular route, never spreading out into native soil, creating a whole host of long-term issues including plant death. A quick online search unearthed a half dozen news articles devoted to the square root of plant (all you math majors out there surely know what the square root of a plant is, haha!). Each parrots the other, saying roots cannot negotiate corners, so they divide at the turn, spreading out densely into the surrounding soil. So, if you have a root bound tree or shrub, dig a square hole to guide the roots outward.
Unfortunately, like in so many fields of endeavor, information spread across the internet is unverified. Mr. Wong, the author of The Guardian article, claims “science proves” through “systematic plant trials” without citing a single published study backing the claim that square holes really aide root growth outward into the soil. After an hour or more of digging, I discovered Mr. Wong might be referring to planting practices supposedly attributed to Kew Gardens, though I could find nothing directly published by Kew to the effect. To date, no published studies support the effect of right angles on growing roots encountering impenetrable substrates. There might some day, but nada thus far.
Three things are critical to transplanting: 1) getting the root system growing in the proper direction at the time of transplanting, 2) what is used to backfill the hole, and 3) how the sides of the hole are treated. Cutting and spreading the roots outward radially is the first key. This is especially true of traditional nursery containers, where circling roots are the norm. Removing girdling roots and other major root imperfections sure to cause long-term harm to the plant should be corrected as well. The makeup of the backfill is the second key. Numerous studies have suggested for years now to backfill the planting hole with the original soil with little soil amending. A bit of compost, slow release organic fertilizer and mycorrhizae fungi is more than enough. Roots, like the rest of us, love to live in the lap of luxury. Who wants to get out of the home and work hard when you can live at home, with a full refrigerator and plenty of video games to keep you busy? Roots grown in highly amended backfill are much slower to establish/anchor into the surrounding soils when they are well fed close to home. The hole proportion and side treatment are the third key. Whether the hole is square or round matters little. What matters is a hole that is wider than it is deep (two or three times in fact), and one which fits the natural crescent shape of the root cross-section. As to the hole side treatment, it all depends on soil type. Roots spread outward radially from the main stem should have little trouble penetrating sandy soils. However, compacted and clay soils present a different challenge. Here, the interface of backfill soil and existing soil conditions is often the deciding factor in transplanting success. Roots have great difficulty breaking through compacted soils and clay soils rubbed smooth by the passing action of the shovel. In this case, roughing up the soil surface with shovel or fork tip gives both root tip and water some place to go. Many trees and shrubs have died in a stagnant bath tub created by poorly prepared holes. If you do not address these issues, I do not care what the shape of the hole happens to be, you will still have a dead plant to replace.
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Josh Steffen, Horticulture and Facilities Manager