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Which Comes First, The Root or The Crack?

The Ohio State University and the City of Cincinnati Park Board conducted a study of street trees in Cincinnati, Ohio, U.S., during the summers of 1999 and 2000. Four genera of trees were examined from four different plant families. Approximately 600 trees planted within the past 20 years were observed. The condition of the nearest sidewalk joint and the incidence of a root for Quercus (oak), Koelreuteria (goldenraintree), Zelkova (zelkova), and Gleditsia (honeylocust) were observed. All genera responded similarly in that roots were more likely to be found under a crack in the sidewalk than under an intact sidewalk. During the first 20 years after planting, tree roots are more likely to be found underneath a sidewalk crack than an intact sidewalk. Of 351 joints observed with no roots, 39% were intact and 61% were cracked. Of the 260 joints where roots were observed, only 16.2% were intact while 83.9% were cracked. Oxygen measurements were made underneath selected sidewalks. Oxygen concentrations were compared for soil underneath cracked and intact sidewalk blocks. The soil underneath cracked sidewalks showed a higher concentration of oxygen. This higher oxygen concentration may contribute to root growth underneath cracked sidewalks.