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A case where the merits were reversed was that of a patriarchal horse chestnut which shaded another client's south terrace. When he built a flagged patio there, he "potted" the tree with a low retaining wall a dozen feet out around the buttress roots. Within this wall he sprinkled topsoil, planted ivy, and diligently watered and fed his tree to keep it flourishing. All went well with the horse-chestnut, apparently, for several years. Then it began to die back throughout its whole crown. What had happened was not obvious, but our explorations exposed it. Unable to find moisture beneath the heavy flagging, the tree's outer roots had atrophied while inner ones had multiplied and massed under the "pot' Here they became self-constricting, and entirely dependent on artificial drink and food, which were not enough. The solution: to drill holes and insert short lengths of pipe down through the flagstones, spaced widely around the "pot"; then, by frequent watering and feeding, to coax the horsechestnut's root system back outward to a normal pattern. (This system, with sieve caps over the pipe inserts and a cutting tool to clear the pipes when rootlets clog them, as they will, can be used to preserve feature trees rooted where a driveway must go.) In the tree's ascending column of water are dissolved minerals from the soil. Chief of these are nitrogen, phosphorus, and potassium, which the tree must have, besides sugars and starches, in forms synthesized by leaf chemistry. At this peak point, in the leaves, the tree's water content becomes enriched sap. Now it must be redistributed downward to impart growth, energy and tensile strength to all parts of the tree. To see how this is done we must re-examine the tree's water column, and now we find that it is a two-way affair. |
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