Joined: July 2004
||Posted: 12 Sep. 2019,18:35
You need to strike a balance between creating a pond or reservoir for moisture, and building a feeding station for the damned tree, so the aim would be to fashion a pit with good-ish moisture retention, plenty of nutrition, but adequate drainage.
The basic idea to use upright sleepers is fine, but how much upstand? A good 'rule-of-thumb' would be to have at least one-third (minimum 2200mm) in the ground fully supported, prefeably with concrete, in the manner of a fence post. The 'inside' can then be lined before backfilling with a good, healthy growing medium.
If you have a well-drained plot, the lining can be root- and watertight(ish) with perforated pipes laid at the base and leading out to a soakaway or similar elsewhere in the garden, but for most applications, simply lining with a good quality root barrier membrane or composite, to inhibit encroachment from the tree roots, would be adequate and by far the simpler construction.
You may have to provide irrigation or regular watering if the tree is going to parch the plot, but that applies to most tree-lined gardens anyway, so no great additional burden.
It'll be interesting to hear how you get on.....
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