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Each monthly issue of the Henry Shaw Cactus Digest includes club updates, columns and articles by members on their favorite aspects of cactus and succulent culture. Follow the link below this item to read select Digest articles -- or join HSCS to receive every article in the print version of the Digest.
Drainage -- Bajadas
By Joe Merkelbach
One of the primary concerns in successfully growing cacti and succulents is providing rapid drainage of water. We generally accomplish this goal in culture with coarse-grained mixtures of growing media -- whether perlite, dynarock, rice hulls or other components. These send water through the root zone but do not allow it to puddle and drown the fine root hairs. We also control volume and timing of watering so we treat our plants to a sort of optimized version of their typical growing conditions and produce beautiful specimen plants.
It is instructive to learn the natural situations that caused our plants to prefer these conditions. Although most, not all, are from deserts with low moisture conditions, they do not grow most successfully on the desert floor. Instead, most grow on the relatively gentle slopes between highlands and the flat pan.
These slopes, called bajadas, consist of overlapping alluvial outflows from intermittent stream washes flowing out of the hills. Mineral substrates broken from the mountains and consisting of rocks through gravel to fine sand are spread as fans, with heavier rocks dropped out first and fine-grained sand and clays carried the furthest into the flats.
The best moisture regime for most cacti and succulents is in the area of medium- to fine-grained gravels; these areas allow rapid drainage but provide purchase for the roots to hold plants in place. This relative porosity also allows penetration of a small amount of organic residue to provide nutrition to the plants. The slopes also are rinsed of the accumulation of salts such as alkalies that are carried into the low, flat areas and accumulate as hard mineral pans beneath the surface or as so-called "desert pavements" on the surface.
Duplicating the natural conditions of bajadas is a good method for successfully culturing cacti and succulents. See http://biology.ed.ac.uk/reasearch/groups/jdeacon/desbiome/bajada.htm for some good illustrations.
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