Knysna’s clayey hillsides.

Posted by on September 30, 2016

Recently I had to give  ‘advice’ to establish a fynbos garden blending  in with existing fynbos of Erica, bietou and kooigoed.  Looking at the natural vegetation on the plot, the gentle slope of the hill and no visible drainage problems, I had no idea what problems would  occur once the house was built.

Cutting into the hillside to establish a platform for the house &  pool unearthed the worse clay I have ever seen. Then the rains came causing havoc with shifting the thin layer of topsoil from the top level, causing landfill to shift.

Subsurface water roughly follows the contour of the surface of the ground. A vertical cut exposes the travel path of subsurface water and any  water discharged from the bank of the hill will flow down the hill to the excavated area. The result is that the plot will be wet for days after rain.

Drainage can be improved, but the clay problem will always be there; heavy clay does not let water pass readily through it, so water does not absorb into the soil.  If the plot is not graded properly, the water will sit for days until it evaporates.

Several solutions are possible for these types of situations. 

  • A French drain is potential solution. Water damage experts can determine if a French drain is the right solution. A French drain involves excavating a ditch, installing a perforated pipe encased in gravel. The pipe is directed to a location away from the house.
  • Adding tons and tons of coarse bark based compost , working it in when the clay is not too wet.
  • Other solutions are plants that tolerate wet areas. As a general rule, vegetation will not totally solve problem, however it can be utilized in certain situations to solve common drainage problems.

Indigenous plants for very wet areas:

 

Zantedeschia aethiopica photo: www.gardeningexpress.co.uk

Zantedeschia aethiopica
photo: www.gardeningexpress.co.uk

Zantedeschia aethiopica, Kniphofia uvaria, Wachendorfia thyrsiflora, Gunnera perpensa, Elegia cuspidata, Elegia equisetacea, Elegia tectorum, Cyperus prolifer.

Indigenous plants for damp areas but not soggy wet:

Crinum macowanii, Crocosmia aurea, Watsonias, Chasmanthe aethiopica, Coleonema, Euryops virgineus, some Ericas, Polygala myrtifolia, Hippia frutescens, Freylinia lanceolata, Polygala virgata, Psoralea pinnata, Aristea capitata, Aristea ecklonii, Helichrysum petiolare, Helichrysum cymosum, Strelitzia reginae.

Buddleja saligna, Buddleja salviifolia, Searsia lucida, Diospyros dichrophylla, Sparrmannia africana, Syzigium cordatum, Chrysanthemoides monilifera,

Exotics include roses, hydrangea, raphiolepis, westringia.  The RHS has a good list of recommended plants and planting techniques.

 

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