Volume 16, Issue 33 - August 14 - August 20, 2008

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The Bay Gardener by Dr. Frank Gouin

Plastic, Fiberglass or Clay

When home gardeners ask me if there are advantages in growing plants in clay pots over plastic or fiberglass, I tell them a story.

Plastic pots were introduced to the greenhouse industry in the 1970s. Before then, all greenhouse crops were grown in porous clay pots using soil-based growing media. The most common potting mix was equal parts by volume of three ingredients: topsoil, which was mostly loam, peat moss and perlite or vermiculite.

When plastic pots were introduced into the trade, growers tried to use the same growing medium — with miserable results. They had to be extremely careful when irrigating because these heavy growing media remained wet longer in pots with walls solid rather than of porous clay. In pots with solid walls, the only means of ridding the growing media of excess water is the drainage holes in the bottom of the pots.

For that reason and more, it proved nearly impossible to include soil in growing media under large-scale production in plastic or fiberglass pots.

The research that followed the large-scale introduction of plastic pots led to the development of soil-less growing media such as Pro-Mix, Metro Mix or Sunshine-Mix — plus many more.

With soil removed from growing media, many other changes had to be made. Soil-less growing media do not retain nutrients as well as soil-based growing media, so it became necessary to develop new fertilizer systems.

Porous clay pots allowed excess water to evaporate from the walls. During evaporation, the walls of the containers remain cooler due to water evaporating from the clay.

Depending on species, it is not uncommon to find no roots growing on the south side of plastic containers exposed to full sun. Growing media on the south side of a container have been measured to be over 100 degrees on bright sunny days. Roots of most plants are killed when soil temperatures climb above 95 degrees.

So scientists went to work studying the effects of high soil temperatures in fiberglass and plastic containers, which also expose roots to higher temperatures.

The lesson from my story? You don’t ever want to use soil-based growing medium in plastic pots. You can use a soil-less growing medium in a clay pot — but be ready to irrigate more often because the porous clay walls will result in a greater loss of water.

Ask Dr. Gouin your questions at frgouin@erols.com. All questions will appear in Bay Weekly. Please include your name and address.

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