Thursday, April 10, 2008

Potting Bench Urban Legends

Every plant enthusiast keeps a mental library of horticultural rules-of-thumb. Some of these rules we learn from other gardeners, some we read about, and some we come up with on our own. Most of these tips and tricks are, in fact, perfectly sensible: you can start new jade plants by sticking a leaf in soil, you shouldn’t water a cactus immediately after transplanting it, you can control mealy bugs by dabbing them with rubbing alcohol. However, some frequently encountered horticultural rules don’t actually hold up to close scrutiny. Cactus and succulent growers are particularly concerned about drainage, and so many C&S urban legends involve insuring that excess water doesn’t stay in pots.

Urban Legend #1: Shallow pots drain better than deep pots.

All else being equal, this one is dead wrong. Soil holds onto water by capillary action, and a certain amount of pressure is needed for excess water to be forced out of the drainage hole of a pot. That pressure comes from standing water in a layer of saturated soil at the bottom of the pot. The depth of the saturated layer that can be supported before water starts draining off depends on the characteristics of the soil, not on the shape of the pot, so in a shallow tray the layer of soil that stays wet after watering is closer to the soil surface and takes up more of the soil volume than in a deep container. Of course, all else is rarely equal, and a shallow pot might actually dry out more quickly that a deep pot, because it holds less soil and presents more soil surface to the air for water to be lost via evaporation. But for efficient initial drainage of water, deeper pots are better.

Urban Legend #2: A layer of gravel or crocking at the bottom of a pot improves drainage.

If your concern is the unimpeded flow of excess water out of the soil after irrigation, a layer of “drainage material” is counterproductive. It’s a basic principle of soil science that a soil layer that is either less or more porous than the layer above it, will act as a barrier to the downward flow of water. Thus, a layer of highly porous gravel or potsherds actually impedes the movement of water in a pot, and leads to the development of a “perched water table” (a layer of soggy, saturated soil) above it. Another way of thinking about it is that filling the bottom of the pot with porous material essentially turns a deep pot into a poorly drained shallow pot. In certain situations, a pot with lots of crocking may dry out faster than one with just soil, but only because the total volume of soil is reduced, not by improving drainage. Minimal usage of stones or potsherds over the drainage holes of a pot is often desirable, however, simply to prevent soil from pouring out.

Urban Legend #3: Adding sand to a soil mix makes it well drained.

This one isn’t always false, but it isn’t as universally true as you might think, either. Some mixes that are heavy on sand are indeed very well drained, but I’ve also experimented with sand-based mixes that water would just sit on top of for half an hour before percolating into the soil. The type of sand makes a difference: sands with larger, sharp-edged grains tend to promote water movement, while sands that include a high percentage of very fine material, or with weathered, rounded grains (like beach sand) can clog a soil mix. Strictly from a drainage-improvement perspective, the best sands are probably the courser types used for sandblasting or filtration systems. However, drainage is not the only consideration when deciding on a soil mix, and sands with some silty, fine material can offer benefits, too, such as sequestering the nutrients and water needed for plant growth. So, experiment with the materials that are available to you, and don’t get thrown off track by horticultural urban legends.