The three photos above show my raised planters between one of my oaks and the bed at the back of the garage. I think these tubs measure 20 inches wide by 17 high. As I said in an earlier post, I added them because the oak roots are so dense here that nothing grows well, groundcover-wise. I had this six-foot space always covered in leaves, and I wanted plants. 

The tubs do more than hold heucheras, bleeding hearts, etc. As planters alone, they don’t need to be as tall as they are. But they serve as a series of masses in the understory of this oak, and that’s beneficial to wildlife, as well as to the goal of a water-retentive, microclimated landscape. (The tubs won’t harm the tree, because the roots grow right up into them, and trees in nature love masses protecting their understory. The woods are full of rocks and fallen logs, full of trees growing right next to other trees, but we plant our yard trees in naked isolation, steal away their fall leaves, then decide anything sitting on their roots is bad for them!)

Masses are anything that has mass, including large rocks, logs, tree stumps, living trees, large containers and planters, certain garden furniture or statuary. Mass functions as a temperature moderator, a collector and distributor of warmth. Mass also acts as a windbreak, sheltering anything leeward from desiccating winds, or severely hot or cold winds. Mass casts shadow, which helps protect plants nearby from foliar evaporation, and lets moisture from rainfall last longer in the soil. Mass provides shelter and homes for wildlife.

Masses are also part of a tiering system, that prevents heavy rains from rushing over the soil surface, so that you lose both the water and a portion of your topsoil. In garden habitats, where human-created structures mix with nature, all our raised beds (whether you like this corrugated metal type, or Corten steel, or railroad ties…), serve as masses—and the more areas of mass we add, the more our garden can defend itself against droughty weather, or too-cold, too-hot, too-rainy weather.