Container gardening: Be careful with temperature

Container gardening can have benefits if you live where you do not have a backyard and have only a small patio or balconies. The two times a year that container plants have big problems is the heat of summer and the freezing cold of winter.

When growing plants in the ground, the soil helps balance everything, like water. It also becomes a huge heatsink balancing out temperatures. During the summer, the soil surface may be too hot to walk on, but a few inches below the surface the soil is much cooler. In winter, the soil may freeze to a few inches, but below that level the soil is still warm.

Plants that are borderline growing here due to cold temperatures often freeze to death when grown in containers. This is because the cold air can completely surround the roots of the plant, turning the soil in the container into an ice cube. This heat deeper in the soil can also slowly rise out of the soil and raise the air temperature for the plants.

In summer, the soil temperature is just the opposite. In the container, the soil could heat up to very hot conditions, where an inch or two into the soil it is again in the 50-degree range. Plants such as junipers and pines can be easily killed by high soil temperatures found in containers. This is due to the fact they need a fungus in the soil to help the root absorb water. The hot temperatures kill the fungus and the plants cannot absorb water and die. So light-colored pots work best in summer, and plants like cactus and succulents are better suited for the hotter soil temperatures.

However, it is now winter, so darker containers work better absorbing heat. You could wrap the containers of your prize containerized plants with trash bags, bubble wrap or even pipe wrap to create an insulated area around the containers. Not very attractive, but keeps your plants alive. Placing containerized plants into larger pots can also create a protection from the cold, but it can also prevent the sun from warming the soil back up.

The larger the container the plant is planted in, the more protection offered to the plant. If the cold freezes the soils two inches deep for a container plants, that means two inches from the top, two inches from the bottom and two inches from all sides. If the container is a six-inch pot or one-gallon container, the entire root system is frozen. If it is a 15-gallon container or 16-inch pot, there is still a large area of soil in the middle that did not freeze. However, a larger container will take much longer or the soil the dry out after winter rains, leading to plants rotting. You just can’t win.

A rule of gardening is it is at least 10 times easier to grow a plant in the ground than in a container.