Keep Cast Iron Containers Cool

A traditional favorite gardening container material is cast iron. Cast iron urns are beautiful and elegant, but are very heavy. Painted cast aluminum is a popular substitute because it does not rust and it is not quite as weighty.

In the deep south, the walls of metal plant containers capture heat in summer. They transfer heat to the soil within, to the detriment of the plants growing within. One way to prevent hot metal from damaging tender roots is to provide an insulation layer between the pot and the potting soil. I have tried several different strategies. The most effective insulator I have found is bubble wrap, the kind used for protecting delicate items during shipping. It is inexpensive, flexible, and malleable to fit the nooks and crannies of elaborate containers. Simply cut a piece of wrap that is large enough to line the inside of the container. Place the wrap around the outside container walls, being careful to avoid blocking any drainage holes. It is easily trimmed to size with scissors, so that there is no excess visible above the soil line. Add regular potting soil inside the bubble wrap layer, and – voila! – the soil will not become scorching hot because it is not in direct contact with the metal. The thin material does not reduce the amount of soil available to whatever plants you choose for your container.

Another tidbit: Reduce the amount of potting soil needed by large containers by adding a layer of pine cones in the bottom, before adding soil.

This iron urn is lined with bubble wrap prior to the addition of potting soil.

A green and white combination looks classy and cool, even in the heat. Container contents: Diamond Snow Euphorbia (a filler), Rosemary (a thriller), and a double Calibrachoa (a second filler). Not shown, an ivy with a white leaf edge (a spiller). The Rosemary is evergreen, so it can carry over to anchor a winter container.

The completed planting. In a couple of weeks, the Rosemary will gain height and be a vertical center accent. The strong pine-like scent of Rosemary repels some insects. I’m hoping that it will keep mosquitos away from the seating area near this container, one of a matching pair.

Container Plants and Heat

Follow a few steps to keep container plants happy during the hottest part of the summer.

First, choose plastic or composite containers rather than clay or metal. Avoid dark-colored pots. Unglazed terracotta allows rapid evaporation of soil moisture, and metal can get hot enough to damage roots. Insulate the inside walls of iron urns or cast aluminum containers with bubble wrap or several layers of newspaper to keep soil cooler. Go big rather than little. Large containers do not dry out as quickly as smaller ones, and close grouping of containers will form a protective community, reducing evaporation in each. “Self-watering” containers have a hidden reservoir that captures water, and allows overflow to be available to the container. Do not allow plants to sit permanently in saucers of water or roots will rot. Mosquitoes breed in standing water, too.

Potting soil with high organic matter will retain moisture better than low-humus soils. For years, I purchased hydrogels and added them to my container soil mix. Hydrogels were first patented by two employees of Union Carbide Company in 1967. They are crystals about the size of large grains of sugar and can hold 600 times their weight in water. When water is absorbed, they look like clear jellybeans. The theory is that they will absorb moisture (from rain or irrigation) and release it back to the roots during dry periods. However, university lab tests (Washington State University, documented by Linda Chalker-Scott – click HERE to read additional info) showed that this retained moisture is not necessarily transferred to plants. In the WSU lab, several brands of hydrogels were used in the testing. The plants were grown in 1-gallon containers and watered when the medium was less than 80% saturated. After all of the plants matured to the same size, watering was stopped and the plants were allowed to dry out. Each plant was tested at regular intervals to determine moisture content. The results showed that none of the hydrogels kept plants supplied with water any better than the plants that had nothing added to the soil. One product actually stunted the growth to the plants to which it was applied. The crystals break down in five years or so, and degrade into carcinogenic neurotoxins. Ugh! A light top-dressing of a fine-textured mulch WILL reduce evaporation. A groundcover in a container will also help keep soil cool and moist – think Creeping Jenny, Ajuga or Dianthus.

Unless they are huge, containers can be repositioned to avoid full sun exposure during periods of punishing heat. Protect pots from excessive wind. Thirsty plants may require twice daily watering in sweltering heat. Ideally, drip irrigation can run to containers, but this restricts their repositioning.

Selecting heat-tolerant and drought-tolerant specimens makes good sense. Succulents are available in different colors and forms. Most are relatively inexpensive. A single Agave or Yucca can be quite attractive. Plants with grayish foliage or wooly leaves are almost always good choices for hot spaces.

This lovely Ixora is in a plastic container which has been inserted into a decorative clay pot. The double potting provides insulation against heat and prevents excess evaporation.