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.

Don't Discard Your Fall Mums

Long-flowering, colorful Mums are ideal for adding spots of bright color to the autumn garden. There are 30 different species in the Chrysanthemum genus, broadly categorized into “florist” mums and “hardy” mums. They are further broken down into different types of blooms: cactus, cushion, anemone form, incurved, intermediate, Korean, pom-pom, spider, reflex, single, others.

All Chrysanthemums demand full sun, rich soil, and regular water. Container specimens must have good drainage to prevent root rot. Plants have a shallow root system, and may need daily irrigation during the hottest part of the summer. Flowers come in a wide range of colors: yellow, orange, burgundy, purple, bi-colors. Good air circulation will prevent leaf issues. Plants are perennial in zones 5-9.

In garden beds, pinch plants back often in spring to keep them compact and get more blooms. Stop pinching in late June so they have time to develop bloom buds. Bud formation is triggered by the shorter days of late summer and early fall. Divide plants in spring, every two or three years, discarding the woody interior section and replanting the vigorous outer portions. Plants can be propagated from cuttings.

Potted (“florist”) Chrysanthemums are found at big box stores and grocery stores in the fall. These have been greenhouse bred and chemically treated to keep them short and light manipulated to make them bloom on cue. They are typically not as cold-hardy as the selections grown for borders. Gardeners in warmer zones can keep them in a freeze-free area through winter, then plant them out into the garden in spring. Some will survive and return the following year, but they won’t ever be compact like their original supermarket form. Plants may bloom much earlier (May or June) and be much taller (24-30”).

Chrysanthemums are toxic to humans, horses, dogs, and cats. Their pungent foliage makes them less likely to be browsed by deer or rabbits.

A Groundcover as Container Specimen

When is a groundcover not used as a live mulch? When it is a container plant, of course. Lysmachia congestiflora ‘Walkabout Sunset’ has a beautiful, multi-colored leaf and clusters of bright yellow blooms. Initially the common name, Loosestrife, frightened me away from using this plant because I feared it might be invasive. I’m happy to report that this is not the case.

Several cultivars are available. All are low growing (to six inches) and will spread readily since they root wherever stem touches soil. This spreading habit makes it a great choice for edging in-ground water features. Lysmachia is a semi-evergreen perennial in zones 7b to 9a  In my 7b garden, I find ‘Walkabout Sunset’ sheds most leaves and looks dead by late winter but makes a rapid recovery when spring arrives. It will not tolerate temperatures below 15 degrees, so I take a small cutting in autumn and overwinter it in the greenhouse as added insurance against plant loss.

‘Walkabout Sunset’ performs best in part-sun, with a little shade in the hottest part of the day. It prefers damp to wet soil and is undemanding as to soil pH. I sited a potted specimen in full sun and found that it needed more additional water than I was willing to donate, so I relocated it to the raised portion of a birdbath. This solution kept the plant roots damp and gave birds and insects a place to perch when they visited the water source. Everyone was happy.

‘Walkabout Sunset’ is equally attractive when used as a sole container specimen or as an undercover to a taller plant. The variegated leaves have a blue-green stripe that pairs well with a similarly toned container.

You will note that I did not include the pronunciation above, as I normally do. Here goes: ly-si-MAK-ee-uh kon-jes-tih-FLOR-uh. I have observed that some well-known plant experts pronounce the third syllable with a long A while other make it a short A. I thought it would be easier to stick with the cultivar name, but then I found some plants tagged as ‘Walkabout Sunset’ and others as ‘Outback Sunset’ or ‘Waikiki Sunset.’ Are they the same? I cannot tell. What I can tell is that ‘Persian Chocolate’ has purple-ish foliage. Can’t wait to try it in a container with pink Begonias.

Foliage color mirrors container

Birdbath keeps soil wet

Heuchera for Year-Round Beauty

When the winter garden is clothed in dormant twigs and fallen leaves, a touch of lively color lifts the spirit and reminds us that spring lurks ahead. Heuchera is a colorful group of perennials that are hardy from zones 4 to 9, depending upon variety and lineage. Modern cultivars have parents in H. villosa and H. americana. In the south, Heucheras (common names: Coral Bell, Alumroot) withstand hot and cold temperatures and look good every month of the year. Tiny blooms, a favorite of butterflies, are held atop wiry stems, but Heucheras are grown for their outstanding foliage. Cultivars have caramel, rose, lime, purple or green leaves marked with silver, red or white.

The lovely chartreuse green ‘Citronelle’ is one of the most heat tolerant cultivars. Grow it in shade to partial shade, where the leaves will be more yellow than green. Full sun leads to crispy edges and bleached leaves. I planted dozens of these in a lightly shaded bed. Within three years, there was only a single survivor. Each one got smaller and smaller, dwindling down to a single leaf before disappearing completely. I finally discovered the key to success – well-drained soil. The native heavy clay soil of upstate South Carolina mandated container culture, enhanced by fertile, well-drained potting soil and frequent watering. I plucked the lone ‘Citronelle’ specimen from the ground and placed it in a container with two other Heucheras, a purple-leaf variety (lost the cultivar name) and ‘Caramel,’ with caramel-colored leaves. Instant success! I eventually moved ‘Citronelle’ to her own container and replaced her with ‘Key Lime Rickey.’ These three live shoulder-to-shoulder in a container which looks as good in February as in August. Two ‘Amber Waves’ plants look especially good in a pair of peachy-toned clay pots along a brick walkway. These are sited in full sun. (Evidently, they did not receive the notification that they were meant for shade.)

A trio of Heucheras (‘Caramel,’ ‘Key Lime Rickey,’ and an unidentified purple leaf cultivar). These three live in harmony all year long. The container looks this good even in winter.

Plant breeders introduce new cultivars every year, with an ever-broadening range of color and vein combinations. One thing I have learned about this wonderful plant: Catalog photos are taken when plants are at their very peak of beauty. Plants that exhibit gorgeous apricot and rose shades in spring (such as ‘Sweet Tea’ or ‘Peach Flambe’) may mature darker, even dirty-looking by August or September. My beloved ‘Berry Smoothie’ is raspberry-rose in spring but darkens to purple in winter. In catalogs, ‘Stop Light,’ is a beauty, with yellow-lime leaves marked with bright red veins. I have never been able to replicate the distinctive markings in my garden and had to settle for a red vein here and there. Purple-leaf forms age a bit better. ‘Obsidian’ is almost black. It looks especially good in containers with pink Begonias or Caladiums. Heucheras also pair well with smaller grasses, such as Lagurus ovatus ‘Bunny Tails.’

If a mature Heuchera develops an elongated stem, it is time to lift it and replant it to position the woody section under soil level. Remove tattered or crispy leaves at any time to keep the plant looking tidy. Heucheras are rarely browsed by deer or rabbits.

Finally, Heuchera is pronounced HEW-ker-ah. This one is mispronounced even more than Liriope. The first syllable rhymes with Yew. Get it right and you have risen to the upper echelon of gardeners.

'Amber Waves' ruffled, peachy leaves

'Berry Smoothie' plum and silver leaves