Hot Plants That are “Cool”

Anyone who has lived in our fair state for some time can attest to the fact that Missouri summers can be brutal. Excessive heat and lack of rain are quite typical of July and August in the “Show Me” state. Several years ago the United States Department of Agriculture released a “heat zone” map--the high temperature equivalent to the plant hardiness zone map. This map shows the majority of Missouri to be in a zone characterized by summers that (normally) contain up to 90 days having temperatures that reach 30o C (86o F). This coupled with the temperature extremes reached during our summers equate to some very harsh conditions for our yards and gardens, not to mention their owners.

Many of the annual ornamental plants traditionally grown in Missouri have been “borrowed” from Europe. Those of us who can trace our heritage back to that continent can thank our ancestors for bringing with them the love of these flowering plants and passing it on to their offspring. With a few exceptions, the climate of European countries is less severe from the standpoint of temperature and precipitation than is Missouri’s climate. Thus, many of the annual flowering plants we attempt to nurse through the summer would really be much happier a few states further north. While they might be quite attractive in the cooler months of spring, they decline significantly when the summer warms.

Fortunately, there are attractive annuals native to warmer climates. These species thrive in high temperatures and (conversely) do not like the cool months of spring and fall. Given proper care, they will be at their showiest when cool-loving plants start to decline because of the heat. Unfortunately, many of us are unfamiliar with these species and have not tried them in our gardens. Here is where the experiences of others can be helpful as we plan our own gardens.

For the past several years, trial plots have been planted at the MU Turf Research Center, located near Columbia, Mo. to evaluate the summer performance of ornamental herbaceous plants. The plants are given the care typical of a home garden and rated on three different dates, to evaluate early, mid-season and late-season performance. From the results of these trials as well as from the experiences of others, it can be concluded that there are a good number of ornamental herbaceous plants that thrive under the high temperatures typical of a Missouri summer. Certain of these are cultivars of species that have been bred for heat tolerance which normally prefer cooler temperatures; others are species of plants whose native habitats are characterized by hot summers. Table 1 lists some of the species quite tolerant of heat and deserving of more attention than they currently are receiving by Missouri gardeners.

A familiar saying goes “everybody talks about the weather but no one does anything about it.” This statement is only partially true for gardeners. Those of us who are serious about enjoying the beauty of ornamental plants during the heat of a Missouri summer can do so by proper selection of our plant material.

Table 1 Herbaceous Ornamentals Tolerant of Hot Weather
Amaranthus	Gazania	Oleander
Bougainvillea	Gomphrena	Portulaca
Canna	Hibiscus	Sanvitalia
Celosia	Mandevilla	Scaevola
Cleome	Lantana	Sweet Potato
Cosmos	Marigold	Tithonia
Cuphea	Melampoduim	Vinca
Flowering purslane	Nicotiana	Zinnia

David Trinklein, Associate Professor, Horticulturist, UMC (573) 882-9631