July is the warmest month of the Wisconsin year, with average high temperatures peaking in the mid-80s. Heat and humidity, intense sunlight, long daylength, drying warm winds, and possible intense rains affect garden growth and survival. Many native plants illustrate adaptations in form, features, leaf orientation, and physiology that allow them to live and thrive under sometimes stressful mid-summer conditions.

In full sun, plants with many narrow leaves and vertical leaf orientation have less surface area exposed directly to the sun. This allows for efficient light capture while reducing heat load. Grasses are an excellent example, but forbs such as prairie coreopsis (Coreopsis palmata), harebell (Campanula rotundifolia), whorled milkweed (Asclepias verticillata), and blazing star (Liatris spp.) illustrate this with finely divided, narrow, or vertical leaf blades.

A plant’s color and surface features may reduce heat load as well. Leaves with waxy surfaces, such as rattlesnake master (Eryngium yuccifolium) and large-flowered penstemon (Penstemon grandiflorus), reflect some sunlight and reduce water loss. Plants with hairs covering leaves or stems reflect sun and reduce heat load. Examples include leadplant (Amorpha canescens), white sage (Artemisia ludoviciana), prairie cinquefoil (Drymocallis arguta), and silky aster (Symphiotrichum sericeum).

Warm season plants produce most of their biomass during the hottest months of the summer, including July. They use a process called the C4 photosynthetic pathway that more efficiently fixes carbon dioxide and helps them reduce energy loss. Think of the rapid growth of prairie grasses like big bluestem (Andropogon gerardii), Indian grass (Sorghastrum nutans), switchgrass (Panicum virgatum), and vigorous ground-hugging summer weeds like purslane (Portulaca oleracea) and crabgrass (Digitaria spp).

During extreme heat and drought, plants with one or more of these adaptations that also grow on exposed rock walls and in gravel soils have appeared to die in July. But this is simply another adaptation – entering complete dormancy mid-season. These same individual plants showed normal growth the following season under wetter conditions.

At ground level, a shaded garden in mid-summer has limited light. By July, most woodland understory leaves are held horizontally. This maximizes light capture for photosynthesis, with little heat load for the shaded leaves.

The native garden in July demonstrates how regional plant species are suited for a changing climate, and also which plants to select when following sustainable gardening practices.

Learn more at an “open garden” summer session with Susan Carpenter in the Native Plant Garden on Saturday, August 15, from 9 a.m. to 12 p.m. This is an opportunity to bring questions from your home garden and get advice and inspiration. To register for this free event, email Susan Carpenter, Native Plant Garden curator, by August 6.

Enjoy the peak of summer observing, and gardening with native plants.

– Susan Carpenter, Native Plant Garden curator