Carol O'Meara - Colorado State University Extension

Carol O’Meara – Colorado State University Extension

If you were planning on picking a peck of peppers this year but the plants are not cooperating, chances are your garden is suffering from heat stress. In this, you’re not alone: reports of poorly performing peppers, and in some cases eggplant, are coming in from around the area.

Even the pros are seeing a difference in pepper growth and lamenting the slow growth of one of our favorite crops.

“I started all of my peppers and had all of these gorgeous transplants I put out. They looked great for the first 10 days and I was thinking ‘man this is going to be a beautiful year.’ But then they went south; they starting going backwards on me and becoming stunted,” says Dr. Mike Bartolo, senior research scientist and manager of the Arkansas Valley Research Center in Rocky Ford. He’s been a leading researcher in the development of chile peppers since 1992.

High temperatures hit us in early June, and that month saw eight days well above 90 degrees (measured at the Boulder airport). This alone shouldn’t have been a problem for the plants, but this is Colorado after all, and May had sent us snowy, wet weather that delayed planting.

The combination of late start and early heat has resulted in many peppers and eggplants stalling. “I remember having this happen about five or six years ago, where we had six or seven days above 100 in May. But that still wasn’t as bad as this year. They’re just not catching up,” he said.

Heat stress reduces plant’s ability to photosynthesize and cool itself efficiently, which in turn limits root development, according to University of Nebraska Extension Water Resources Engineer Suat Irmak. Persistently higher temperature s this summer – 22 days as of July 24 – plus limited root growth has left these plants languishing.

“As hot as it’s been everywhere, we’re seeing a lot of this.  But it seems like it’s impacting the sweet and bell peppers more; they’re really affected. Chiles are handling it a lot better,” said Bartolo. “In some cases the stems got sunscald and developed woody callous tissue over the burn. Now that’s interfering with the flow of water through the plant and making things worse.”

Bartolo’s research into preventing sunscald on the fruit of peppers led him to using 30-percent shade barrier over some plants to protect the fruit from harmful rays. But now that shade is proving effective in shielding plants from high temperatures as well – the transplants he covered with shade in early June aren’t showing the stress the exposed peppers are.

Locally, peppers covered with shade cloth or floating row covers are also in much better shape. Popping a tent of 30-percent cloth over your crop might help get things growing again, Bartolo said. “As long as the stem is okay and not injured by sunscald, things should get moving again once things cool down or you shade the plant. It’s possible that they’ll recover.”

Shade cloth is available at garden centers in various lengths. It’s a durable fabric that can be used for several years. When covering your plants, remember that this is to be suspended above them, unlike floating row covers that can sit right on the plants. Create a frame for holding the shade barrier above the plants, taking care that it’s secure for wind.

Floating row covers provide about 15 to 19-percent shade, which might not be enough at this time to break the grip of heat. Early in the season it would help for root development, so keep it handy for next year.

Colorado State University Extension, together with Boulder County Parks and Open Space, provides unbiased, research-based information about consumer and family issues, horticulture, natural resources, agriculture and 4-H youth development. For more information contact Extension at the Boulder County Fairgrounds, 9595 Nelson Rd., Box B, Longmont, 303.678.6238, e-mail [email protected] or visit