Heat-tolerant vegetable crops and cultivars for the changing climate

Our food-growing spaces allow us to grow healthy produce, connect with Nature, and hopefully save money. They are also a solid response to climate change and COVID.

My blog articles this year will be about climate-resilient food gardening. Each month I’ll address one or more aspects of how climate change is affecting our food gardens and changes we can make to reduce global warming and ensure a future of healthy harvests.

HGIC has a new Climate-Resilient Gardening section (thanks to Christa Carignan!) where you’ll find more information on these topics. We plan to continually update content and add new pages. And please check out the University of Maryland Extension’s new Healthy Garden, Healthy You project that connects food gardening and human health.

This first installment includes an overview of how our mid-Atlantic climate is changing and a look at heat-tolerant crops and cultivars. Future articles will explore low-dig soil prep, composting food scraps, peat alternatives, heat stress in plants, reducing plastics, and “hardening” our garden spaces.

Resiliency is mentioned a lot with respect to climate change. A climate-resilient garden can both withstand and recover from warmer, more extreme weather. Resiliency can also mean transforming how we grow food by creating and sharing a community knowledgebase of new ideas and techniques.

Warmer, wetter, wilder

Severe or unexpected weather has always been the biggest “beyond our control” challenge for farmers and gardeners around the world. Recent scientific reports show that climate change effects are “widespread, rapid, and intensifying” (IPCC- 6th Assessment Report). In the mid-Atlantic, the number of frost-free days is increasing, winters are warmer, “intense precipitation events” (>2 in. /24 hrs.) are becoming more frequent (warmer air holds more moisture), and coastal farmers are battling saltwater intrusion of cropland.

Here are highlights from the Capitol Weather Gang’s 2021 summary of Washington, D.C. weather:

  • 7 days of wind gusts >50 mph during March-May; some damaging >60 mph wind gusts
  • Numerous tornadoes from severe storms and Hurricane Ida
  • Coastal flooding partly from slow moving storms
  • 5th warmest year on record; Oct. and Dec. were each the 2nd warmest months on record
  • 48 days >90 ⁰F., 8 more days than the 1991-2020 average
  • 8 record-high minimum day temps which reflects the fact that nights are warming faster than days

It’s remarkable that the small, steady increases in average temperatures caused by humans over the past 200 years can produce such profound changes! 

Resilient crops and cultivars

Heatwaves, drought, hail, strong winds, and heavy downpours can all stress plants. Crops such as snap and lima bean, squash, pepper, and tomato are especially sensitive to heat stress at flowering and fruiting. Climate change resiliency in specific vegetable crops and cultivars often refers to heat tolerance, but can also be the ability to grow in low-moisture soil, or mature quickly before prolonged hot weather sets in. Selecting heat-tolerant crops and cultivars is one strategy for addressing warming temperatures. Other approaches include moving crops to shadier garden spots, planting earlier or later, and covering plants with shade cloth materials. Pay close attention to seed catalog descriptions. Some companies have a “heat-tolerant” page or section.

Heat-tolerant warm-season crops to try

Southern peas (cowpeas) and yardlong (asparagus) beans come in a variety of fruit and seed colors and patterns. They tolerate hot, dry weather and fix nitrogen from the air, providing your soil with “free” nitrogen after plants decompose. Look for cultivars that can be trellised to save space.

blackeye pea plants
Blackeye pea plants

Okra makes beautiful flowers and an abundance of fruit pods through frost. All parts are edible.

Sweet potato is a durable storage crop plus you can harvest and eat young leaves and shoot tips during the entire growing season. Save space by growing plants vertically. (Also, see the video: How to Start and Multiply Sweet Potato Plants.)

Sweet potato plants growing on a metal support structure in the
Master Gardener Learning Garden at the Maryland State Fair.

Heat-tolerant tomato cultivars

Hybrids: Summer Set, Sun Leaper, Solar Set, Sun Sugar, Red Bounty, Phoenix, Heatmaster, Solar Fire, Sanibel, Florida 91

Open-pollinated: Creole, Homestead, Roma, Arkansas Traveler, Porter

Some commercial tomato growers in the mid-Atlantic are observing reduced fruiting and fruits with yellow shoulders and white internal tissue caused in large part by heat stress. For home gardeners, this is probably more likely to occur in heavily pruned determinate cultivars grown in full sun, especially in urban/suburban locations with a pronounced heat-island effect. There is much research and breeding work underway to develop cultivars that can tolerate heat stress.

Quick-maturing tomato cultivars

Is your goal is to start harvesting long before sweltering summer weather? There are many fast-maturing (55-65 days from transplanting) cultivars that will typically produce a lot of fruit by late July. Early Girl, 4th of July, Moskvich, and cultivars with “Oregon” in their name are a few examples. Cherry and pear tomatoes are often fast maturing. Juliet is a 65-day, grape-shaped hybrid tomato that produces big crops of perfect fruits.

Heat-tolerant lettuces

Most lettuces will bolt when temperatures are >85 ⁰F. Crisphead (iceberg), oakleaf type lettuces, Merlot, Bronze Arrow, Bronze Beauty, and Jericho are more heat-tolerant. Dutch breeders developed the Salanova series that includes some heat-tolerant cultivars, such as Muir, Nevada, and Cherokee. Johnny’s Selected Seeds is the sole U.S. distributor.

Heat-tolerant broccoli

Cold weather can force spring-planted broccoli to bolt and high heat damages broccoli buds. The Eastern Broccoli Project is a decades-long effort to increase commercial broccoli production in the Eastern U.S. A number of heat-tolerant cultivars have been developed. University of Delaware researchers found good heat tolerance in Eastern Crown, Millennium, and Green Magic.

Heat-tolerant greens

Check seed catalogs for mild-flavored leafy Asian mustards like Vitamin Green that hold up well in warm weather. Callaloo (Amaranthus viridis) leaves and succulent stems grow abundantly throughout the summer and early fall and can be prepared and used like spinach.

Callaloo growing at the UMES Education and Demonstration Farm in Princess Anne.

Heat-tolerant beans

High temperatures are interfering with the pollination/fertilization of lima bean and snap bean flowers and reducing yields. University of Delaware researchers are finding that high night temperatures are more responsible than day-time warming for this problem. See research results in the references below.

Speaking of seeds…

January 29th is National Seed Swap Day! Some area seed swaps have already happened. There is one upcoming event, the seed swap hosted by Kathy Jentz and Washington Gardener Magazine, at Brookside Gardens on February 22. Find details and registration information here: http://seedswapday.blogspot.com/.

Past blog articles about vegetable seeds

Seeds and Seed Catalog Jargon
Are Cheaper Vegetable and Flower Seeds Just As Good As More Expensive Seeds?
Seeds to Die For

References

Climate Change in Maryland (UME)
D.C.’s second-warmest December on record caps fifth-warmest year
Mid-Atlantic Regional Climate Impacts Summary and Outlook: Fall 2021
NOAA State Climate Summaries
UDEL- Heat Stress Trial With Tomato
UDEL- Heat Tolerant Vegetable Varieties
Genetic and Molecular Mechanisms Conferring Heat Stress Tolerance in Tomato Plants

By Jon Traunfeld, Extension Specialist, University of Maryland Extension, Home & Garden Information Center. Read more posts by Jon.

5 thoughts on “Heat-tolerant vegetable crops and cultivars for the changing climate

  1. kent phillips January 21, 2022 / 9:42 am

    Great blog Jon. I’ve grown Green Magic broccoli the last two years in the spring and had great success with it.

  2. grahambackyard February 5, 2022 / 11:25 am

    Thank you for your article on heat-tolerant vegetable plants. I got a lot of information from your writing which will be useful for my gardening.

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