Heat-tolerant greens

Tender spring lettuce and spinach leaves are just a memory for many Maryland gardeners. As we move into the summer season the types and flavors of garden greens expands significantly. While some, like Swiss chard and kale, can be cut or torn and dropped into fresh salads and dishes, most will benefit from some level of cooking, like sautéing on their own or being added to stovetop or baked dishes. Most of the summer greens below grow quickly and have a long harvest period. They help us improve food security and adapt to climate change.

General growing tips for summer greens:

  • Water, water, water and fertilize to promote rapid, healthy growth and  maintain leaf and stem succulence
  • No row cover! They can cause a heat build-up. Instead, use insect netting to exclude insect pests
  • Plant summer greens in containers and move them to shady spots close to your front or back door 
  • Create some shade for lettuces and other marginal crops like cilantro … plant on the north side of taller crops or try 30% to 50% shade cloth material 
  • Most leafy greens below can be treated as cut-and-come-again crops: they put on new growth below each harvesting cut. Older, stressed foliage is less palatable
  • Explore seed racks and online seed catalogs for heat-tolerant crops and varieties

Leafy amaranth

Two well-adapted species for Maryland gardens are Amaranthus viridis (callaloo, also known as slender amaranth) and Amaranthus tricolor (Chinese spinach; leaves somewhat smaller than A. viridis).

Several species are very popular in Central and South America, India, Southeast Asia, and Africa. Nutritionally, they compare favorably with spinach and Swiss chard. Plants in this family use a special C4 photosynthetic pathway, also present in corn and sugarcane, which allows for vigorous growth under hot, dry growing conditions. 

Leafy amaranth is basically a tasty and productive pigweed. Flowering accelerates with shorter days after the summer solstice. Frequent harvesting delays flowering and promotes branching. Immediately remove any flower stalks that emerge to prevent re-seeding.

Callaloo growing in 3-gallon bags in a high tunnel. UME Small Farms Program
Photo credit: Jon Traunfeld

Callaloo can quickly become a weed problem. Don’t let plants flower. Each plant can produce >100,000 tiny seeds dispersed by water, wind, tools, and animals. Photo credit: Jon Traunfeld

Tri-color amaranth. Photo credit: Jon Traunfeld

Other heat-tolerant greens in the Amaranthus family:

Swiss chard and Perpetual Spinach (a.k.a. leaf beet) fall within the beet species- Beta vulgaris- and will produce large amounts of leafy goodness from spring through early fall. Orach (Atriplex hortensis) is another family member that grows best in spring and fall but can tolerate summer heat.

Two varieties of Swiss chard in the UMD Community Learning Garden. Photo credit: Jon Traunfeld

Leafy Brassicas

Several crops in the Brassicaceae plant family tolerate Maryland summers. Collard plants produce reliable and abundant harvests from summer through fall. ‘Morris Heading’ is an heirloom “cabbage-collard” variety found in Baltimore City community gardens throughout the growing season. ‘Green Glaze’ is touted as being heat-tolerant but I am not aware of studies that looked at temperature effects on the productivity of collard varieties. 

Collard plant in late July surrounded by common purslane, and edible weed. Photo credit: Jon Traunfeld

Mustard and kale are somewhat less heat-tolerant than collards. ‘Green Wave’ and ‘Red Giant’ mustards and ‘Lacinato’ kale are common varieties grown in summer gardens in Maryland. I’m very curious about Portuguese kale (Couve tronchuda). It resembles collard and is described as sweet and tender and more heat-tolerant than other kales. If you grow it please leave a comment about your experience.

‘Lacinato’ kale (a.k.a Tuscan kale, dinosaur kale, black kale) growing in Baltimore City in late July. Leaves are harvested from the bottom. Photo credit: Jon Traunfeld

Check seed catalogs for mild-flavored leafy Asian greens that hold up well in warm weather like Tokyo Bekana (Brassica rapa var. Chinensis), ‘Komatsuna’ (Brassica rapa var. perviridis), Vitamin Green and Tatsoi (Brassica rapa Napa group), and ‘Chijimisai’ (tatsoi x komatsuna). 

Malabar spinach

Basella alba (green stem) and Basella rubra (red stem) below are “summer spinaches” that produces a vigorous leafy vine. Leaves and stems can be sautéed or used to thicken soups and stews.

New Zealand spinach

New Zealand spinach (Tetragonia tetragonoides) below is a low growing annual with a spreading habit that has somewhat fuzzy, arrow shaped leaves, and mild spinach flavor.

Photo credits: Jon Traunfeld

Molokhia (Corchorus olitorius), known as Egyptian spinach and jute leaf, is an important food plant in the Middle East, Africa, and Asia. It’s higher in vitamins and minerals than most other leafy greens. This is the jute plant, known for its strong stem fibers. Young leaves can be eaten fresh, sautéed, or used to thicken soups and stews.

Sweet potato (Ipomoea batatas) – shoot tips, young leaves, and tender stems are excellent in many top-of-the-stove dishes. Harvesting young foliage, even on a regular basis, will not reduce your harvest of sweet potato roots later in the season.

Sweet potato plants with a less typical cut-leaf shape. Photo credit: Jon Traunfeld

Hibiscus as a Leafy Green?

Sunset hibiscus (Abelmoschus manihot) and roselle hibiscus (Hibiscus sabdariffa) leaves have a compelling lemon-sour flavor similar to garden sorrel. These plants are in the Malvaceae family along with cotton and okra, planted throughout the tropics and sub-tropics. Roselle is also grown for its strong fibers and its fleshy calyx which farmers and gardeners use to make tea, juice, and preserves.

Green stem hibiscus plant harvested for its leaves used in many Indian foods. 
Photo credit: Jon Traunfeld

Don’t Dismiss the Lettuces!

Lettuces are generally a cool-season crop but these varieties have demonstrated some level of heat tolerance:

‘Merlot,’ ‘Speckled Bibb,’ ‘Adriana,’ ‘Jericho,’, ‘Coastal Star,’ ‘New Red Fire,’ ‘Starfighter,’ ‘Tropicana,’ ‘Red Cross,’ ‘Magenta,’ ‘Cherokee,’ ‘Green Star,’ ‘Summer Crisp,’ ‘Little Gem,’ ‘Muir,’ ‘Burgundy,’ ‘Bronze Beauty,’ ‘Forlina,’ and most oakleaf types of leaf lettuce. Asian sword leaf lettuces, (pointed lettuce) have long, thin leaves and are described as crisp and tender with a mild bitterness.

One of the sword lettuces growing mid-summer in a Howard Co. community garden. Photo credit: Jon Traunfeld

An Auburn University study found that ‘Aerostar’, ‘Monte Carlo’, ‘Nevada’, ‘Parris Island’, ‘Rex’, ‘Salvius’, and ‘Sparx’ grown in a “hot greenhouse” out-performed 10 other heat-tolerant lettuce cultivars and received the highest flavor and texture ratings.

Give some of these greens a try this summer. The investment in seed, space, and time is minimal and you may discover some surprising new textures and flavors.

Resources:

Callaloo recipes– Dr.Nadine Burton, Alternative Crops Specialist, UMES Extension

The Difference Between C3 and C4 Plants– University of Illinois

The Heirloom Collard Project collects and increases collard seed and documents the history of many American South varieties 

Malabar spinach, Basella alba. University of Wisconsin

Lettuce All Year in a Changing Climate– Sustainable Market Farming- Pam Dawling

World Vegetables: Principles, Production, and Nutritive Values. V. Rubatzky, M. Yamaguchi. 1997

By Jon Traunfeld, Extension Specialist, University of Maryland Extension, Home & Garden Information Center.

Read more posts by Jon.

3 thoughts on “Heat-tolerant greens

  1. Cristina Sovereign June 17, 2022 / 5:07 pm

    What an excellent topic for adapting to global warming! I will try these ideas next year! Could these be added to the planting calendar? BTW malabar spinach readily self sows so be careful about letting it go to seed.

    • jontraunfeld June 27, 2022 / 10:45 am

      Yes- we’ll look into adding them.
      Thanks for the heads-up on the Malabar spinach.

  2. M. Boley July 7, 2022 / 9:44 am

    Great post! Some really new and relevant information in the face of our warming climate.

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