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.


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.

Vegetable Crops to Plant Now

Mid-September is a transition time for vegetable gardeners. You may be doing some garden clean-up, recording the successes and failures you were too busy to think about when they happened, and deciding how to manage the soil this fall and winter. We’re also slowly removing the warm season plants that are well-past their prime and wondering how to keep the fresh produce coming! It’s too late to plant peas (they rarely do well in the fall) or a late crop of broccoli or cabbage. So what to do with the open beds and spaces that won’t be planted in garlic, leeks, and shallots or cover crops?

There’s a long list of crops, mostly leafy greens, which you can plant now and harvest before and after frost arrives. And you can probably find seed packets for some crops in local garden centers, hardware stores, and food markets:

  • Leaf lettuce, spinach, radish
  • Broccoli raab (rapini), kales, collards, mustards, arugula
  • All sorts of Asian greens such as tatsoi, mizuna, mibuna, komatsuna, hon tsai tai, autumn poem, and Chinese broccoli
  • Turnips produce delicious fall greens. Some of the quicker maturing cultivars like ‘Hakurei,’ planted in early-mid September, will have enough time to make turnips before frost

Mixed fall greens in a Salad Box
Mixed fall greens in a Salad Box. Photo credit: Jon Traunfeld, UME

Making it Work

Start by removing crop debris and raking the soil so that it’s relatively smooth. Spread an inch or so of compost or rake in 2 lbs. of cottonseed meal (6-2-1) or equivalent fertilizer per 100 sq. ft. of growing area. If you practice no-till techniques simply move mulch to the side and drag a garden tool, tool handle, or stick through the soil to make a shallow furrow. Wet the ground prior to planting if the soil is dry. Seeds will germinate quickly in warm, moist soil.

No-till planting- mulch is moved to the sides and a stick makes the furrow Photo credit: Jon Traunfeld, UME
No-till planting- mulch is moved to the sides and a stick makes the furrow.
Photo credit: Jon Traunfeld, UME

Fall gardening means shorter days, lower sun angle, and less intense sunlight so it helps to increase the recommended space between seeds. Cover the planted rows, beds, and containers with a floating row cover to reduce insect feeding, increase plant growth, protect against frost injury, and extend the harvest period. The cover can float on top of the crop or be draped over a frame. Secure the cover to the ground with rocks, pins, bricks, or boards. Consider using heavier floating row covers in October/November, and for overwintering crops, that give 5-10⁰ F. of frost protection. Type “floating row covers” into an online search to find a variety of sellers.

: #9 wire makes an excellent support for floating row covers Photo credit: Jon Traunfeld, UME
#9 wire makes an excellent support for floating row covers.
Photo credit: Jon Traunfeld, UME

Floating row covers protect fall crops at the Howard Co. Conservancy Community Garden Photo credit: Jon Traunfeld, UME
Floating row covers protect fall crops at the Howard Co. Conservancy Community Garden.
Photo credit: Jon Traunfeld, UME

Water regularly, thin plants if needed, and start harvesting leafy greens, either by breaking off outer leaves or cutting plants to the ground to re-grow. Kale, spinach, and arugula, will overwinter reliably with floating row cover protection in all but the coldest areas of Maryland (would love to hear from Western Maryland gardeners about their experiences). These crops will re-grow impressively in spring.

Overwintered spinach coming back to life in spring Photo credit: Jon Traunfeld, UME
Overwintered spinach coming back to life in spring. Photo credit: Jon Traunfeld, UME

Don’t delay- the time is now! Taking advantage of longer, warmer fall weather for growing vegetable crops is a smart climate change adaption strategy.

By Jon Traunfeld, Extension Specialist