heat

Heat-tolerant greens

June 17, 2022June 17, 2022Maryland Grows3 Comments

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.

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.

Plants not behaving as expected: vegetable garden edition

October 1, 2021January 28, 2022Maryland GrowsLeave a comment

Two of the vegetable crops I grew this year are known for loving the heat: okra and eggplant. I grow eggplant in pots on my deck, to avoid flea beetle infestation, and okra directly in the ground in my community garden plot. Both of them produced adequately over the summer. Now it’s fall; we’re having days in the 70s and nights in the 50s, and there are fewer hours of sunlight in the day. Time to pull the summer crops, right?

Except – boom! Both the okra and the eggplant are going gangbusters. More flowers, more fruits than in the hot summer months, by far.

‘Bride’ eggplant on a cool autumn morning

So why aren’t these plants following the rulebook? Do they not know how to read? Or have the rules changed?

Continue reading →

How plants cope with soaring temperatures

July 23, 2021December 17, 2021Maryland GrowsLeave a comment

Hot enough for ya? It’s only July, but we’ve had more than our share of relentless heat.

Have you ever wondered how plants cope with heat? It’s not as if they can turn on their air conditioners or pour themselves a cold one. In fact, plants have myriad adaptations that help them survive high temperatures. Some involve managing heat while others focus on conserving water.

Many coping mechanisms are structural. Plants such as sedum have waxy leaves to conserve water. Fuzzy lamb’s ear has reflective leaf hairs.
Ornamental grasses’ rolled leaves give them an advantage as does threadleaf coreopsis’ smaller, finer leaves. Less surface area means leaves lose less water.
Lavender, Russian sage and other plants with bluish leaves are summer survivors, too.
Plants with thick roots such as iris, peonies and daylilies store water better. And native plants’ deeper roots find water more easily.

Orange butteryfly weed flower with butterfly — Drought-resistant native plants like this butterfly weed (Aesclepias tuberosa) also attract pollinators such as this zebra swallowtail. Photo credit – Joan Willoughby

Purple bee balm flower — Drought-resistant native plants like this butterfly weed (Aesclepias tuberosa) also attract pollinators such as this zebra swallowtail. Photo credit – Joan Willoughby

All of these evolutionary adaptations help plants tolerate hot, dry conditions. We know that summer’s heat comes every year and that global warming is bringing more temperature extremes. So it makes sense to help our gardens adapt by incorporating plants with these characteristics.

Okay, science geeks. Here’s one for you. Did you know that some plants can make special “heat-shock proteins” to help them recover from heat stress? When you cook an egg, you are unfolding proteins. When you melt butter, you are disrupting cell membranes. These same disruptions can happen when plants get too hot. Cell membranes can literally melt, leaking plant’s vital fluids. Heat-shock proteins act like “molecular chaperones,” preventing these bad things from happening at a cellular level. They beef up membranes and collapsing proteins. Plants survive.

Purple iris flower — Iris’ fleshy rhizomes help it hold water and thrive in high temperatures. Photo credit – Marie Bikle, Washington County Master Gardener

But don’t make plants go it alone, relying only on their adaptations. Help them when it’s hot by watering them more often and deeply. Newer plantings of trees and shrubs need slow, deep soaks once a week. Use a hose on a trickle, a soaker hose, drip irrigation or a 5-gallon bucket with nail holes in the base.

Container plants heat up and dry out faster, so check and water them once or twice a day. Soak them until water runs out the drainage holes.

Keep plants mulched to conserve moisture. Consider shade covers on vegetable crops. And be vigilant, watching your plants for signs of heat stress such as wilting.

Water moves constantly from the soil to roots, stems and leaves. There it escapes through leaf pores. When the rate of water lost is greater than the water absorbed, plants wilt and need water.

Trees often jettison some leaves to conserve water when it’s hot. Fewer leaves need less water. Unless leaf loss is dramatic, there is no cause for concern.

Vegetable plants slow production in high heat. Blossoms drop when temps top 80 degrees. Without blossoms, plants can’t make fruit.

Tomatoes, squash, peppers, melons, cucumbers and beans are likely to drop blossoms. It’s a passing phase. Plants will make flowers and fruit again when temperatures cool.

Green peppers — Summer’s high temperatures cause blossoms on some vegetables such as these peppers to drop, temporarily slowing production. Photo credit – Home & Garden Information Center

Plants have developed miraculous adaptations to high temperatures, but sometimes need our help. So watch, water and marvel at the many ways nature finds ways to beat the heat.

Annette Cormany, horticulture educator, University of Maryland Extension – Washington County

Food gardening in summer heat

August 7, 2020June 24, 2022Maryland Grows1 Comment

“Hot enough for you?”

That used to be a summer joke, right? I haven’t heard it in a while. We all know it’s way too hot out there. This time of year, with the heat and humidity and bugs and weeds, it’s a challenge even to step into the garden and do what needs to be done. But if we ignore our garden tasks they just get more overwhelming. I’m overwhelmed myself, but let me try to give you a few hints on making summer in the vegetable garden more bearable.

Even plants like eggplant that like the heat can fail to form flowers or fruits when it gets really hot. A little afternoon shade helps.