Garden Notes: Leguminous trees

Will we need their nitrogen-fixing properties more in the future?

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Honey Locust tree. –Susan Safford

Snowdrops and early crocus cheer, red-winged blackbirds sing, buds on lilacs enlarge. Stray flowers on flowering quince open. Look upward: Under a first-quarter moon, buds are visibly swelling and branch tips coloring. This has been quietly underway since January — a subtle process initially not much evident.

The ornamental trees and shrubs we add to landscapes are often chosen for their colorful bark and the interest they add to otherwise drab winter gardens. One of the most eye-catching is coral-bark maple, Acer palmatum ‘Sango Kaku.’ Shrubs with colorful canes — such as shrubby dogwoods and willows — are often coppiced (pruned hard) now, to produce young growth with the most colorful bark.

However, another group of trees, not particularly striking in winter, make an entirely different contribution to landscapes, and beyond. These are leguminous trees with symbiotic relationships to nitrogen-fixing bacteria, which allow them to harvest nitrogen from the air.

The bacteria inhabiting nodules on the leguminous trees’ roots facilitate this nitrogen-fixing ability, enabling them to draw gaseous nitrogen from the atmosphere and putting it into the soil and substance of the tree, just as a pea vine does in the vegetable garden. Thorny or spiny, many are native to harsh habitats that challenge their survival. I have been thinking about them.

I consulted “The Botanical Garden” (Vol. 1, Trees and Shrubs), by Roger Phillips and Martyn Rix (Firefly Books) on the Leguminosae. I also read “Trees and Carbon: What Is the Link?”

In some places these trees are already critical. For example, the gao tree (Faidherbia albida), a kind of acacia of sub-Saharan Niger and Mali, holds back the desert. Farmers plant their crops among them and feed the pods to their livestock. Their leaves add nitrogen to the lean soil through compost action, while the roots fix it deeply in the ground. Contrary to other trees, the gao lose their leaves during the rainy season, so the interplanted crops receive enough light to grow.

Visitors to the arid parts of the American West know mesquite, the stereotypical scrub vegetation of the Wild West range. How does mesquite (40-plus species of small leguminous trees in the genus Prosopis) survive? It takes nitrogen from the air and fixes it with the help of bacteria in its root nodules.

Mesquite is not unique, sharing with grander trees the ability to harvest nitrogen and, like trees and plants in general, carbon dioxide from the atmosphere. It provides nectar and pollen for pollinators, and edible pods for livestock and humans.

If you vacation in warmer places, you have probably admired flame-flowered poinciana (Delonix regia) or ethereal, mauve-flowered jacaranda (Dalbergia spp.), the rosewood of cabinetry. Exotic, but well-known on the Vineyard, is the pagoda tree (Styphnolobium japonicum, ex. Sophora japonica) of South Water Street. All three are leguminous.

Closer to home, familiar leguminous trees include salt-tolerant honey locust (Gleditsia triacanthos), the successional pasture invader, black locust (Robinia pseudoacacia), and American redbud (Cercis canadensis). Less well-known but admirable are Kentucky coffeetree (Gymnocladus dioica), yellowwood (Cladrastis lutea), and, looking like a transplant from the African veldt, the mimosa or albizia (Albizia julibrissin). For warmer U.S. hardiness zones, the carob tree (Ceratonia siliqua) provides many of the same amenities the gao tree does in Niger.

The “sequestration” refers to the drawing of gases out of the air and their incorporation into soil and plants. They are reaching imbalance, harmful to weather and climate, in Earth’s atmosphere. It is only by taking the excess carbon dioxide and excess nitrogen out of the atmosphere that unbalanced carbon and nitrogen cycles can normalize. Nitrogen and carbon sequestration address rebalancing the proportions of these gases.

There are many species of nitrogen-fixing trees that also provide numerous useful products and functions, including food, pollinator support, wind protection, shade, animal fodder, fuel, living fence, and timber, in addition to delivering nitrogen to the soil system. With their toughness and special properties, how much more important might these legume-y trees become?

What to grow

As has been observed before, some grow what is unobtainable, or too choice, to be commercially viable. Some just like to grow, and will choose curiosities that have no discernable kitchen value, for their sheer novelty, such as four o’clocks or ornamental gourds.

Some like to grow what is in daily or weekly use in the kitchen and menu. Garden-worthy plants lead to kitchen-worthy recipes lead to better cooking leads to better eating. Based on garden staples, Curried Butternut Squash Soup, from the “Silver Palate Cookbook,” is an uncomplicated, tasty standby:

Curried Butternut Squash Soup

Serves 4-6

4 Tbsp. sweet butter
2 cups finely chopped yellow onions
4-5 tsp. curry powder
Approximately 3 pounds altogether: butternut squash, peeled and chopped; two apples, peeled, cored, and chopped
3 cups chicken stock
1 cup apple juice
Salt and freshly ground pepper to taste
1 shredded, unpeeled Granny Smith apple for garnish

Melt butter in a pot. Add chopped onions and curry powder and cook, covered, over low heat until onions are tender, about 25 mins.

Meanwhile, peel the squash, remove seeds, and chop the flesh. When onions are tender, pour in the stock, add butternut squash and apples. Bring to a boil, reduce heat and simmer, partially covered, until butternut and apples are very tender, about 25 mins.

Pour the soup through a strainer, reserving liquid, and transfer the solids to a food processor bowl fitted with a steel blade, or use a food mill fitted with medium disc. Add one cup of the cooking stock and process until smooth.

Return puréed soup to pot and add apple juice and additional cooking liquid, about 2 cups, until soup is of the desired consistency. Season to taste with salt and pepper, simmer briefly to heat through, and serve immediately, garnished with shredded apple.