Green Scene

Get a whiff while you can: Exploring black locust trees


You know how some mornings you open the front door to pick up the newspaper lying on your front steps and the first thing that you notice is a heavenly scent on the air? 

Right now is one of those days. The wisteria, which contributes to the spring perfume, has finished blooming, but those magnificent bundles of purple flowers — racemes — are being duplicated by the white flowers on the black locust trees (Robinia pseudoacacia). 

If we are lucky, we will be enjoying that perfume for several days. But invariably it rains, and those petals drop off the trees creating a thick, white carpet on the ground. Then we must patiently wait another 12 months for the next whiff.

Wisteria is one of those beautiful vines that can be dangerous, however, to your gardening health. They are extremely invasive, and in return for that fabulous spring display, you will spend hours pulling the travelling vines down out of your nearby trees. Right now, we have a neighborhood vine that is attempting to gobble up the entire block.

Wisteria and black locust trees are all members of the Fabaceae — formerly Leguminosae — family, commonly referred to as the pea or bean family. 

The Fabaceae are the third largest family of land plants after the Orchidaceae and the Asteraceae (think daisies, dandelions and marigolds). It is second in economic importance for human sustenance only to the Poaceae (grasses), the family containing our important cereal grains of wheat, barley, corn and rice.

It is not called the pea/bean family for nothing. Important food crops from this family include Pisum sativum (peas), Cicer arietinum (chickpea), Arachis hypogaea (peanut) and Glycine max (soybean). Other important crops include Medicago sativa (alfalfa), Ceratonia siliqua (carob) and Glycyrrhiza glabra (licorice). 

The Fabaceae plant forms include trees, shrubs, vines and herbaceous plants — all of which can be either perennials or annuals.

The name Fabaceae comes from the Latin faba, meaning “bean.” The older name Leguminosae refers to the fruits that we know as legumes. Those are fruits that grow inside pods, which dehisce on two sides — open along a seam.

The family is divided into three subfamilies, the Caesalpinoideae (includes the redbud, the sophoras and the carob), the Mimosoideae (acacias, for example), and the Papilionoideae or Faboideae (the aforementioned wisteria and locust trees). 

Current genetic research indicates that the Caesalpinioideae are the root group from which the other two groups diverged. The Caesalpinioideae and the Mimosoideae are found primarily in tropical and subtropical environments, while the Papilionoideae are mainly found in temperate zones of South and Central America, Africa and Australia.

I am particularly taken with the Papilionoideae subgroup because of the bilaterally symmetrical, distinctive “pea flower,” as well as their delicate, feathery leaves. The French word for butterfly is “le papillon” and the pea flowers remind me of butterflies with their five large petals. 

These include an upper large banner petal lying outside the two reduced lateral wings. The underside of the flower is formed by two elongated fused keel petals which enclose the stamens.

The compound leaves are composed of paired leaflets growing along a secondary stem, a rachis. If the leaf has a final single leaflet at the terminal end, it is “odd-pinnate,” or minus that final leaflet it is “even-pinnate.”

But Fabaceae have another important role. Despite Earth’s atmosphere containing 78 percent nitrogen gas, that does not translate into great quantities available to plants. Chlorophyll, the engine of photosynthesis, needs nitrogen. It is an essential element in amino acids, which are the building blocks of proteins. 

DNA and RNA are composed of nucleic acids that also require nitrogen.

There are two non-organic pathways that move gaseous nitrogen from the atmosphere into the soil in chemical forms useful to plants. The first is through the catalytic action of lightning upon atmospheric nitrogen and oxygen. The resulting compound then reacts with atmospheric water, finally seeping into the soil. 

The second pathway has only been recently elucidated. Ancient decomposed organic matter becomes absorbed into material destined to become sedimentary rock. As that rock breaks down over the eons, it releases nitrogen back into the soil.

Diazotroph is the designation of organisms, primarily bacteria, that can convert free nitrogen to a form that is useful to plants. 

Although there are many diazotrophs, leguminous plants and their Rhizobia symbionts, play the major role in converting atmospheric nitrogen to forms useful to plants. 

These bacteria form nodules on the Fabaceae roots. Not only do the Rhizobia provide nitrogen to the host plants, but after the harvest the crop residue — which includes underground structures — will provide this natural fertilizer for next year’s crops.

Soon my mimosa trees will bloom, and I can hardly wait!

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Sura Jeselsohn